Hydrologic Analysis Report

Home , Bitterroot River, John Long Mountains, Rock Creek (Montana)

RESTORING OUR ENVIRONMENT  DESIGNING OUR FUTURE

Missoula-Granite PMR, MAS No. 2019-02 Missoula and Granite Counties, Montana Hydrologic Analysis Report

Montana DNRC July 9, 2020

Pioneer Technical Services, Inc. 106 Pronghorn Trail, Bozeman Montana 59718 www.pioneer-technical.com

Missoula-Granite PMR MAS No. 2019-02 Missoula and Granite Counties, Montana Hydrologic Analysis Report

Prepared for: State of Montana, Department of Natural Resources and Conservation 1424 9th Avenue Helena, Montana 59620

Prepared by: Pioneer Technical Services, Inc. 106 Pronghorn Trail Bozeman, Montana 59718

Missoula-Granite PMR MAS No. 2019-02

Missoula and Granite Counties, Montana

Hydrologic Analysis Report

July 9, 2020

I hereby certify that all work products (maps, reports, etc.) prepared for this project were done so under my direct supervision and that I am a duly Licensed Professional Engineer under the laws of the State of Montana.

George Austiguy P.E.

Date: ______07/09/20______Montana Registration No. 9528

Pioneer Technical Services, Inc. 106 Pronghorn Trail, Suite A Bozeman, MT 59718

Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

TABLE OF CONTENTS

Table of Contents ...... i Figures ...... iii Tables...... iv Appendices ...... v 1 Introduction ...... 1 1.1 Background Information ...... 1 2 Clark Fork River ...... 4 2.1 Clark Fork River Introduction ...... 4 2.1.1 Basin Description ...... 4 2.1.2 Flood History ...... 6 2.2 Past Studies and Existing Flood Data ...... 15 2.2.1 City of Missoula and Missoula County Flood Insurance Study ...... 16 2.2.2 Granite County and Incorporated Areas Flood Insurance Study ...... 17 2.2.3 Water-Resources Investigations Report 03-4308 ...... 18 2.2.4 Scientific Investigations Report 2015-5019...... 19 2.2.5 Scientific Investigations Report 2018-5046...... 20 2.2.6 Letter of Map Revision – Clark Fork River and Blackfoot River Near Milltown, Montana – Appendix A ...... 21 2.2.7 Restoration Plan for the Clark Fork River and Blackfoot River near Milltown Dam – October 2005: Appendix A – Hydrology and Flood Series Analysis ...... 22 2.2.8 Additional Previous Studies ...... 23 2.2.9 Flood Protection Measures ...... 25 2.3 Hydrologic Analyses and Results ...... 26 2.3.1 USGS Stream Gage Analysis ...... 26 2.3.1.1 1% + Peak Flow Estimates ...... 28 2.3.2 Flow Change Node Locations ...... 32 2.3.3 Flood Frequency Estimates at Ungaged Sites ...... 37 2.3.3.1 Two-Site Logarithmic Interpolation Method ...... 38 2.4 Clark Fork Mainstem Discussion ...... 42 3 Bitterroot River ...... 53 3.1 Bitterroot River Introduction ...... 53 3.1.1 Basin Description ...... 53 3.1.2 Flood History ...... 55 3.2 Past Studies and Existing Flood Data ...... 57 3.2.1 City of Missoula and Missoula County Flood Insurance Study ...... 58 3.2.2 Water Resources Investigations Report 03-4308 ...... 59 3.2.3 Scientific Investigations Report 2015-5019...... 60 3.2.4 Scientific Investigations Report 2018-5046...... 60 3.2.5 Additional Previous Studies ...... 61

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3.2.6 Flood Protection Measures ...... 61 3.3 Hydrologic Analyses and Results ...... 62 3.3.1 USGS Stream Gage Analysis ...... 62 3.3.1.1 1%+ Peak Flow Estimates ...... 64 3.3.2 Flow Change Node Locations ...... 68 3.3.3 Flood Frequency Estimates at Ungaged Sites ...... 71 3.3.3.1 Two-Site Logarithmic Interpolation Method ...... 72 3.3.3.2 Drainage Area Gage Transfer Method ...... 75 3.3.4 Bitterroot River Discussion ...... 77 4 Rock Creek and Tributaries ...... 85 4.1 Introduction...... 85 4.1.1 Basin Description ...... 85 4.1.1.1 Rock Creek Mainstem ...... 85 4.1.1.2 Rock Creek Tributaries ...... 85 4.1.2 Flood History ...... 87 4.2 Past Studies and Existing Flood Data ...... 90 4.2.1 City of Missoula and Missoula County Flood Insurance Study ...... 91 4.2.2 Granite County and Incorporated Areas Flood Insurance Study ...... 92 4.2.3 Water Resources Investigations Report 03-4308 ...... 92 4.2.4 Scientific Investigations Report 2015-5019...... 93 4.2.5 Scientific Investigations Report 2018-5046...... 94 4.2.6 Additional Previous Studies ...... 94 4.2.7 Flood Protection Measures ...... 95 4.3 Hydrologic Analysis and Results ...... 96 4.3.1 Rock Creek Mainstem ...... 96 4.3.1.1 USGS Stream Gage Analysis ...... 96 4.3.1.2 1%+ Peak Flow Analysis ...... 99 4.3.1.1 Flow Change Node Locations ...... 103 4.3.1.2 Flood Frequency Estimates at Ungaged Sites ...... 106 4.3.1.3 Two-Site Logarithmic Interpolation Method ...... 106 4.3.2 Rock Creek Mainstem Discussion ...... 110 4.3.3 Rock Creek Tributaries ...... 116 4.3.3.1 USGS Stream Gage Analysis ...... 116 4.3.3.2 1%+ Peak Flow Analysis ...... 117 4.3.3.3 Flow Change Node Locations ...... 120 4.3.3.4 Flood Frequency Estimates at Ungaged Sites ...... 125 4.3.3.5 Two Site Logarithmic Interpolation ...... 125 4.3.3.6 Regional Regression Equations Method ...... 128 4.3.4 Rock Creek Tributaries Discussion...... 134 5 References ...... 140

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FIGURES

Figure 1-1 Missoula-Granite Study Area...... 3 Figure 2-1 Clark Fork River Study Area ...... 5 Figure 2-2 Clark Fork River below Missoula (12353000) ...... 8 Figure 2-3 Clark Fork at Missoula (12341500) ...... 9 Figure 2-4 Clark Fork above Missoula, MT (12340500) ...... 10 Figure 2-5 Clark Fork at Turah Bridge near Bonner (12334550) ...... 11 Figure 2-6 Clark Fork near Clinton (12331900) ...... 12 Figure 2-7 Clark Fork near Drummond (12331800) ...... 13 Figure 2-8 Clark Fork at Drummond (12331600) ...... 14 Figure 2-9 Clark Fork River USGS Watersheds ...... 27 Figure 2-10 USGS Flood Frequency Estimates Systematic Record through 2018 ...... 29 Figure 2-11 USGS Flood Frequency Estimates Extended Record through 2018 ...... 30 Figure 2-12 Clark Fork River Flow Node 100 to 12334550 ...... 35 Figure 2-13 Clark Fork River Flow Node 12334550 to 2700 ...... 36 Figure 2-14 Clark Fork River Log Interpolation Gage Analysis Results ...... 41 Figure 2-15 Updated Clark Fork River below Missoula (12353000) ...... 47 Figure 2-16 Updated Clark Fork above Missoula, MT (12340500) ...... 48 Figure 2-17 Updated Clark Fork at Turah Bridge near Bonner (12334550)...... 49 Figure 2-18 Updated Clark Fork near Drummond (12331800) ...... 50 Figure 2-19 Clark Fork River Recommended 1-Percent Annual Discharge Flow Node 100 to 12334550 ...... 51 Figure 2-20 Clark Fork River Recommended 1-Percent Annual Discharge Flow Node 12334550 to 2700 ...... 52 Figure 3-1 Bitterroot River Study Area ...... 54 Figure 3-2 Bitterroot River near Missoula, MT (12352500) ...... 56 Figure 3-3 Bitterroot River near Florence MT (12351200) ...... 57 Figure 3-4 Bitterroot River USGS Watersheds ...... 63 Figure 3-5 USGS Flood Frequency Estimates Systematic Record through 2018 ...... 65 Figure 3-6 USGS Flood Frequency Estimates Extended Record though 2018 ...... 66 Figure 3-7 Bitterroot River Flow Node Locations ...... 70 Figure 3-8 Bitterroot River Log Interpolation Gage Analysis Results ...... 74 Figure 3-9 Bitterroot River Drainage Area Gage Transfer Results ...... 77 Figure 3-10 Updated Bitterroot River near Missoula, MT (12352500) ...... 82 Figure 3-11 Updated Bitterroot River near Florence, MT (12351200) ...... 83 Figure 3-12 Bitterroot River Recommended 1-Percent Annual Discharge ...... 84 Figure 4-1 Rock Creek Mainstem and Tributaries Study Areas ...... 86 Figure 4-2 Rock Creek near Clinton (12334510) ...... 88 Figure 4-3 Middle Fork Rock Creek near Phillipsburg (12332000) ...... 89 Figure 4-4 Rock Creek Mainstem USGS Watersheds ...... 98 Figure 4-5 USGS Flood Frequency Estimates Systematic Record through 2018 ...... 100

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Figure 4-6 USGS Flood Frequency Estimates Extended Record though 2018 ...... 101 Figure 4-7 Rock Creek Mainstem Flow Node Locations ...... 105 Figure 4-8 Rock Creek Mainstem Two-Site Logarithmic Interpolation Flow Node Results ...... 109 Figure 4-9 Updated Rock Creek near Clinton (12334510) ...... 114 Figure 4-10 Rock Creek Mainstem Recommended 1-Percent Annual Discharge ...... 115 Figure 4-11 Rock Creek Tributaries USGS Watersheds...... 118 Figure 4-12 Rock Creek Tributaries Flow Node Locations 100 to 400 ...... 123 Figure 4-13 Rock Creek Tributaries Flow Node Locations 500 to 900 ...... 124 Figure 4-14 Middle Fork Rock Creek Two-Site Logarithmic Interpolation Flood Frequency Analysis ...... 127 Figure 4-15 Upper Willow Creek Regional Regression Flood Frequency Analysis ...... 132 Figure 4-16 West Fork Rock Creek Regional Regression Flood Frequency Analysis ...... 133 Figure 4-17 Updated Middle Fork Rock Creek near Phillipsburg (12332000) ...... 137 Figure 4-18 Rock Creek Tributaries Recommended 1-Percent Annual Discharge Flow Node 100 to 400 ...... 138 Figure 4-19 Rock Creek Tributaries Recommended 1 Percent Annual Discharge Flow Node 500 to 900 ...... 139

TABLES

Table 2-1 Clark Fork River Floodplain Mapping Summary ...... 15 Table 2-2 Missoula County FIS Summary of Discharges ...... 16 Table 2-3 Granite County FIS Summary of Discharges ...... 17 Table 2-4 WRIR 03-4308 Clark Fork River Peak Discharge Summary ...... 18 Table 2-5 SIR-2015-5019-C Clark Fork River Peak Discharge Summary ...... 19 Table 2-6 SIR 2018-5046 Clark Fork River Peak Discharge Summary ...... 20 Table 2-7 Results of Peak Flow Frequency Analysis for selected stream gages in the Clark Fork Basin ...... 20 Table 2-8 Appendix A –2012 LOMR Peak Flows ...... 21 Table 2-9 Appendix A – Hydrology and Flood Series Analysis Summary of Discharges ...... 22 Table 2-10 Levees on the Clark Fork Mainstem ...... 25 Table 2-11 Clark Fork River USGS Gage Summary ...... 28 Table 2-12 Clark Fork River Gage Flood Frequency Estimates ...... 31 Table 2-13 USGS and GIS Model Watershed Comparison ...... 33 Table 2-14 Flow Node and USGS Gage Station Information Used in Hydrologic Analyses ...... 34 Table 2-15 Two-Site Logarithmic Interpolation Flow Node Peak Flow Estimates ...... 39 Table 2-16 Recommended AEP Peak Flow Estimates ...... 44 Table 2-17 Recommended Peak Flow Estimates vs Existing Missoula and Granite County FIS Report Peak Flow Estimates ...... 46 Table 3-1 Bitterroot River Floodplain Mapping Summary ...... 58 Table 3-2 Missoula County FIS Summary of Discharges ...... 59 Table 3-3 WRIR 03-4308 Bitterroot River Peak Flow Summary ...... 59

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Table 3-4 SIR 2015-5019-C Bitterroot River Peak Discharge Summary ...... 60 Table 3-5 Bitterroot River USGS Gage Summary ...... 64 Table 3-6 Bitterroot River Gage Flood Frequency Peak Flow Estimates ...... 67 Table 3-7 USGS and GIS Model Watershed Comparison ...... 69 Table 3-8 Flow Node and USGS Gage Station Information Used in Hydrologic Analysis ...... 69 Table 3-9 Two-Site Logarithmic Interpolation Flow Node Peak Flow Estimates ...... 73 Table 3-10 Drainage Basin Ratios ...... 75 Table 3-11 Drainage Area Gage Transfer Regression Coefficients ...... 76 Table 3-12 Drainage Area Gage Transfer Method Results for Flow Node Locations near USGS gage 12352500 Bitterroot River near Missoula, MT ...... 76 Table 3-13 Recommended AEP Peak Flow Estimates ...... 80 Table 3-14 Recommended Peak Flows vs Existing Missoula County FIS ...... 81 Table 4-1 Rock Creek Mainstem and Tributaries Floodplain Mapping Summary ...... 90 Table 4-2 Missoula County FIS Summary of Discharges ...... 91 Table 4-3 Granite County FIS Summary of Peak Flows ...... 92 Table 4-4 WRIR 03-4308 Rock Creek Mainstem and Tributaries Peak Flow Summary ...... 93 Table 4-5 SIR 2015-5019-C Rock Creek Mainstem and Tributaries Peak Flow Summary ...... 94 Table 4-6 SIR 2018-5046 Rock Creek Mainstem Peak Flow Summary ...... 94 Table 4-7 Rock Creek Mainstem USGS Gage Summary ...... 99 Table 4-8 Rock Creek Mainstem Gage Flood Frequency Estimates ...... 102 Table 4-9 USGS and GIS Model Watershed Comparison ...... 104 Table 4-10 Flow Node and USGS Gage Station Information Used in this Hydrologic Analysis .... 104 Table 4-11 Rock Creek Mainstem Two-Site Logarithmic Interpolation Flow Node Peak Flow Estimates ...... 108 Table 4-12 Recommended AEP Peak Flow Estimates ...... 112 Table 4-13 Recommended Rock Creek Mainstem AEP Peak Flows vs Missoula and Granite Existing FIS Reports ...... 113 Table 4-14 Rock Creek Tributaries USGS Gage Summary ...... 117 Table 4-15 Rock Creek Tributaries Gage Flood Frequency Peak Flow Estimates ...... 119 Table 4-16 USGS and GIS Model Watershed Comparison ...... 121 Table 4-17 Flow Node and USGS Gage Station Information Used in this Analysis ...... 122 Table 4-18 Two-Site Logarithmic Interpolation Method Results for Flow Node 500 ...... 126 Table 4-19 Regional Regression Variables ...... 130 Table 4-20 Regional Regression Flood Frequency Peak Flow Estimates ...... 131 Table 4-21 Recommended Flood Discharge Estimates Using Regional Regression and Gage Transfer Analysis ...... 136

APPENDICES

Appendix A. USGS Gage Flood Frequency Analysis Appendix B. Ungaged Flow Node Flood Frequency Calculations

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EXECUTIVE SUMMARY

Flood flow frequency calculations were conducted for a 109-mile reach of the Clark Fork River, a 22-mile reach of the Bitterroot River, and a 53-mile reach of Rock Creek mainstem and 6 tributary reaches. The study reach extends from the Mineral\Missoula County border, 1 mile east of Alberton, Montana, to the Granite\Powell County border, approximately 7 miles east of Drummond, Montana. Information gathered from this analysis will be used for future floodplain studies and mapping projects.

The hydrology of the Clark Fork River, Bitterroot River, and Rock Creek basins are primarily snowmelt driven, although significant flows can result from precipitation events. Land use in the Clark Fork River basin is primarily agricultural with irrigated farming and ranching operations. Most of the intensely farmed land is in the Deer Lodge Valley within the Clark Fork River floodplain. Missoula is the primary community in the study area.

The Clark Fork River is a major tributary to the Pend Oreille River and upper headwaters of the Columbia River located west of the continental divide in western Montana. The river is formed by the confluence of Silver Bow Creek and Warm Springs Creek. The river tributaries originate in the Deerlodge National Forest near the Continental Divide. The watershed is formed by the Bitterroot Mountains to the west, Deer Lodge Mountains to the east, and the Pintler and Highland Ranges to the south. The watershed area at the downstream study boundary is 9,548 square miles. The Bitterroot River and Rock Creek are significant tributaries to the Clark Fork River.

The primary cause of flooding on the Clark Fork River is spring snowmelt and ice jams (according to historical records). There are historical records from several U.S. Geological Survey (USGS) stream gages on the river that date back to 1908, which document basin flood history.

There are numerous past flood studies for the Clark Fork River within Missoula County. Past flood studies for the Clark Fork River within Granite County are limited. Past studies on the Bitterroot River, Rock Creek mainstem and tributary study reaches are limited within Missoula and Granite Counties. FEMA Flood Insurance Rate Maps (FIRM) and a FIS exist for Missoula County unincorporated areas and the City of Missoula. FEMA FIRMs and a FIS exist for Granite County unincorporated areas, the City of Drummond, and the City of Phillipsburg.

For this hydrologic analysis, flood flow frequency analysis was conducted to develop peak flow discharge estimates for the 50, 10, 4, 2, 1, and 0.2% annual chance events. The 1%+ (plus) annual chance event was also calculated. Peak flow estimates were calculated at 52 flow node locations and 10 gage sites within and adjacent to the study area. Flood frequency peak flow estimates at the gaged sites with more than 10 years of record were conducted by the USGS using Bulletin #17C methodologies (England, et al., 2018). At the ungaged and gaged sites with less than 10 years of record, peak flow estimates were calculated using the two-site logarithmic

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interpolation method, the drainage area gage transfer method or the regional regression method. These methods conform to standard engineering practice.

In the Clark Fork River, Bitterroot River and Rock Creek study reach’s, the flood flow frequency estimates are generally less than or equal to the existing effective mapping values.

The hydrologic analysis documented in this report conforms to FEMA standards for detailed/enhanced level studies, and the recommended flows of this analysis are deemed reliable and suitable for future floodplain studies and hydraulic analyses.

The recommended AEP peak flows for each study reach are shown in the following tables.

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Clark Fork River Recommended AEP Peak Flow Estimates

Peak Flow (cfs) 50% Annual 10% Annual 4% Annual 2% Annual 1% Annual 0.2% Annual 1% + Annual Node/USGS Chance Chance Chance Chance Chance Chance Chance Station ID Location Description 2-year 10-year 25-year 50-year 100-year 500-year 100-year + 27001 Perkins Creek 2,800 6,410 8,540 10,200 12,000 16,400 18,200 26001 Lower Flint Creek 2,820 6,450 8,610 10,300 12,200 16,700 18,500 123316001 Clark Fork at Drummond, MT 3,190 7,220 9,880 12,100 14,600 21,300 24,400 24001 Rattler Gulch 3,220 7,300 10,000 12,200 14,800 21,800 25,100 23001 Mulkey Gulch 3,240 7,330 10,100 12,300 14,900 22,000 25,400 123318002 Clark Fork near Drummond, MT 3,280 7,420 10,200 12,500 15,200 22,600 26,100 21001 Harvey Creek 3,330 7,510 10,300 12,600 15,300 22,700 26,200 20001 Tyler Creek 3,450 7,710 10,500 12,800 15,600 22,900 26,300 19001 Dry Gulch 3,500 7,780 10,600 12,900 15,600 23,000 26,300 123319001 Clark Fork near Clinton, MT 3,580 7,930 10,800 13,100 15,800 23,200 26,400 17001 Rock Creek-Kitchen Gulch 3,670 8,060 10,900 13,300 16,000 23,300 26,500 16001 Schwartz Creek 6,020 11,700 14,800 17,300 20,000 26,700 28,400 15001 Wallace Creek 6,180 11,900 15,100 17,600 20,300 26,900 28,400 123345502 Clark Fork at Turah Bridge near Bonner, MT 6,260 12,000 15,200 17,700 20,400 27,000 28,500 13001 Clark Fork upstream of Blackfoot River 6,350 12,200 15,400 17,900 20,600 27,300 28,700 12001 Lower Rattlesnake Creek 15,000 26,600 32,100 36,100 39,900 48,400 46,600 11001 Grant Creek 15,300 27,100 32,600 36,700 40,500 49,000 47,200 10001 Clark Fork upstream of Bitterroot River 15,600 27,500 33,100 37,100 41,000 49,600 47,800 123530002 Clark Fork below Missoula, MT 29,300 47,200 54,900 60,200 65,000 75,200 73,500 8001 Deep Creek 29,500 47,500 55,200 60,600 65,400 75,600 73,900 7001 Rock Creek 29,800 48,000 55,800 61,200 66,000 76,300 74,600 6001 Mill Creek 30,000 48,300 56,100 61,500 66,400 76,700 75,000 5001 Roman Creek 30,200 48,600 56,500 61,900 66,800 77,200 75,400

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Bitterroot River Recommended AEP Peak Flow Estimates

Peak Flow (cfs) 50% Annual 10% Annual 4% Annual 2% Annual 1% Annual 0.2% Annual 1% + Annual Node/USGS Chance Chance Chance Chance Chance Chance Chance Station ID Location Description 2-year 10-year 25-year 50-year 100-year 500-year 100-year + 4002 Bitterroot River-North Woodchuck Creek 14,200 18,500 21,000 23,900 26,100 32,800 33,100 3002 Lower Lolo Creek 14,600 21,900 25,300 27,900 30,400 36,100 35,900 123525003 Bitterroot River near Missoula, MT 14,700 22,700 26,400 28,900 31,400 36,900 36,600 1001 Bitterroot River at junction with Clark Fork River 15,200 23,500 27,300 29,800 32,400 38,000 37,800 1. Analyzed with USGS gage transfer method. 2. Analyzed with USGS two-site logarithmic interpolation Method. 3. Analyzed by USGS MOVE.3 extended record analysis. USGS: U.S. Geological Survey. cfs: cubic feet per second.

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Rock Creek Mainstem Recommended AEP Peak Flow Estimates

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Rock Creek Tributaries Recommended AEP Peak Flow Estimates

Peak Discharge (cfs) 50% 10% 4% 2% 1% 0.2% 1% + Annual Annual Annual Annual Annual Annual Annual Chance Node/USGS Chance Chance Chance Chance Chance Chance Station ID Location Description 2-year 10-year 25-year 50-year 100-year 500-year 100-year + 9001 Maukey Gulch at junction with West Fork Rock Creek 13 39 56 72 89 134 139 8001 Ross Fork at junction with West Fork Rock Creek 355 723 908 1,060 1,230 1,590 1,920 7001 Lower West Fork Ross Creek 374 749 935 1,090 1,250 1,620 1,950 6001 West Fork Rock Creek at junction with Rock Creek 688 1,330 1,640 1,900 2,160 2,750 3,370 5002 Middle Fork Rock Creek at junction with Rock Creek 1,270 2,130 2,500 2,750 2,980 3,480 3,450 4001 Upper Upper Willow Creek 159 348 448 533 622 828 970 3001 Middle Upper Willow Creek 229 509 657 784 917 1,230 1,430 2001 Willow Creek at junction with Rock Creek 297 668 868 1,040 1,220 1,640 1,900 1001 Ranch Creek at junction with Rock Creek 206 418 524 613 705 911 1,100 1. Analyzed with USGS regional regression west region equations 2. Analyzed with USGS gage transfer method. USGS: U.S. Geological Survey. cfs: cubic feet per second.

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1 INTRODUCTION As part of the Missoula and Granite PMR Project, the Montana Department of Natural Resources and Conservation (DNRC) contracted Pioneer Technical Services, Inc. (Pioneer) to complete a comprehensive peak flow hydrologic analysis for Missoula-Granite study reaches. Pioneer conducted flood flow frequency calculations on 4 primary study areas. The first study area was a 109.6-mile reach of the Clark Fork River mainstem. The second study area was a 21.9-mile reach of the Bitterroot River mainstem. The third and fourth study areas included a 53.0-mile reach of the Rock Creek mainstem and 6 of its tributaries, respectively. All study reaches are in Missoula and Granite Counties. The Missoula-Granite study area encompassed a combined 9,548 square miles. The hydrologic analysis in this report has been organized into 4 hydrologic study areas:

1. Clark Fork River Mainstem. 2. Bitterroot River Mainstem. 3. Rock Creek Mainstem. 4. Rock Creek Tributaries.

Information gathered from this analysis will be used for both enhanced level and enhanced level with floodway hydraulic analyses and floodplain mapping. Figure 1-1 shows the project study reach.

1.1 Background Information The FEMA administers the National Flood Insurance Program (NFIP). As part of this program, FEMA supports flood hazard studies and prepares flood hazard maps and related documents. The study reaches in Missoula County are heavily populated and located in predominantly urban environments. The study reaches in Granite County are sparsely populated and located in predominately rural environments. Study reaches flow through or near the communities of Drummond, Clinton, Turah, East Missoula, Missoula, Frenchtown, Huson, and Lolo. The existing floodplain mapping for Missoula County includes 94.8 miles of Detailed Zone AE. The existing floodplain mapping for Granite County includes 85.3 miles of Approximate Zone A and 21.4 miles of Detailed Zone AE on sections of Rock Creek.

Approximate Zone A flood maps are developed using approximate methodologies and are not based on detailed hydraulic analysis. This level of flood mapping is often used in rural areas with low populations. Base Flood Elevations (BFEs) or flood depths are not identified in Approximate Zone A mapping (a BFE is the computed elevation to which floodwater is estimated to rise during the base flood). As a result, areas designated with Zone A flood mapping are difficult for local communities to manage and administer.

Enhanced and base level mapping are similar in that both use standard hydrologic and hydraulic modeling methods to estimate BFEs and flood inundation areas. Both require the same topographic accuracy. However, base level analysis may not include mapping the 500-year

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floodplain delineation, BFEs, or developing flood profiles. Base level analysis may allow some flexibility in the acquisition and modeling of bathymetric and structure survey data, and generally does not include floodway delineation. Although enhanced level analysis often includes floodway delineation, it is not always required (FEMA, 2016a).

The DNRC, in partnership with FEMA and Missoula and Granite Counties, initiated work to produce a new floodplain study along existing study reaches in Missoula County and Granite County. The Missoula-Granite floodplain study will provide the groundwork for completing floodplain mapping in Missoula and Granite Counties. This report documents the hydrologic analysis methodology and results completed on the study areas. The analysis includes peak discharge estimate calculations for the 50, 10, 4, 2, 1, and 0.2% annual exceedance probability (AEP) peak discharges at key flow change locations (such as Hydrologic Unit Code [HUC] 12 [HUC 12], significant tributary confluences, stream gages, and population centers) along the study reach. This report conforms to FEMA standards for enhanced level studies (FEMA, 2019: SID #84).

As part of the study, the DNRC contracted with the USGS to perform a Bulletin 17C flood frequency analysis for 17 selected USGS stream gages in Missoula, Granite, Mineral, Powell and Ravelli Counties (Sando, et al., 2020); the results of the 2020 USGS gage analysis were from a courtesy data release while the information is being processed for publication. Ten of the 17 selected USGS gages were used to support the flood frequency analyses in this report. This courtesy data release will be updated when the final USGS data release is published. A detailed summary of the 2019 USGS gage analysis is included in the subsequent Hydrologic Analysis sections for each study area.

Page 2 35 Choteau Polson «¬ LAKE COUNTY TETON COUNTY SANDERS FLATHEAD COUNTY COUNTY

«¬28 89 Ronan «¬ ¤£87 200 «¬ Great Falls LEWIS & CLARK CASCADE 135 83 ¬287 «¬ «¬ COUNTY « COUNTY ¤£93

MINERAL 68 COUNTY «¬

POWELL COUNTY ¨¦§90 MISSOULA COUNTY ¤£200

Missoula Bonner ¨¦§15 Clark Fork River Bitterroot River MEAGHER Lolo COUNTY «¬141

Ranch Creek

Helena East Helena 12 Rock Creek GRANITE ¤£ COUNTY

Deer Lodge RAVALLI Upper Willow Creek COUNTY West Fork Hamilton 1 Townsend Rock Creek «¬ Maukey Gulch 90 BROADWATER Ross Fork ¨¦§ COUNTY 93 ¤£ Middle Fork Anaconda 287 Rock Creek SILVER BOW JEFFERSON ¤£ COUNTY COUNTY «¬69 DEER LODGE Butte GALLATIN COUNTY COUNTY ¨¦§15 «¬2

LEGEND DISPLAYED AS: FIGURE 1-1 MISSOULA-GRANITE TOWNS NHS INTERSTATE PROJECTION/ZONE: MONTANA STATE PLANE DATUM: NAD1983 MISSOULA-GRANITE STUDY REACHES NHS NON-INTERSTATE UNITS: INT'L FEET STUDY AREA SOURCE: DNRC/MSL/PIONEER COUNTY BOUNDARIES PRIMARY STREAMS N 0 7.5 15 30 HUC 8 BOUNDARIES DATE: 7/2/2020 Miles Path: Z:\Shared\Active Projects\DNRC\Missoula-Granite\GIS\Hydrology\MSAGNT-FIG-PLN-1-1-SA.mxd Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

2 CLARK FORK RIVER

2.1 Clark Fork River Introduction 2.1.1 Basin Description The Clark Fork River is a major tributary to the Pend Oreille River and upper headwaters of the Columbia River located west of the continental divide in western Montana. The river is formed by the confluence of Silver Bow Creek and Warm Springs Creek. The river tributaries originate in the Deerlodge National Forest near the Continental Divide. The watershed is formed by the Bitterroot Mountains to the west, Deer Lodge Mountains to the east, and the Pintler and Highland Ranges to the south. The mainstem Clark Fork River begins at Warm Springs, Montana, and flows north for approximately 20 miles through the Deer Lodge Valley before tuning west. The Blackfoot and Bitterroot Rivers join the Clark Fork River near Missoula. Approximately 213 miles downstream of Missoula, the Clark Fork River terminates at Lake Pend Oreille. The entire Clark Fork River watershed encompasses approximately 22,905 square miles. The Clark Fork River study reach is shown in Figure 2-1.

The Clark Fork River basin elevations within the study area range from 10,463 feet in the Pintler Mountains to approximately 2,600 feet at Alberton. The overall basin elevations range from over 10,000 feet in the Pintler Mountains to 2,060 feet near the confluence with Pend Oreille Lake (USACE, 1967). The terrain varies from a high alpine environment in its headwaters to a heavily cultivated landscape in the Deer Lodge valley with expansive irrigated pasture lands, bracketed by rolling foothills. The majority of peak flows along the Clark Fork River gages occurred in May, June, or July, suggesting the hydrology of the basin is primarily snowmelt driven.

Land use in the Clark Fork River basin is primarily agricultural with irrigated farming and ranching operations. Most of the intensely farmed land is located in the Deer Lodge Valley within the Clark Fork River floodplain. Missoula is the primary community in the Clark Fork River study area.

Page 4 MINERAL COUNTY «¬83

Alberton Frenchtown ¤£93 12353300

MISSOULA COUNTY ¨¦§90

H W1YW0

Clark Fork River

W BROADWAY 12340500 12353000 12341500 Missoula ¤£200 Bonner

RESERVE ST BROOKS ST H W1 Y0E POWELL COUNTY HWY 93 S 12334550

Lolo Clinton Bitterroot River

Clark Fork River

12331900 12331800 ¨¦§90

Drummond Florence Rock Creek 12331600 GRANITE COUNTY «¬1 Ranch Creek

12324680 RAVALLI COUNTY Stevensville

LEGEND DISPLAYED AS: ACTIVE USGS GAGE MISSOULA-GRANITE STUDY REACHES NHS NON-INTERSTATE PROJECTION/ZONE: MONTANA STATE PLANE FIGURE 2-1 CLARK FORK RIVER DATUM: NAD1983 INACTIVE USGS GAGE COUNTY BOUNDARIES PRIMARY UNITS: INT'L FEET STUDY AREA SOURCE: DNRC/MSL/PIONEER TOWNS HUC 8 BOUNDARIES SECONDARY URBAN N 0 2.5 5 10 CLARK FORK MAINSTEM STUDY REACH NHS INTERSTATE DATE: 7/2/2020 STREAMS Miles Path: Z:\Shared\Active Projects\DNRC\Missoula-Granite\GIS\Hydrology\MSAGNT-FIG-PLN-2-1-SA.mxd Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

2.1.2 Flood History The primary cause of flooding on the Clark Fork River is spring snowmelt and ice jams (according to historical records). The greatest flood on the Clark Fork River during the past 100 years occurred in June 1908 at Missoula. Newspapers reported it was greater than any flood known to the oldest residents at that time. In early June, the ground was already saturated from weeks of hard rain when temperatures dropped, and several inches of snow covered the area. As temperatures warmed, the drenched earth began to flood, sending torrents into the river. The flood peaked at 4.5 feet above major flood stage, forcing residents to flee and railroads to shut down—even the mines in Butte, Montana, closed (Bonner Milltown History Center, n.d.).

There are historical records from USGS stream gages in the study area that document flooding history. The gages are listed below:

1. Clark Fork below Missoula, MT (12353000) 2. Clark Fork at Missoula, MT (12341500) 3. Clark Fork above Missoula, MT (12340500) 4. Clark Fork at Turah Bridge near Bonner, MT (12334550) 5. Clark Fork near Clinton, MT (12331900) 6. Clark Fork near Drummond, MT (12331800) 7. Clark Fork at Drummond (12331600)

The USGS stream gage Clark Fork below Missoula, MT (12353000) has a 90-year period of record (1930-2019). The annual peak flow record for the Clark Fork below Missoula gage is shown in Figure 2-2. The gage is located on the Clark Fork study reach.

The USGS stream gage Clark Fork at Missoula, MT (12341500) has a 9-year period of record (1899-1907). The annual peak flow record for the Clark Fork at Missoula gage is shown in Figure 2-3. The gage is located on the Clark Fork study reach.

The USGS stream gage Clark Fork above Missoula, MT (12340500) has a 91-year period of record (1899-1908,1930-2018). The annual peak flow record for the Clark Fork above Missoula gage is shown in Figure 2-4. The gage is located on the Clark Fork study reach.

The USGS stream gage Clark Fork at Turah Bridge near Bonner, MT (12334550) has a 33-year period of record (1986-2018). The annual peak flow record for the Turah Bridge gage is shown in Figure 2-5. The gage is located on the Clark Fork study reach.

The USGS stream gage Clark Fork near Clinton, MT (12331900) has a 14-year period of record (1980-1990, 1992-1994). The annual peak flow record for the Clinton gage is shown in Figure 2-6. The gage is located on the Clark Fork River study reach.

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The USGS stream gage Clark Fork near Drummond, MT (12331800) has a 26-year period of record (1993-2018). The annual peak flow record for the Drummond gage is shown in Figure 2-7. The gage is located on the Clark Fork River study reach.

The USGS stream gage Clark Fork at Drummond, MT (12331600) has a 12-year period of record (1980-1990, 1992-1994). The annual peak flow record for the Drummond gage is shown in Figure 2-8. The gage is located on the Clark Fork River study reach.

Peak flow recurrence intervals are shown in Figure 2-2, Figure 2-5, Figure 2-6, Figure 2-7 and are based on previously published flood frequency analysis through Water Year 2011 (SIR 2015- 5019-C) (Sando, et al., 2015a).

Peak flow recurrence intervals shown in Figure 2-4 are based on previously published flood frequency analysis through Water Year 2015 (SIR 2018-5046) (Sando & McCarthy, 2018).

Peak flow recurrence intervals shown in Figure 2-8 are based on flood frequency analysis through Water Year 1998 (Water Resources Investigations Report [WRIR] 03-4308) (Parret & Johnson, 2004).

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Figure 2-2 shows that the peak flood of record for the Clark Fork below Missoula (12353000) gage occurred in 1997 with a flow of 55,100 cubic feet per second (cfs), exceeding the 10% (10-year) AEP flow of 44,600 cfs. The second highest flood on record occurred in 1948 with a flow of 52,800 cfs, exceeding the 10-year flow. The third highest flood on record occurred in 1972 with a flow of 52,200 cfs, exceeding the 10-year flow. In the 90-year period of record at the Clark Fork below Missoula gage, the 10% AEP flow has been equaled or exceeded 9 times. All the annual peak flows occurred in April, May, and June. The peak flow of record occurred in May.

Figure 2-2 Clark Fork River below Missoula (12353000)

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0 1930 1933 1936 1939 1942 1945 1948 1951 1954 1957 1960 1963 1966 1969 1972 1975 1978 1981 1984 1987 1990 1993 1996 1999 2002 2005 2008 2011 2014 2017 Year 1. Flood Frequency Based on Data through 2011

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Figure 2-3 shows the peak flood of record for the Clark Fork at Missoula (12341500) gage occurred in 1899 with a flow of 36,400 cfs. The second highest flow occurred in 1902 with a flow of 31,400 cfs. The third highest flow occurred in 1900 with a flow of 25,600 cfs. All the annual peak flows occurred in the months of May and June. The peak flow of record occurred in May. The USGS has not performed a flood frequency analysis on this gage.

Figure 2-3 Clark Fork at Missoula (12341500)

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Figure 2-4 shows that the peak flood of record for the Clark Fork above Missoula (12340500) gage occurred in 1908 with a flow of 48,000 cfs, exceeding the 1% (100-year) AEP flow of 39,500 cfs. The second highest flood on record occurred in 2018 with a flow of 32,500 cfs, exceeding the 10% (10-year) AEP flow of 26,100 cfs. The third highest flood on record occurred in 1975 with a flow of 32,300 cfs, exceeding the 10-year flow. In the 91-year period of record at the Clark Fork above Missoula gage, the 10% AEP flow has been equaled or exceeded 10 times. All the annual peak flows occurred in April, May, and June. The peak flow of record occurred in June.

Figure 2-4 Clark Fork above Missoula, MT (12340500)

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0 1908 1932 1935 1938 1941 1944 1947 1950 1953 1956 1959 1962 1965 1968 1971 1974 1977 1980 1983 1986 1989 1992 1995 1998 2001 2004 2007 2010 2013 2016 Year 1. Flood Frequency Based on Data through 2015

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Figure 2-5 shows that the peak flood of record for the Clark Fork at Turah Bridge (12334550) gage occurred in 2011 with a flow of 13,300 cfs, exceeding the 10% (10-year) AEP flow of 10,200 cfs. The second highest flood on record occurred in 1996 with a flow of 12,400 cfs, exceeding the 10-year flow. The third highest flood on record occurred in 2018 with a flow of 12,300 cfs, exceeding the 10-year flow. In the 33-year period of record at the Clark Fork at Turah Bridge gage, the 10% AEP flow has been equaled or exceeded 3 times. All the annual peak flows occurred in February, April, May, and June. The peak flow of record occurred in June.

Figure 2-5 Clark Fork at Turah Bridge near Bonner (12334550)

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0 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 Year 1. Flood Frequency Based on Data through 2011

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Figure 2-6 shows that the peak flood of record for the Clark Fork near Clinton, MT (12331900) gage occurred in 1981 with a flow of 16,000 cfs, exceeding the 10% (10-year) AEP flow of 9,140 cfs. The second highest flood on record occurred in 1986 with a flow of 8,760 cfs. The third highest flood on record occurred in 1980 with a flow of 6,410 cfs. In the 14-year period of record at the Clark Fork near Clinton, MT gage, the 10% AEP flow has been equaled or exceeded once. All the annual peak flows occurred in February, March, April, May, June, and July. The peak flow of record occurred in May.

Figure 2-6 Clark Fork near Clinton (12331900)

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0 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1992 1993 1994 Year 1. Flood Frequency Based on Data through 2011

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Figure 2-7 shows that the peak flood of record for the Clark Fork near Drummond (12331800) gage occurred in 1996 with a flow of 9,800 cfs, exceeding the 10% (10-year) AEP flow of 7,620 cfs. The second highest flood on record occurred in 2011 with a flow 8,220 cfs, exceeding the 10-year flow. The third highest flood on record occurred in 2018 with a flow of 6,260 cfs. In the 26-year period of record at the Clark Fork near Drummond gage, the 10% AEP flow has been equaled or exceeded 2 times. All the annual peak flows occurred in February, March, April, May, and June. The peak flow of record occurred in February.

Figure 2-7 Clark Fork near Drummond (12331800)

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0 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 Year 1. Flood Frequency Based on Data through 2011

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Figure 2-8 shows that the peak flood of record for the Clark Fork at Drummond (12331600) gage occurred in 1981 with a flow of 15,800 cfs, exceeding the 2% (50-year) AEP flow of 13,900 cfs. The second highest flood on record occurred in 1975 with a flow 8,490 cfs. The third highest flood on record occurred in 1980 with a flow of 6,030 cfs. In the 12-year period of record at the Clark Fork at Drummond gage, the 10% AEP flow has been equaled or exceeded 1 time. All the annual peak flows occurred in January, February, May, June, and December. The peak flow of record occurred in May.

Figure 2-8 Clark Fork at Drummond (12331600)

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2.2 Past Studies and Existing Flood Data

There are numerous past flood studies for the Clark Fork River within Missoula County. Past flood studies for the Clark Fork River within Granite County are limited. FEMA Flood Insurance Rate Maps (FIRM) and a FIS exist for Missoula County unincorporated areas and the City of Missoula. FEMA FIRMs and a FIS exist for Granite County unincorporated areas, the City of Drummond, and the City of Phillipsburg. Studies relevant to this hydrologic study are those that include peak flow frequency analyses. Table 2-1 shows a summary of the mainstem Clark Fork River floodplain mapping.

Table 2-1 Clark Fork River Floodplain Mapping Summary

Map Panel Summary Study Details # of # of FIRM Panel County Approx Detailed Total Community FIRM FBFM Effective FIS Date Stream (mi) (mi) (mi) Panels Panels Date Missoula Co. Unincorporated Missoula 66 24 07/06/2015 03/07/2019 Clark Fork 0 69.6 69.6 Areas, City of Missoula Granite County Unincorporated Granite 52 7 04/19/2016 04/19/2016 Clark Fork 27.4 14.2 41.6 Areas, City of Drummond Source: FEMA Map Service Center FIRM: Flood Insurance Rate Map. FIS: Flood Insurance Studies. FBFM: Flood Boundary and Floodway Map. mi: Miles measured along channel alignment

The FIS dates reflect recent revisions in both Missoula and Granite Counties in 2019 and 2016, respectively. None of the study reaches analyzed in this report were updated in these revisions.

The USGS WRIR 03-4308 (Parret & Johnson, 2004), Scientific Investigations Report (SIR) 2015- 5019-C (Sando, et al., 2015b) and USGS SIR 2018-5046 (Sando & McCarthy, 2018) document the flood frequency analysis on several gages along the mainstem Clark Fork River. These studies and investigations are discussed in more detail in the following sections.

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2.2.1 City of Missoula and Missoula County Flood Insurance Study The City of Missoula and Missoula County FIS was initially issued August 16, 1988, and revised on March 7, 2019 (FEMA, 2019a). This Clark Fork River FIS was studied using detailed methods. The study area included the City of Missoula and the unincorporated areas of Missoula County. The FIS identified a major flood on the Clark Fork in 1908 with an estimated peak flow of 48,000 cfs in the City on Missoula.

Table 2-2 provides the City of Missoula and Missoula County FIS peak discharge summary for ungaged flow change locations and USGS gages on the Clark Fork. Peak discharge values shown below were not updated in the 2019 revision.

Table 2-2 Missoula County FIS Summary of Discharges

Drainage Peak Flow, Return Interval (years) USGS USGS Station FIS Flooding Source and Area (cfs) Station ID Name Location (mi2) 10-Year 50-Year 100-Year 500-Year Clark Fork at NA NA downstream Limit of 9,272 49,250 61,000 67,000 86,000 Detailed Study Clark Fork Clark Fork at USGS 12353000 below Gage No. 3530 below 9,003 47,000 58,000 64,000 82,000 Missoula, MT Missoula Clark Fork Clark Fork at USGS 12340500 above Gage No. 3405 above 5,999 27,000 38,200 42,500 56,000 Missoula, MT Missoula Clark Fork Just upstream of NA NA 3,668 15,000 22,500 26,000 35,500 confluence of Blackfoot River Source: FEMA, 2019a. mi2: square miles. cfs: cubic feet per second. NA: not applicable.

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2.2.2 Granite County and Incorporated Areas Flood Insurance Study The Granite County FIS was initially issued January 5, 1982 and revised on April 19, 2016 (FEMA, 2016). As part of the FIS, the Clark Fork River was studied using detailed and approximate methods. The study area included the City of Drummond and the unincorporated areas of Granite County. The FIS identified several major flood events. The largest flood event occurred in 1908 with a peak reported stage of 15.5 feet. Other floods referenced occurred in 1975 (10.6 feet), 1981 (12.44 feet), 1996 (10.03 feet), and 2011 (9.79 feet).

Table 2-3 provides the Granite County FIS peak discharge summary for flow change locations on the Clark Fork River. Peak discharge values shown below were not updated in the 2016 revision.

Table 2-3 Granite County FIS Summary of Discharges Peak Flow, Return Interval (years) (cfs) USGS USGS Station Drainage 10- 50- 100- 500- Station ID Name FIS Flooding Source and Location Area (mi2) Year Year Year Year NA NA Clark Fork below Rock Creek 3,553 15,000 22,500 26,000 35,500 NA NA Clark Fork above Rock Creek 2,668 14,000 21,000 24,000 32,500 NA NA Clark Fork below Flint Creek 2,378 9,850 16,900 20,500 30,000 NA NA Clark Fork above Flint Creek 1,888 8,300 14,400 17,500 25,700 Source: FEMA, 2016. mi2: square miles. cfs: cubic feet per second. NA: not applicable.

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2.2.3 Water-Resources Investigations Report 03-4308 The USGS WRIR 03-4308 developed annual peak discharges with recurrence intervals of 2, 5, 10, 25, 50, 100, 200, and 500 years (T-year floods) for 660 gaged sites in Montana and in adjacent areas of Idaho, Wyoming, and Canada, based on data through Water Year 1998 (Parret & Johnson, 2004). The flood-frequency information was used in regression analyses to develop equations relating T-year floods to various basin and climatic characteristics, active-channel width, and bankfull width. The equations can be used to estimate flood frequency at ungaged sites. Flood-frequency data typically were determined by fitting a log-Pearson Type III probability distribution using methods described by the Interagency Advisory Committee on Water Data (IACWD), Bulletin 17B (IACWD, 1982).

Table 2-4 provides the WRIR 03-4308 peak discharge summary for the USGS gages on the Clark Fork River. Table 2-4 shows that, in general, peak flows increase with an increase in drainage area and return interval.

Table 2-4 WRIR 03-4308 Clark Fork River Peak Discharge Summary Peak Flow (cfs), Return Interval (years) USGS Drainage Years (cfs) Station Area of 5- 10- 25- 50- 100- 200- 500- Number USGS Station Name (mi2) Record year year year year year year year

12354500 Clark Fork at St. Regis 10,709 83 49,500 57,000 65,300 70,700 75,600 80,000 85,200

Clark Fork Below 12353000 9,003 69 38,900 45,200 52,400 57,300 61,700 65,900 70,900 Missoula

Clark Fork above 12340500 5,999 70 21,600 25,900 31,200 35,000 38,600 42,200 46,800 Missoula

Clark Fork at Turah 12334550 3,641 13 8,440 10,800 13,900 16,300 18,600 21,000 24,100 Bridge, near Bonner

Clark Fork near 12331900 2,629 14 5,380 7,600 11,000 14,000 17,400 21,200 27,000 Clinton

Clark Fork at 12331600 2,378 18 6,700 8,850 11,700 13,900 16,200 18,400 21,500 Drummond

Clark Fork at 12324680 1,704 21 4,600 6,250 8,610 10,500 12,600 14,800 18,000 Goldcreek Based on systematic data through 1998. mi2: square miles. cfs: cubic feet per second.

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2.2.4 Scientific Investigations Report 2015-5019 The USGS SIR 2015-5019-C updated annual peak discharges with AEPs of 66.7, 50, 42.9, 20, 10, 4, 2, 1, 0.5, and 0.2% (return intervals of 1.5, 2, 2.33, 5, 10, 25, 50, 100, 200, and 500 years, respectively) for 725 gaged sites in or near Montana, based on data through Water Year 2011 (Sando, et al., 2015a). Flood-frequency data typically were determined by fitting a log-Pearson Type III probability distribution using methods described by the IACWD, Bulletin #17B (IACWD, 1982). The study was part of a larger study to develop an online StreamStats application for Montana, in conjunction with computing streamflow characteristics at gage stations, and estimate peak flow flood frequency at ungaged sites. Table 2-5 provides the SIR 2015-5019-C discharge summary for the Clark Fork River gages.

Table 2-5 SIR-2015-5019-C Clark Fork River Peak Discharge Summary Peak Flow (cfs) , Return Interval (years) USGS Drainage Years (cfs) Station Area of 5- 10- 25- 50 100- 200- 500- Number USGS Station Name (mi2) Record year year year year year year year Clark Fork at St, 12354500 10,728 96 48,700 56,200 64,600 70,200 75,200 79,800 85,400 Regis Clark Fork below 12353000 9,017 82 38,300 44,600 51,900 56,800 61,400 65,600 70,900 Missoula Clark Fork above 12340500 6,021 92 22,000 26,700 32,400 36,500 40,600 44,500 49,600 Missoula Clark Fork at Turah 12334550 3,657 26 8,200 10,200 12,700 14,500 16,300 18,000 20,200 Bridge, near Bonner Clark Fork near 12331900 2,650 14 6,210 9,140 13,900 18,200 23,200 29,100 38,400 Clinton Clark Fork near 12331800 2,516 31 5,710 7,620 10,600 13,400 16,700 20,600 26,800 Drummond Clark Fork at 12324680 1,774 34 4,310 5,780 8,050 10,000 12,400 15,000 19,100 Goldcreek Based on systematic data through 2011. mi2: square miles. cfs: cubic feet per second.

The USGS SIR 2015-5019-F (Sando, et al., 2015b) selected 537 gaging stations from the gage study. The 537 gaging stations were segregated based on the following criteria: contributing drainage area less than about 2,750 square miles, peak-flow records unaffected by major regulation, small redundancy with nearby stations, and representation of peak-flow frequencies at sites within Montana. Most of the gaging stations on the Clark Fork River within Missoula and Granite County were excluded from the data set because the contributing basin drainage area for these gages was greater than 2,750 square miles. The only gaging station used within Missoula and Granite County was USGS gage Clark Fork at Drummond (12331600) The study used regression analyses to develop equations relating AEP flows to various basin and climatic characteristics. The relationships developed for this study resulted in lower mean standard errors of prediction than previous regression analyses.

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2.2.5 Scientific Investigations Report 2018-5046 The USGS SIR 2018-5046 included updated AEP peak discharges with AEPs of 50, 42.9, 20, 10, 4, 2, 1, 0.5, and 0.2% (return intervals of 2, 2.33, 5, 10, 25, 50, 100, 200, and 500 years, respectively) for 99 gages sites in or near Montana, based on Water Year 2015 (Sando & McCarthy, 2018). Flood-frequency data typically were determined by fitting a log-Pearson Type III probability distribution using an Expected Moments Algorithm analysis and other methods described by England and others in Bulletin 17C (England, et al., 2018). The AEP peak discharges for Clark Fork above Missoula gage were updated as part of the report. Table 2-6 provides the SIR 2018-5046 peak discharge summary for the gage.

Table 2-6 SIR 2018-5046 Clark Fork River Peak Discharge Summary Peak Flow (cfs) , Return Interval (years) USGS Drainage Years (cfs) Station Area of 10- 25- 50- 100- 200- 500- Number USGS Station Name (mi2) Record 5-year year year year year year year Clark Fork above 12340500 6,021 96 21,600 26,100 31,700 35,600 39,500 43,200 48,100 Missoula Based on systematic data through 2015. mi2: square miles. cfs: cubic feet per second.

Supplementing this report, additional peak flow frequency analysis data were released for 14 stream gages in the Beaverhead River and Clark Fork Basins (McCarthy, et al., 2018). Estimates of peak flow magnitudes for 50, 42.9, 20, 10, 4, 2, 1, 0.5, and 0.2% AEPs were reported. Table 2-7 provides the peak flow discharge summary for the Clark Fork River gages.

Table 2-7 Results of Peak Flow Frequency Analysis for selected stream gages in the Clark Fork Basin

USGS Drainage Years Peak Flow (cfs) , Return Interval (years) Station USGS Station Area of (cfs) Number Name (mi2) Record 5-year 10-year 25-year 50-year 100-year 200-year 500-year Clark Fork below Nixon 12391400 21,709 79 98,000 110,000 123,000 132,000 139,000 146,000 154,000 Rapids Dam, near Nixon Clark Fork near 12389000 19,964 105 97,600 110,000 123,000 131,000 139,000 146,000 154,000 Plains Clark Fork at St. 12354500 10,728 101 48,200 55,100 62,900 68,100 72,800 77,200 82,400 Regis Clark Fork 12353000 9,017 87 38,100 43,900 50,500 54,900 58,900 62,600 67,200 below Missoula Clark Fork 12340500 6,021 97 21,500 26,000 31,600 35,700 39,600 43,500 48,500 above Missoula Clark Fork at 12334550 Turah Bridge, 3,657 31 7,890 9,830 12,300 14,100 15,900 17,700 20,000 near Bonner Clark Fork near 12331800 2,516 44 5,350 7,360 10,400 13,000 15,800 19,100 23,900 Drummond Based on systematic data through 2016. mi2: square miles. cfs: cubic feet per second.

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2.2.6 Letter of Map Revision – Clark Fork River and Blackfoot River Near Milltown, Montana – Appendix A The United States Environmental Protection Agency (EPA) and the State of Montana Department of Justice Natural Resource Damage Program (NRDP) funded a project to create a Letter of Map Revision (LOMR) following the removal of the Milltown dam in 2008 (River Design Group, Inc., 2013). The purpose of the LOMR was to determine how the removal of the dam changed the Water Surface Elevation (WSEL) in the event of a flood.

Table 2-8 summarizes the 2012 LOMR Discharges for three USGS gages and three ungaged flow change locations determined from the LOMR – Clark Fork River and Blackfoot River Near Milltown, Montana – Appendix A

Table 2-8 Appendix A –2012 LOMR Peak Flows Peak Flow, Return Interval (years) USGS USGS Station Drainage (cfs) Station ID Name LOMAR Name Area (mi2) 10-Year 50-Year 100-Year 500-Year Clark Fork at DS Detailed NA NA 9,272 49,250 61,000 67,000 86,000 Study Limit Clark Fork below Clark Fork at Gage #3530 12353000 9,003 47,000 58,000 64,000 82,000 Missoula, MT blw Missoula Clark Fork above Clark Fork at Gage #3405 12340500 5,999 27,000 38,200 42,500 56,000 Missoula, MT abv Missoula Clark Fork at Blackfoot Not NA NA 31,800 45,000 51,000 66,700 River Confluence* Determined Clark Fork abv Blackfoot NA NA 3,668 15,000 22,500 26,000 35,500 River Confluence Blackfoot River 12340000 near Bonner, Blackfoot at Gage #3400 2,290 16,800 22,500 25,000 31,200 MT * Peak discharge determined by coincident peak (sum of Blackfoot River at Gage #3440 and Clark Fork River above Blackfoot River Confluence). Source: River Design Group, Inc, 2013. mi2: square miles. cfs: cubic feet per second. NA: not applicable.

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2.2.7 Restoration Plan for the Clark Fork River and Blackfoot River near Milltown Dam – October 2005: Appendix A – Hydrology and Flood Series Analysis The Restoration Plan for the Clark Fork River and Blackfoot River near Milltown Dam, Appendix A, document lists calculated flood frequencies for USGS stream gage stations surrounding the Milltown Dam, located on the confluence of the Blackfoot and Clark Fork Rivers (River Design Group, Inc.; Westwater Consulants, Inc., 2005). The USGS gages Clark Fork at Turah Bridge near Bonner (12334550) and Clark Fork above Missoula (12340500) were analyzed. The flood frequency analysis was extended 6 years past the USGS WRIR 03-4308 (Parret & Johnson, 2004). The WRIR 03-4308 calculated flood frequencies through water year 1998. This extension was made to incorporate the most recent flood history through 2004. Flood frequencies were calculated from a flood frequency curve using the log-Pearson Type III distribution method described in Bulletin 17B (IACWD, 1982). The study also calculated bankfull flows (Qbf), at the above referenced USGS gage stations. The study identified a major flood on the Clark Fork in 1908 as the largest flood of record with a flow of 48,100 cfs, greater than the 500-year event based on the updated flood frequency calculations for the Clark Fork above Missoula (12340500) gage. The report also referenced major floods at the Clark Fork above Missoula gage in 1947 (22,000 cfs), 1948 (28,800 cfs), 1953 (27,200 cfs), 1964 (30,500 cfs), 1972 (26,000 cfs), 1975 (30,800 cfs), 1976 (24,800 cfs), 1981 (28,900 cfs), and 1997 (26,400 cfs).

Table 2-9 provides the summary of peak discharges for two USGS stream gages on the Clark Fork River determined from the Restoration Plan for the Clark Fork River and Blackfoot River near Milltown Dam – October 2005: Appendix A – Hydrology and Flood Series Analysis report Rivers (River Design Group, Inc.; Westwater Consulants, Inc., 2005).

Table 2-9 Appendix A – Hydrology and Flood Series Analysis Summary of Discharges Peak Flow, Return Interval (years) (cfs) USGS Study 10- 25- 50- 100- 500- Qbf 2-year Station ID USGS Station Name Name year year year year year Clark Fork River at Turah CFR at 12334550 3,200 4,600 9,700 12,600 14,700 16,900 22,100 Bridge, near Bonner, MT Turah Clark Fork River above CFR above 12340500 10,400 14,600 25,700 30,800 34,400 37,800 45,200 Missoula, MT Missoula Source: River Design Group, Inc. & Westwater Consultants, Inc. 2005. Qbf: calculated bankfull flows. cfs: cubic feet per second.

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2.2.8 Additional Previous Studies

Additional related previous studies conducted along the Clark Fork River within Missoula and Granite County involve mining impacts, hydrology, water management, fisheries management, or sediment management:

• USGS, 1949. Floods of May-June 1948 in Columbia River Basin: Geological Survey Water- Supply Paper 1080, Washington: United States Government Printing Office. • USACE, 1967. Floodplain Information: Clark Fork, Missoula, Montana. Prepared for Montana State Resources Board., s.l.: U.S. Army Corps of Engineers (USACE). • USGS, 1982. Floods of May 1981 in West-Central Montana: Water-Resources Investigations 82-33, Helena: USGS. • Applied Geomorphology, Inc., 2009. Technical Memorandum: Clark Fork River CMZ Pilot, s.l.: Missoula County. • Applied Geomorphology, Inc., 2012. Technical Memorandum: Phase 1 Clark Fork River Channel Migration Zone Map Development, s.l.: US Fish and Wildlife Service. • Tetra Tech, 2013. State of Montana Multi-Hazard Mitigation Plan & Statewide Hazard Assessment, Helena: Tetra Tech.

• CDM & Applied Geomorphology, Inc., 2010. Part:2 Geomorphic, Hydrologic, and Hydraulic Investigation for Phase 1 Remedial Design/Remedial Action. Helena: Montana Department of Environmental Quality (CDM & Applied Geomorphology, Inc., 2010). • Clark, K. W., 1986. Interactions between the Clark Fork River and Missoula Aquifer, Missoula County, Montana. Student Thesis (Clark, 1986). • Clark Fork Coalition, 2011. Aquatic Restoration Strategy for the Upper Clark Fork Basin. Clark Fork Coalition (Clark Fork Coalition, 2011). • EMC2, 2004. 2004 Flood Frequency Analysis Calc Brief for the Milltown Reservoir Sediments Operable Unit (MRSOU) of the Milltown Reservoir/Clark Fork Rover Superfund Site (EMC2, 2004). • EPA, 2011. Integrating the "3 Rs": Remediation, Restoration and Redevelopment, The Milltown Reservoir Sediments Site and Missoula County, Montana. Helena (EPA, 2011). • EPA, 2004. Milltown Reservoir Sediments Operable Unit: Record of Decision. Helena: U.S. Environmental Protection Agency Region 8 (EPA, 2004). • EPA and Montana Department of Environmental Quality (DEQ), 2003. Superfund Program Clean-up Proposal: Milltown Reservoir Sediments Operable Unit of the Milltown Reservoir/Clark Fork River Superfund Site. (EPA and DEQ, 2003). • Evans, E., & Wilcox, A., 2013. Fine Sediment Infiltration Dynamics in a Gravel-Bed River Following A Sediment Pulse. Wiley Online Library. (Evans & Wilcox, 2013) . • Geldon, A. L., 1979. Hydrogeology and water resources of the Missoula basin Montana. Ann Arbor: ProQuest LLC. (Geldon, 1979). • Hydrosolutions, Inc., 2011. Clark Fork - Pend Oreille Watershed Water Quality Monitoring Program. Sandpoint: Tri-State Water Quality Council. (Hydrosolutions, Inc, 2012).

Page 23 Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

• Kendy, E., & Tresch, R. E., 1996. Geographic, Geologic, and Hydrologic Summaries of Intermontane Basins of the Northern Rocky Mountains. Helena: USGS. (Kendy & Tresch, 1996). • KirK Engineering & Natural Resources, Inc., 2015. Water Supply Report Series I: Water Availability and Mitigation Options in the Clark Fork Basin. DNRC. (KirK Engineering & Natural Resources, Inc., 2015). • Lambing, J. H., & Sando, S. K., 2009. Estimated Loads of Suspended Sediment and Selected Trace Elements Transported through the Milltown Reservoir Project Area Before and After the Breaching of Milltown Dam in the Upper Clark Fork Basin, Montana, Water Year 2008: USGS SIR 2009-5095. Helena (Lambing & Sando, 2009). • DEQ and EPA, 2015. Clark Fork River Operable Unit (OU #3) Milltown Reservoir/Clark Fork River Superfund Site: Explanation of Significant Differences. (DEQ and EPA, 2015). • Smith, C. A., 1992. Hydrogeology of the central and northwestern Missoula Valley. Student Thesis. (Smith, 1992). • Smith, J. D., Lambing, J. H., Nimick, D. A., Parret, C., Ramey, M., & Schafer, W., 1998. Geomorphology, Flood Plain Tailings, and Metal Transport in the Upper Clark Fork Valley, Montana: Water Resources Investigations Report 98-4170. USGS. (Smith, et al., 1998). • U.S. Fish and Wildlife Service, 2002. Chapter 3: Clark Fork River Recovery Unit, Montana and Idaho. Portland. (USFWS, 2002).

Page 24 Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

2.2.9 Flood Protection Measures The City of Missoula and Missoula County Unincorporated Areas FIS (FEMA, 2019a) described non-levee flood protection measures within Missoula county as dams, jetties, and dikes. The report identified dikes and a berm/riprap structure on the Clark Fork River. A system of dikes around settling ponds was identified at the Smurfit-Stone Paper Mill northwest of Missoula. Areas of the Reserve Street Bridge west of Missoula have berms and riprap for shaping and stabilization.

The City of Missoula and Missoula County Unincorporated Areas FIS (FEMA, 2019a) provided an inventory of USACE levees that meet standards in compliance with comprehensive floodplain management criteria. Levees are designed to reduce the risk from the 1% AEP flow and are approved by FEMA. Table 2-10 shows the levees on the Clark Fork River.

Table 2-10 Levees on the Clark Fork Mainstem Flooding Levee USACE Community Levee Owner Levee ID FIRM Panel(s) Source Location Levee 30063C1195E, Missoula County, Clark Fork South Missoula County Yes 5505000201 30063C1455E, Unincorporated Areas River Bank 30063C1460E Clark Fork North Missoula, City of City of Missoula Yes 550500005 30063C1480E River Bank Clark Fork North Missoula, City of City of Missoula Yes 550400108 30063C1195E River Bank Source: FEMA, 2019a.

The USACE maintains the National Inventory of Dams (NID), a database that keeps record of all the dams across the country. The NID identifies hazard potential, dam height, ownership, purpose, dam type, and administrative data. There are 28 dams in the Clark Fork River study area. No dams were identified in the Clark Fork River study reach (USACE, 2019).

Page 25 Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

2.3 Hydrologic Analyses and Results The purpose of the hydrologic analyses conducted as part of this project was to develop peak flow discharge estimates for the 50, 10, 4, 2, 1, and 0.2% AEP and 1%+ AEP events at key flow change locations (such as at significant tributaries confluences, stream gages, and population centers) along the Clark Fork River study reach. The hydrologic analysis was organized into 2 sub- sections:

1. USGS Stream Gage Analysis. 2. Ungaged Flow Node Analysis.

Throughout the study area, 27 flow nodes on the Clark Fork River study reach were identified as having significant changes in streamflow or being at critical locations. Of the 27 flow nodes on the study reach, 4 are located at active USGS stream gages and 2 are located on inactive USGS stream gages. The study extents in this report were based on the Missoula-Granite County Modernization Study Area Map provided by the DNRC. The river stations used in this report were based on the Clark Fork River S_Wtr_Ln, delineated by Pioneer using 2019 light detection and ranging (LiDAR) and channel bathymetry. The Clark Fork River S_Wtr_Ln alignment begins at the Missoula-Mineral County line. The upstream extent of the study reach ends at the Granite- Powell County line.

2.3.1 USGS Stream Gage Analysis The USGS performed peak flow analysis, using data through 2018, for selected stream gages in Deer Lodge, Powell, Granite, Missoula, Ravalli, and Mineral counties as part of this hydrologic analysis (Sando, et al., 2020). This longer period of record should produce more accurate peak discharge estimates than those based on a shorter period of record. Four Clark Fork River gages were analyzed by USGS within Missoula and Granite Counties (the study reach), and 2 additional gages located in Mineral and Powell Counties were used in this analysis. The USGS gage flood frequency analysis summary, data, and calculations are provided in Appendix A. Bulletin 17C methods (England, et al., 2018) were used to estimate the 50, 42.9, 20, 10, 4, 2, 1, 0.5, and 0.2% AEPs and the 1%+ AEP (Sando, et al., 2020). The USGS methodologies used for the gage analyses presented in this report are documented in USGS SIR 2018-5046 (Sando & McCarthy, 2018).

The oldest records used in this study date back to 1899 at USGS gaging station Clark Fork above Missoula, MT (12340500). This gage remains active. The FEMA guidance document (FEMA, 2019) indicates that gage station records equal or exceeding 10 years in length are applicable to all types of studies. The gages used in this study meet this criterion. Figure 2-9 shows the study reach and the USGS gaging station locations along and near the study reach.

Page 26 Charlo Augusta «¬200 LEWIS & Saint Regis 12354500 Saint 83 CLARK 287 135 «¬ «¬ «¬ Ignatius COUNTY Dixon ¤£93

Superior

MINERAL COUNTY Arlee Seeley Lake

POWELL COUNTY ¨¦§90 Frenchtown Alberton MISSOULA COUNTY Clark Fork River ¤£200 Lincoln 12340500 Missoula Bonner 15 12353000 ¨¦§ 12334550

Lolo Bitterroot River Clinton Clark Fork River «¬141 12331800

Florence Rock Creek Drummond

Ranch Creek 12324680

Stevensville ¤£12 IDAHO GRANITE Upper Willow COUNTY Victor Creek

Pinesdale Deer Lodge RAVALLI Philipsburg COUNTY Rock Creek Hamilton «¬1 ¨¦§90

Boulder

LEGEND DISPLAYED AS: ACTIVE USGS GAGE COUNTY BOUNDARIES SECONDARY 12331800 PROJECTION/ZONE: MONTANA STATE PLANE FIGURE 2-9 CLARK FORK RIVER DATUM: NAD1983 TOWNS HUC 8 BOUNDARIES URBAN 12334550 UNITS: INT'L FEET USGS WATERSHEDS SOURCE: DNRC/MSL/PIONEER CLARK FORK MAINSTEM STUDY REACH NHS INTERSTATE STREAMS 12340500 12353000 MISSOULA-GRANITE STUDY REACHES NHS NON-INTERSTATE USGS WATERSHED N 0 5 10 20 12354500 DATE: 7/2/2020 PRIMARY 12324680 Miles Path: Z:\Shared\Active Projects\DNRC\Missoula-Granite\GIS\Hydrology\MSAGNT-FIG-PLN-2-9-UW.mxd Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

Table 2-11 summarizes the USGS stream gages analyzed by the USGS along the Clark Fork River study reach and the additional gages outside the study reach used in this hydrologic analysis. Figure 2-10 plots the systematic or at-site flood frequency results as a function of drainage area.

Table 2-11 Clark Fork River USGS Gage Summary

Number Regulation Period of of USGS Station Status as of Systematic Annual Number USGS Station Name 2018 Record Peaks 2018 Status 12324680* Clark Fork at Goldcreek, MT U 1978-2018 41 Active 1967, 1973- 12331800 Clark Fork near Drummond, MT U 1990, 1992- 46 Active 2018 12334550 Clark Fork at Turah Bridge near Bonner, MT U 1986-2018 33 Active 1899-1908, 12340500 Clark Fork above Missoula, MT U 99 Active 1930-2018 12353000 Clark Fork below Missoula, MT U 1930-2018 89 Active 1911-1923, 12354500* Clark Fork at St. Regis, MT U 103 Active 1929-2018 * Denotes USGS gage outside of study reach Source: (Sando, et al., 2020). USGS: U.S. Geological Survey. U: unregulated.

2.3.1.1 1% + Peak Flow Estimates The 1%+ AEP event was calculated by the USGS (Sando, et al., 2020) to provide a confidence range that the 1% flood frequency peak flow estimates are likely to fall within. FEMA guidance defines the 1%+ as “…a flood elevation derived by using discharges that are at the upper 84- percent confidence limit as calculated in the gage analysis for the 1-percent-annual-chance event for the Flood Risk Project. Methods for estimating synthetic statistics outlined in Bulletin 17C Appendix 7 are used to estimate the upper 84 percent confidence limit of the Log Pearson III frequency Curve at the 1-percent-annual-chance event” (FEMA, 2019). The USGS used the methods for estimating synthetic statistics outlined in Bulletin 17C, Appendix 7.

Page 28 Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

Figure 2-10 USGS Flood Frequency Estimates Systematic Record through 2018

90,000 1%+ 500-yr 12354500

80,000 below 100-yr at St. Regis 12353000 Missoula 50-yr 25-yr 70,000 10-yr 2-yr

60,000 12340500 above 12340500 above Missoula

50,000 at

40,000 Bridge Peak Flow Flow (cfs) Peak 12331800 near Drummond 12334550 at Turah 12324680 Goldcreek 30,000

20,000

10,000

2,000 4,000 6,000 8,000 10,000 12,000 Drainage Area (mi2)

Figure 2-10 indicates some peak discharges do not increase with increasing drainage area, as typically expected. Specifically, this can be observed in the 500-year recurrence interval and 1%+ peak flows between the Drummond and Turah Bridge gage. These inconsistences can be related to non-congruent periods of record, variability in flood storage, flow diversions, and variations in confidence interval limits between 2 gaging stations (in the 1%+ case).

Regional Regression Weighted and Maintenance of Variance Extension, Type III (MOVE.3) are 2 procedures that sometimes can be used to improve at-site frequency analyses. In this Clark Fork River gage analysis, the USGS employed the MOVE.3 method (England, et al., 2018) to extend the historical gage record for the Clark Fork River USGS gage stations to improve the at-site frequency analysis. Per Bulletin 17C guidance, a minimum correlation of 0.8 is required when using MOVE.3 extended record analysis. The correlation for the gages analyzed by USGS ranged from 0.81 to 0.98 (Appendix A) and therefore meet or exceed the Bulletin 17C guidance. Figure 2-11 plots the extended record results for the Clark Fork River extended record analysis. Table

Page 29 Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

2-12 shows the results for systematic and MOVE.3 flood frequency estimates for gages on the Clark Fork River.

Figure 2-11 USGS Flood Frequency Estimates Extended Record through 2018

100,000 1%+ Missoula 12353000 500-yr below

90,000 12354500 100-yr at St. Regis 50-yr 80,000 25-yr Missoula 12340500 above 10-yr 70,000 2-yr

60,000 Drummond 12331800 near Turah 12334550 at 50,000 Bridge Goldcreek 12324680

Peak Flow Flow (cfs) Peak 40,000

30,000 at at

20,000

10,000

2,000 4,000 6,000 8,000 10,000 12,000 Drainage Area (mi2)

Figure 2-11 indicates the extended record data set eliminates the flow attenuation, observed in the systematic record between the Drummond and Turah Bridge gages, and improves the increasing flow with increasing drainage areas consistency for the other AEP peak flow estimates. Using the extended record data set will help minimize the potential error associated with the at-site, non-congruent periods of record. For these reasons, flood flow frequency estimates using the 2020 extended record data set were selected to represent the annual chance flood potential at the Clark Fork River gaged locations.

Page 30 Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

Table 2-12 Clark Fork River Gage Flood Frequency Estimates Peak Flow (cfs) 50% 10% 4% 2% 1% 0.2% 1% + Annual Annual Annual Annual Annual Annual Annual Water years of Chance Chance Chance Chance Chance Chance Chance Node/USGS Location peak flows used in 10- 25- 50- 100- 500- 100- Station ID Description Peak Flood Frequency Method the analysis 2-year year year year year year year + At-Site 1978-2018 2,380 5,640 7,840 9,740 11,900 17,800 22,900 Clark Fork at 12324680* 1899-1908, 1930- Goldcreek, MT MOVE.3 2,730 6,260 8,300 9,900 11,600 15,600 17,200 2018 1967,1973-1990, At-Site 3,040 7,580 10,400 12,800 15,200 21,600 26,200 Clark Fork near 1992-2018 12331800 Drummond, MT 1899-1908, 1911- MOVE.3 3,280 7,420 10,200 12,500 15,200 22,600 26,100 1923, 1929-2018 Clark Fork at At-Site 1986-2018 5,300 10,500 13,200 15,200 17,200 21,700 24,700 12334550 Turah Bridge, 1899-1908, 1911- MOVE.3 6,260 12,000 15,200 17,700 20,400 27,000 28,500 near Bonner, MT 1923, 1929-2018 1899-1908, 1930- At-Site 14,700 26,300 32,000 36,100 40,100 49,100 46,900 Clark Fork above 2018 12340500 Missoula, MT 1899-1908, 1911- MOVE.3 15,000 26,600 32,100 36,100 39,900 48,400 46,600 1923, 1929-2018 At-Site 1930-2018 28,200 44,000 50,400 54,800 58,700 66,700 65,600 Clark Fork below 12353000 1899-1908, 1911- Missoula, MT MOVE.3 29,300 47,200 54,900 60,200 65,000 75,200 73,500 1923, 1929-2018 1911-1923, 1929- At Site 36,200 55,300 63,100 68,200 73,000 82,500 81,200 Clark Fork at St. 2018 12354500* Regis, MT 1899-1908, 1911- MOVE.3 36,700 59,000 68,300 74,500 80,200 91,900 90,100 1923, 1929-2018 * Denotes USGS gage outside of study reach. Source: (Sando, et al., 2020). USGS: U.S. Geological Survey cfs: cubic feet per second. At-site: Peak flow frequency analysis on recorded data. MOVE.3: Peak flow frequency analysis on combined recorded and synthesized data; synthesized data from Maintenance of Variance Extension Type III (MOVE.3) record extension procedure.

Page 31 Missoula-Granite PMR Missoula-Granite County Hydrologic Analysis

2.3.2 Flow Change Node Locations Future flood studies will use hydraulic models that are composed of geometric data and streamflow data. To accurately model the Clark Fork River, the locations of major tributary confluences and other flow change locations must be identified. The hydrologic analysis results will be used as the streamflow data input at the tributary confluences within the hydraulic model.

Generally, the hydraulic models simulate flood events using steady-state conditions and, therefore, the peak flow rate calculated at a flow node is projected to the next upstream flow node. This method was followed for the hydrologic analysis calculations. Flow nodes were assigned immediately upstream of major tributaries; this method of locating the flow nodes was employed so that the additional flow resulting from the tributary confluence was accurately reflected to the reach downstream of the confluence.

A detailed review of the study area was performed to identify all potential flow change locations (flow nodes). HUC 12-digit watershed boundaries were used to initially locate the flow nodes. The HUC 12-digit watershed boundaries represent the smallest USGS-delineated watershed areas available in Geographical Information System (GIS) format. Using ArcGIS, flow nodes were located just upstream of the HUC 12 boundary intersection with the Clark Fork River. To avoid excessive flow changes between HUC 12 boundary nodes, additional flow nodes were located immediately downstream of towns, at the beginning of study reaches, or where intermediate tributaries within the HUC 12 boundaries intersected the study reach.

At each flow node, a drainage basin area was delineated, and streamflow values were calculated for the various recurrence interval floods. Two methods were used to generate flow node drainage basins. The primary method, used for most flow nodes, involved calculating drainage basins with ArcGIS, an ESRI software that produces drainage basin areas. Complex, low-gradient topographic conditions in the lower study reach required additional methods to calculate drainage basins for nodes 300, 1200, and 2600. These drainage basin areas were calculated with StreamStats, a USGS software that estimates peak flow data and drainage basin area.

As an accuracy check, the USGS gaging station watershed areas calculated in ArcGIS were compared to the USGS National Watershed Information System (NWIS) (http://waterdata.usgs.goc/nwis) published gaging station areas. Figure 2-9 shows the USGS gaging stations analyzed and the correlating GIS model-generated watershed areas along the study area. Table 2-13 shows the results of this comparison. Based on the 6 gaging stations analyzed, the project GIS model calculates watershed areas that are within 1% of the USGS NWIS published areas (Appendix A).

This study used the nearest Geographic Names Information System (GNIS) hydrographic feature name for the ungaged flow node names. In most cases, these features (typically tributary

Page 32 Missoula-Granite PMR Missoula-Granite County Hydrologic Analysis

streams) flow into the Clark Fork River just downstream of the flow node. Nodes were also named for nearby towns and USGS gage station numbers if placed at gaged locations.

Table 2-13 USGS and GIS Model Watershed Comparison USGS Published Relative Station Drainage Area1 GIS Calculated Percent Accuracy of Number Station Name (mi2) Basin Area2 (mi2) Change Areas 12324680* Clark Fork at Goldcreek, MT 1,774 1,789 0.84% 99.2% 12331800 Clark Fork near Drummond, MT 2,516 2,513 0.12% 99.9% 12334550 Clark Fork at Turah Bridge near Bonner, MT 3,657 3,661 0.10% 99.9% 12340500* Clark Fork above Missoula, MT 6,021 6,013 0.13% 99.9% 12353000 Clark Fork below Missoula, MT 9,017 9,007 0.11% 99.9% 12354500* Clark Fork at St. Regis, MT 10,728 10,715 0.12% 99.9% 1. Source: (Sando, et al., 2020). 2. Basin areas (watershed areas) used for hydrological analysis. * Denotes USGS gage used for ungaged flow node analysis but not used as a flow node in this analysis. USGS: U.S. Geological Survey. mi2: square miles. GIS: Geographical Information System.

Table 2-14 lists the flow node and gage station information used in the analyses. A total of 27 flow nodes were identified throughout the study reach, including 5 gaged locations and 22 ungaged locations. Figure 2-9 shows the USGS gaging stations analyzed and the correlating watershed areas within the study area. Figure 2-12 and Figure 2-13 map the flow node locations and corresponding watershed areas from Table 2-14.

Page 33 Missoula-Granite PMR Missoula-Granite County Hydrologic Analysis

Table 2-14 Flow Node and USGS Gage Station Information Used in Hydrologic Analyses Calculated Node/USGS Study River Basin Area2 Station ID Location Description County Reach Station1 (mi) (mi2) 12354500* Clark Fork at St. Regis MT Mineral NA NA 10,715 100 Missoula-Mineral County Boundary Missoula Clark Fork 0.0 9,548 200 Petty Creek Missoula Clark Fork 0.3 9,463 3003 Ninemile Creek Missoula Clark Fork 6.2 9,269 400 SixmileCreek Missoula Clark Fork 9.6 9,238 500 Roman Creek Missoula Clark Fork 13.5 9,210 600 Mill Creek Missoula Clark Fork 15.9 9,165 700 Rock Creek Missoula Clark Fork 19.1 9,123 800 Deep Creek Missoula Clark Fork 21.9 9,052 12353000 Clark Fork below Missoula MT Missoula Clark Fork 28.9 9,007 Clark Fork upstream of Bitterroot 1000 Missoula Clark Fork 29.6 6,149 River 11003 Grant Creek Missoula Clark Fork 32.8 6,087 1200 Lower Rattlesnake Creek Missoula Clark Fork 38.2 6,018 123405004 Clark Fork above Missoula MT Missoula Clark Fork 41.5 6,013 Clark Fork upstream of Blackfoot 1300 Missoula Clark Fork 44.6 3,691 River Clark Fork at Turah Bridge near 12334550 Missoula Clark Fork 49.8 3,661 Bonner MT 1500 Wallace Creek Missoula Clark Fork 52.6 3,633 1600 Schwartz Creek Missoula Clark Fork 58.3 3,580 1700 Rock Creek-Kitchen Gulch Missoula Clark Fork 61.3 2,682 12331900 Clark Fork near Clinton MT Missoula Clark Fork 66.0 2,646 1900 Dry Gulch Granite Clark Fork 4.1 2,608 2000 Tyler Creek Granite Clark Fork 6.5 2,589 2100 Harvey Creek Granite Clark Fork 11.9 2,537 12331800 Clark Fork near Drummond MT Granite Clark Fork 14.8 2,513 2300 Mulkey Gulch Granite Clark Fork 18.5 2,455 24003 Rattler Gulch Granite Clark Fork 22.8 2,434 12331600 Clark Fork at Drummond MT Granite Clark Fork 27.8 2,383 2600 Lower Flint Creek Granite Clark Fork 28.8 1,900 2700 Perkins Creek Granite Clark Fork 38.0 1,875 12324680* Clark Fork at Goldcreek MT Powell NA NA 1,789 1. River miles start at the downstream extent of each study reach. 2. Source: Esri ArcGIS and USGS StreamStats. 3. Watershed area generated with USGS StreamStats. 4. Not a flow node, used in ungaged flow node calculations. * Denotes USGS gage outside of study reach, not used as a flow node in this analysis. USGS: U.S. Geological Survey. mi: mile. mi2: square miles.

Page 34 Gold Creek

400 300 MINERAL COUNTY Frenchtown Alberton ¤£93 500 600 100 200

O'Keefe Creek

700 Valle Creek, La Albert Creek

90 Butler Creek

Butler IrrigationDitch ¨¦§ West Fork Petty Creek Rock Creek Rattlesnake Creek

MISSOULA COUNTY Grant Creek

Petty Creek HW 800 Y 1 0 W

Clark Fork River

Marshall Creek Gilman Creek

Deep Creek

Johnson Gulch W BROADWAY Mittower Gulch

Blackfoot River

12353000 12340500 200 1100 ¤£ Missoula 1000 Bonner 1200 1300 OBrien Creek

South Fork Petty Creek

RESERVE ST Grave Creek BearCreek East Fork Grave Creek BROOKS ST H W Y 10 Deer Creek E

Hayes Creek HWY 93 S Bitterroot River Moose Can Gully

12334550

LEGEND DISPLAYED AS: ACTIVE USGS GAGE MISSOULA-GRANITE STUDY REACHES URBAN 400 1000 PROJECTION/ZONE: MONTANA STATE PLANE FIGURE 2-12 CLARK FORK RIVER DATUM: NAD1983 FLOW NODE COUNTY BOUNDARIES STREAMS 500 1100 UNITS: INT'L FEET FLOW NODE 100 to 12334550 TOWNS HUC 8 BOUNDARIES FLOW CHANGE NODE 600 1200 SOURCE: DNRC/MSL/PIONEER 700 12340500 100 HUC 12 BOUNDARIES NHS INTERSTATE 800 1300 NHS NON-INTERSTATE 200 N 0 1 2 4 CLARK FORK MAINSTEM STUDY REACH 12353000 12334550 DATE: 7/9/2020 SECONDARY 300 Miles Path: Z:\Shared\Active Projects\DNRC\Missoula-Granite\GIS\Hydrology\MSAGNT-FIG-PLN-2-12-FN.mxd 12334550

Donovan Creek

Union Creek

H W Y 10 E

1500 MISSOULA COUNTY Allen Creek Deep Creek

Greenough Creek West Fork Cramer Creek Douglas Creek Clinton POWELL COUNTY Cramer Creek Tenmile Creek

Bear Creek 1600

Mocassin Creek

Clark Fork River Y

W 1700 H G I

H Swartz Creek 12331900 2300 Clark Fork River 90 Dry Gulch 2100 ¨¦§ 2000 12331800

Gilbert Creek 1900

Gillispie Creek GRANITE COUNTY 2400

Antelope Creek

Spring Creek Tigh Creek Rock Creek Drummond Tyler Creek 12331600 Bert Creek RAVALLI COUNTY Unnamed trib to Antelope Cr RM 2.9 2600

Sawmill Creek Brewster Creek

Welcome Creek

Cow Creek Harvey Creek «¬1 2700 Cinnabar Creek

Ranch Creek North Fork Lower Willow Creek Flint Creek

Upper Willow Creek Lower Willow Creek

Cinnamon Bear Creek

LEGEND DISPLAYED AS: FLOW NODE MISSOULA-GRANITE STUDY REACHES PRIMARY 12334550 1900 2400 PROJECTION/ZONE: MONTANA STATE PLANE FIGURE 2-13 CLARK FORK RIVER DATUM: NAD1983 TOWNS COUNTY BOUNDARIES SECONDARY 1500 2000 12331600 UNITS: INT'L FEET FLOW NODE 12334550 to 2700 SOURCE: DNRC/MSL/PIONEER HUC 12 BOUNDARIES HUC 8 BOUNDARIES STREAMS 1600 2100 2600 1700 12331800 2700 CLARK FORK MAINSTEM STUDY REACH NHS INTERSTATE FLOW CHANGE NODE N 0 1.25 2.5 5 12331900 2300 DATE: 7/9/2020 12340500 Miles Path: Z:\Shared\Active Projects\DNRC\Missoula-Granite\GIS\Hydrology\MSAGNT-FIG-PLN-2-13-FN.mxd Missoula-Granite PMR Missoula-Granite County Hydrologic Analysis

2.3.3 Flood Frequency Estimates at Ungaged Sites To calculate peak flood discharge estimates at the ungaged flow nodes, Pioneer considered methods described in USGS SIR 2015-5109-F (Sando, et al., 2015b). These methods included estimating flood frequency using regional flood-frequency relations (regression analysis) and estimating flood frequency on gaged streams by translating gaged data to ungaged locations (drainage-area ratio adjustment or logarithmic interpolation between 2 gaged sites).

The hydrologic regions defined in SIR 2015-5019-F (Sando, et al., 2015b) indicate the Clark Fork River flows through the west region. All the Clark Fork River flow nodes are classified as unaffected by major regulation. The SIR 2015-5019-F report states that regression equations are possibly not reliable for an ungaged site that is outside the range of gage drainage areas used to develop the equations. The ungaged Clark Fork River flow node drainage areas are not within the range of values used to develop the west region regressions equations. Therefore, the regional regression equations were determined to be not applicable to the ungaged sites on the Clark Fork River.

Six USGS gaging stations were used on the Clark Fork River study reach to calculate peak flows. Nodes 100 through 900 were located between gaging stations Clark Fork at St. Regis (12354500) and Clark Fork below Missoula (12353000). Nodes 1100 through 1300 were located between gaging stations Clark Fork below Missoula (12353000) and Clark Fork above Missoula (12340500). Node 1500 was located between gaging stations Clark Fork above Missoula (12340500) and Clark Fork at Turah Bridge near Bonner (12334550). Nodes 1700 through 2300 were located between gaging stations Clark Fork at Turah Bridge near Bonner (12334550) and Clark Fork near Drummond (12331800). Nodes 2500 through 2900 were located between gaging stations Clark Fork near Drummond (12331800) and Clark Fork at Goldcreek (12324680).

All the ungaged flow nodes on the study reach are located between two gaging stations. Therefore, the two-site logarithmic interpolation method was used to estimate peak flows at ungaged flow nodes on the Clark Fork River study reach. Six USGS gages, all analyzed by the 2020 USGS peak flow frequency analysis (Sando, et al., 2020), were used to calculate AEP flows with the two-site logarithmic interpolation method.

The drainage area gage transfer method can be used to calculate peak flow frequency estimates at ungaged flow nodes near a single gaging station. Since the two-site logarithmic interpolation method is generally considered the more accurate peak flow method, the drainage area gage transfer method was not used to calculate peak flow frequency estimates at ungaged flow nodes on the Clark Fork River study reach.

Page 37 Missoula-Granite PMR Missoula-Granite County Hydrologic Analysis

2.3.3.1 Two-Site Logarithmic Interpolation Method Pioneer used the log interpolation method listed in SIR 2015-5019-F (Sando, et al., 2015b) for analysis on ungaged nodes between two gaged sites. All ungaged nodes on the Clark Fork River study reach fall between two gaging stations. In this method, the logarithm of the flood- frequency discharge estimates at the ungaged site is linearly interpolated based on discharge estimates and drainage basin areas of the upstream and downstream gaged sites. This method is listed in the equation below from SIR 2015-5019-F:

log Q , log Q , log , = log Q , + (log DA log DA ) log𝐴𝐴𝐴𝐴𝐴𝐴DA𝐺𝐺2 log DA𝐴𝐴𝐴𝐴𝐴𝐴 𝐺𝐺1 𝐴𝐴𝐴𝐴𝐴𝐴 𝑈𝑈 𝐴𝐴𝐴𝐴𝐴𝐴 𝐺𝐺1 − 𝑈𝑈 𝐺𝐺1 𝑄𝑄 𝐺𝐺2 𝐺𝐺1 − where − log is the base 10 logarithm.

QAEP,U is the AEP-percent peak flow at the ungaged site, in cfs.

QAEP,G1 is the AEP-percent peak flow at the upstream gaged site, in cfs.

QAEP,G2 is the AEP-percent peak flow at the downstream gaged site, in cfs.

DAG2 is the drainage area at the downstream gaged site, in square miles.

DAG1 is the drainage area at the upstream gaged site, in square miles.

DAU is the drainage area at the ungaged site, in square miles.

Table 2-15 shows the calculation results using extended record estimates. Figure 2-14 plots the relationship between the calculated discharge estimates and correlating drainage area. Except for the 1%+ peak flow estimates between Drummond and Turah, results indicate estimated flow at the ungaged flow nodes increases with increasing drainage area. The small decrease (5%±) in the 1%+ peak flow estimates between Drummond and Turah are attributed to local variances between the gages and the more complex methods required to calculate Bulletin 17C-based confidence intervals.

Page 38 Missoula-Granite PMR Missoula-Granite County Hydrologic Analysis

Table 2-15 Two-Site Logarithmic Interpolation Flow Node Peak Flow Estimates

Log Interpolation of Gaged Analysis Peak Flow (cfs) 50% Annual 10% Annual 4% Annual 2% Annual 1% Annual 0.2% Annual 1% + Node/USGS Chance Chance Chance Chance Chance Chance Station ID Location Description 2-year 10-year 25-year 50-year 100-year 500-year 100-year 12324680*1 Clark Fork at Goldcreek, MT 2,730 6,260 8,300 9,900 11,600 15,600 17,200 2700 Perkins Creek 2,800 6,410 8,540 10,200 12,000 16,400 18,200 2600 Lower Flint Creek 2,820 6,450 8,610 10,300 12,200 16,700 18,500 12331600 Clark Fork at Drummond, MT 3,190 7,220 9,880 12,100 14,600 21,300 24,400 2400 Rattler Gulch 3,220 7,300 10,000 12,200 14,800 21,800 25,100 2300 Mulkey Gulch 3,240 7,330 10,100 12,300 14,900 22,000 25,400 123318001 Clark Fork near Drummond, MT 3,280 7,420 10,200 12,500 15,200 22,600 26,100 2100 Harvey Creek 3,330 7,510 10,300 12,600 15,300 22,700 26,200 2000 Tyler Creek 3,450 7,710 10,500 12,800 15,600 22,900 26,300 1900 Dry Gulch 3,500 7,780 10,600 12,900 15,600 23,000 26,300 12331900 Clark Fork near Clinton, MT 3,580 7,930 10,800 13,100 15,800 23,200 26,400 1700 Rock Creek-Kitchen Gulch 3,670 8,060 10,900 13,300 16,000 23,300 26,500 1600 Schwartz Creek 6,020 11,700 14,800 17,300 20,000 26,700 28,400 1500 Wallace Creek 6,180 11,900 15,100 17,600 20,300 26,900 28,400 123345501 Clark Fork at Turah Bridge near Bonner, MT 6,260 12,000 15,200 17,700 20,400 27,000 28,500 1300 Clark Fork upstream of Blackfoot River 6,350 12,200 15,400 17,900 20,600 27,300 28,700 123405001,2 Clark Fork above Missoula, MT 15,000 26,600 32,100 36,100 39,900 48,400 46,600 1200 Lower Rattlesnake Creek 15,000 26,600 32,100 36,100 39,900 48,400 46,600 1100 Grant Creek 15,300 27,100 32,600 36,700 40,500 49,000 47,200 1000 Clark Fork upstream of Bitterroot River 15,600 27,500 33,100 37,100 41,000 49,600 47,800 123530001 Clark Fork below Missoula, MT 29,300 47,200 54,900 60,200 65,000 75,200 73,500 800 Deep Creek 29,500 47,500 55,200 60,600 65,400 75,600 73,900 700 Rock Creek 29,800 48,000 55,800 61,200 66,000 76,300 74,600

Page 39 Missoula-Granite PMR Missoula-Granite County Hydrologic Analysis

Page 40 Missoula-Granite PMR Missoula-Granite County Hydrologic Analysis

Figure 2-14 Clark Fork River Log Interpolation Gage Analysis Results

100,000 1%+

90,000 500-yr 100-yr

80,000 50-yr

25-yr 70,000 10-yr 2-yr 12340500 Above Missoula*

60,000

50,000

12334550 Bridge Turah

Peak Flow Flow (cfs) Peak 40,000 12331800 Near Drummond

30,000 12324680 Goldcreek* St Regis* St 12340500 Below Missoula Below 12340500 20,000

10,000

0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 9,000 10,000 11,000 Drainage Area (mi2)

* Denotes USGS gage used in calculations but not used as flow change node in this analysis.

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2.4 Clark Fork Mainstem Discussion Pioneer conducted a peak discharge frequency analysis for the Clark Fork River study reach. The Clark Fork River study reach extends 110 miles upstream from the Missoula/Mineral County Boundary to the Powell County boundary. Information gathered from this analysis will be used to support the Missoula-Granite County hydraulic analyses and floodplain mapping studies.

There are numerous previous flood studies on the Clark Fork River. The most relevant earlier flood studies were the City of Missoula and Missoula County FIS (FEMA, 2019a) and the Granite County and Incorporated Areas FIS (FEMA, 2016), The USGS SIR 2015-5019-C, published in 2015 (Sando, et al., 2015a) was also an important study that included flood frequency analysis for multiple Clark Fork River gaging stations. The results of these previous studies were compared with the results of this study.

This hydrologic analysis included conducting flood frequency estimates for both gaged and ungaged sites. Flood frequency estimates at the gaged sites were conducted by USGS (Sando, et al., 2020) using Bulletin 17C methodologies (England, et al., 2018).

The Clark Fork River systematic record data set had variable and discontinuous periods of recorded information between the stream gages. To address these non-congruent periods of record, USGS employed methods to extend the historical gage records and create an extended record data set (Sando, et al., 2020). Five of the 6 Clark Fork River gage records were extended to include water years 1899-1908, 1911-1923, and 1929-2018. The gage record at USGS station 12324680 Clark Fork at Goldcreek did not have a sufficient gage records for the 13 years between 1911 and 1923 to match the 113-year extended period of record achieved with the other 5 Clark Fork River gages used in the analysis. The Goldcreek gage was extended to include water years 1899-1908 and 1930-2018 (99-years). The flood flow frequency estimates based on the gage extended record data set were determined to provide the most accurate peak flow estimates for the Clark Fork River basin, due to the long, mostly congruent period of record, which minimized errors associated with the non-congruent, systematic flood frequency analysis. The gage-based AEP estimates meet standard hydrologic flood frequency practice guidance including the following:

• A 99-year synthesized period of record (Goldcreek gage) and a 113-year synthesized period of record (all other gages) that exceed the minimum FEMA guidance (FEMA, 2019).

Peak flow estimates were calculated at 27 locations (flow nodes) within the watershed (3 gaged sites and 24 ungaged sites). The ungaged sites (flow nodes) were located at HUC 12 boundaries, major tributaries, population centers, and at the end of study reaches. Peak flow 1%+ estimates were developed for all gaged locations using standard FEMA methodologies.

Page 42 Missoula-Granite PMR Missoula-Granite County Hydrologic Analysis

Three methods were considered for estimating peak flood discharges at ungaged flow nodes: 1) regional regression; 2) two- station logarithmic interpolation method; and 3) the drainage area gage transfer method.

The regional regression method for ungaged flow nodes was not selected for the Clark Fork River Study reach due to the size of the flow node drainage areas, which were not within the range of values recommended for the western region.

The two-site logarithmic interpolation method (Sando, et al., 2015b) was used on 24 ungaged flow nodes on the Clark Fork River. The drainage area gage transfer method was not used to calculate flood frequency estimates, as all the flow nodes on the Clark Fork River study reach were located between 2 gage stations. Higher confidence is typically associated with flood frequency estimates that are based on measured stream flows, specifically the two-site logarithmic interpolation method. For these reasons, the two-site logarithmic interpolation method was selected for the recommended flood discharge estimates on all ungaged flow nodes and 2 gaged flow nodes which had period of records less than 10 years.

Table 2-16 summarizes the recommended flood frequency peak flows for the Clark Fork River Study reach. Table 2-17 compares the recommended peak flows in this study with the existing effective mapping peak flows in Missoula and Granite Counties at comparable drainage areas. The comparison shows flood peak flows are generally less than or equal to the existing effective mapping values. Figure 2-15 through Figure 2-18 plot the recorded peak flow data and the recommended AEP flow estimates for the Clark Fork River gages updated in this analysis.

Figure 2-19 and Figure 2-20 shows the recommended 1% AEP peak flow for each flow node location. The hydrologic analysis results provided in Table 2-16 represent the recommended peak flow at each flow node location throughout the study reach. The methods used for hydrological analysis are industry accepted methods [Bulletin #17C (England, et al., 2018) and SIR 2015-5019-F (Sando, et al., 2015b)] based on the Clark Fork River basin characteristics. This hydrologic analysis conforms to FEMA standards for detailed/enhanced level studies, and the recommended flows of this analysis are deemed reliable and suitable for future floodplain studies and hydraulic analyses.

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Table 2-16 Recommended AEP Peak Flow Estimates

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Page 45 Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

Table 2-17 Recommended Peak Flow Estimates vs Existing Missoula and Granite County FIS Report Peak Flow Estimates

Peak Flow (cfs)

10% 2% 1% 0.2% Annual Annual Annual Annual Node/USGS Drainage Chance Chance Chance Chance Station ID Location Description County Peak Flood Frequency Method Area 10-year 50-year 100-year 500-year Granite Recommended AEP Discharge 1,900 6,450 10,300 12,200 16,700 2600 Lower Flint Creek (Above Flint Creek) Granite Granite County Existing FIS 1,888 8,300 14,400 17,500 25,700 Granite Recommended AEP Discharge 2,383 7,220 12,100 14,600 21,300 12331600 Clark Fork at Drummond MT (Below Flint Creek) Granite Granite County Existing FIS 2,378 9,850 16,900 20,500 30,000 Granite Recommended AEP Discharge 2,682 8,060 13,300 16,000 23,300 1700 Rock Creek-Kitchen Gulch (Above Rock Creek) Granite Granite County Existing FIS 2,668 9,850 16,900 20,500 30,000 Granite Recommended AEP Discharge 3,580 11,700 17,300 20,000 26,700 1600 Schwartz Creek (Below Rock Creek) Granite Granite County Existing FIS 3,553 15,000 22,500 26,000 35,500 Clark Fork Upstream of Blackfoot River (Just Missoula Recommended AEP Discharge 3,691 12,200 17,900 20,600 27,300 1300 upstream of confluence with Bitterroot River) Missoula Missoula County Existing FIS 3,668 15,000 22,500 26,000 35,500 Clark Fork above Missoula, MT (At USGS Gage Missoula Recommended AEP Discharge 6,013 26,600 36,100 39,900 48,400 12340500 No. 3405 above Missoula) Missoula Missoula County Existing FIS 5,999 27,000 38,200 42,500 56,000 Clark Fork below Missoula MT (At USGS Gage Missoula Recommended AEP Discharge 9,007 47,200 60,200 65,000 75,200 12353000 No. 3530 below Missoula) Missoula Missoula County Existing FIS 9,003 47,000 58,000 64,000 82,000 Ninemile Creek (At downstream Limit of Missoula Recommended AEP Discharge 9,269 49,000 62,400 67,300 77,700 Node 300 Detailed Study) Missoula Missoula County Existing FIS 9,272 49,250 61,000 67,000 86,000 Location description in parenthesis is name of flow change node in the existing FIS. Source: Pioneer, Granite County Flood Insurance Study (FEMA, 2016), Missoula County Flood Insurance Study (FEMA, 2019a). USGS: U.S. Geological Survey. cfs: cubic feet per second.

Page 46 Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

Figure 2-15 Updated Clark Fork River below Missoula (12353000)

70000 10 Yr Flood1 50 Yr Flood1 100 Yr Flood1

60000

50000

40000

30000 Peak Flow Flow (cfs) Peak

20000

10000

Page 47 Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

Figure 2-16 Updated Clark Fork above Missoula, MT (12340500)

50000 10 Yr Flood1 50 Yr Flood1 100 Yr Flood1 45000

40000

35000

30000

25000

Peak Flow Flow (cfs) Peak 20000

15000

10000

5000

Page 48 Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

Figure 2-17 Updated Clark Fork at Turah Bridge near Bonner (12334550)

25000 10 Yr Flood1 50 Yr Flood1 100 Yr Flood1

20000

15000

Peak Flow Flow (cfs) Peak 10000

5000

Page 49 Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

Figure 2-18 Updated Clark Fork near Drummond (12331800)

16000 10 Yr Flood1 50 Yr Flood1 100 Yr Flood1

14000

12000

10000

8000

Peak Flow Flow (cfs) Peak 6000

4000

2000

0 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 Year 1. Flood Frequency Based on Data through 2018

Page 50 Gold Creek

400 (67000 CFS)

300 MINERAL COUNTY (67300 CFS) Frenchtown Alberton 200 ¤£93 (69000 CFS) 500 (66800 CFS) 600 (66400 CFS) 100 (69800 CFS)

O'Keefe Creek

Valle Creek, La Albert Creek 700 (66000 CFS)

90 Butler Creek

Butler IrrigationDitch ¨¦§ West Fork Petty Creek Rock Creek Rattlesnake Creek

MISSOULA COUNTY Grant Creek

Petty Creek 800 (65400 CFS) HW Y 1 0 W

Clark Fork River

Marshall Creek Gilman Creek

Deep Creek

Johnson Gulch W BROADWAY Mittower Gulch

Blackfoot River

12353000 (65000 CFS) ¤£200 1100 (40500 CFS) Missoula 1000 (41000 CFS) Bonner 1200 (39900 CFS) 1300 (20600 CFS) OBrien Creek

South Fork Petty Creek

RESERVE ST Grave Creek BearCreek East Fork Grave Creek BROOKS ST H W Y 10 Deer Creek E

Hayes Creek HWY 93 S Bitterroot River Moose Can Gully

12334550 (20400 CFS)

LEGEND DISPLAYED AS: CLARK FORK RIVER FLOW NODE MISSOULA-GRANITE STUDY REACHES NHS NON-INTERSTATE FLOW CHANGE NODE 500 1000 PROJECTION/ZONE: MONTANA STATE PLANE FIGURE 2-19 DATUM: NAD1983 RECOMMENDED 1-PERCENT TOWNS COUNTY BOUNDARIES SECONDARY 100 600 1100 UNITS: INT'L FEET SOURCE: DNRC/MSL/PIONEER ANNUAL DISCHARGE HUC 12 BOUNDARIES HUC 8 BOUNDARIES URBAN 200 700 1200 STREAMS 300 800 1300 FLOW NODE 100 to 12334550 CLARK FORK MAINSTEM STUDY REACH NHS INTERSTATE N 0 1 2 4 400 12353000 12334550 DATE: 7/9/2020 Miles Path: Z:\Shared\Active Projects\DNRC\Missoula-Granite\GIS\Hydrology\MSAGNT-FIG-PLN-2-19-RD.mxd 12334550 (20400 CFS)

Donovan Creek

Union Creek

H W Y 10 E

1500 (20300 CFS) Allen Creek MISSOULA COUNTY Deep Creek

Greenough Creek West Fork Cramer Creek Douglas Creek Clinton POWELL COUNTY Cramer Creek Tenmile Creek

Bear Creek

1600 (20000 CFS) Mocassin Creek

Clark Fork River Y

G 1700 (16000 CFS) I

Swartz Creek 12331900 (15800 CFS) 2300 (14900 CFS) Clark Fork River 90 Dry Gulch 2100 (15300 CFS) ¨¦§ 12331800 (15200 CFS) 2000 Gilbert Creek 1900 (15600 CFS) (15600 CFS) 2400 (14800 CFS)

Gillispie Creek GRANITE COUNTY

Antelope Creek

Spring Creek Tigh Creek Rock Creek Drummond Tyler Creek

12331600 (14600 CFS) Bert Creek RAVALLI COUNTY Unnamed trib to Antelope Cr RM 2.9 2600 (12200 CFS)

Sawmill Creek Brewster Creek

Welcome Creek Harvey Creek Cow Creek «¬1 2700 (12000 CFS) Cinnabar Creek

Ranch Creek North Fork Lower Willow Creek Flint Creek

Upper Willow Creek Lower Willow Creek

Cinnamon Bear Creek

LEGEND DISPLAYED AS: CLARK FORK RIVER FLOW NODE MISSOULA-GRANITE STUDY REACHES PRIMARY 12334550 1900 2400 PROJECTION/ZONE: MONTANA STATE PLANE FIGURE 2-20 DATUM: NAD1983 RECOMMENDED 1-PERCENT TOWNS COUNTY BOUNDARIES SECONDARY 1500 2000 12331600 UNITS: INT'L FEET SOURCE: DNRC/MSL/PIONEER ANNUAL DISCHARGE HUC 12 BOUNDARIES HUC 8 BOUNDARIES STREAMS 1600 2100 2600 1700 12331800 2700 FLOW NODE 12334550 to 2700 CLARK FORK MAINSTEM STUDY REACH NHS INTERSTATE FLOW CHANGE NODE N 0 1.25 2.5 5 12331900 2300 DATE: 7/9/2020 1300 Miles Path: Z:\Shared\Active Projects\DNRC\Missoula-Granite\GIS\Hydrology\MSAGNT-FIG-PLN-2-20-RD.mxd Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

3 BITTERROOT RIVER

3.1 Bitterroot River Introduction 3.1.1 Basin Description The Bitterroot River is a major tributary to the Clark Fork River and upper headwaters of the Columbia River basin located west of the continental divide in western Montana. The river is formed by the confluence of the West Fork and East Fork Bitterroot Rivers. The river tributaries originate in Lolo and Bitterroot National Forests and the Anaconda-Pintler Wilderness. The watershed is formed by the Bitterroot Mountains to the west and the Sapphire Mountains to the east and south. The Bitterroot mainstem begins south of Darby, Montana, and flows 84 miles north to its confluence with the Clark Fork River. The entire Bitterroot River watershed encompasses 2,859 square miles. The Bitterroot River study reach is shown in Figure 3-1.

The Bitterroot River basin elevations within the study reach range from 3,194 feet at the Missoula-Ravalli County Border to approximately 3,092 feet at the confluence with the Clark Fork River. The overall basin elevations range from over 10,000 feet in the Bitterroot Range to 3,092 feet at the confluence with the Clark Fork River. The terrain varies from a high alpine environment at its headwaters to a heavily cultivated landscape in the Bitterroot Valley with expansive irrigated pasture lands, bracketed by rolling foothills.

Land use in the Bitterroot Basin is primarily agricultural with irrigated farming and ranching operations. Most of the intensely farmed land is located north of Hamilton, Montana, and south of Florence, Montana, in Ravalli County. Missoula is the primary community in the Bitterroot River study area.

Page 53 W BROADWAY 200 Missoula ¤£ Bonner

BROOKS ST Clark Fork River ¨¦§90

RESERVE ST

12352500

HWY 10 E

HW Y 12 Lolo Bitterroot River ¤£93

MISSOULA COUNTY

RAVALLI COUNTY

Florence 12351200 GRANITE COUNTY E ASTSIDE HWY

LEGEND DISPLAYED AS: ACTIVE USGS GAGE MISSOULA-GRANITE STUDY REACHES NHS NON-INTERSTATE PROJECTION/ZONE: MONTANA STATE PLANE FIGURE 3-1 BITTERROOT RIVER DATUM: NAD1983 TOWNS COUNTY BOUNDARIES SECONDARY UNITS: INT'L FEET STUDY AREA SOURCE: DNRC/MSL/PIONEER HUC 12 BOUNDARIES HUC 8 BOUNDARIES URBAN STREAMS N 0 1 2 4 BITTERROOT MAINSTEM STUDY REACH NHS INTERSTATE DATE: 7/2/2020 Miles Path: Z:\Shared\Active Projects\DNRC\Missoula-Granite\GIS\Hydrology\MSAGNT-FIG-PLN-3-1-SA.mxd Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

3.1.2 Flood History The primary cause of flooding on the Bitterroot River is spring snowmelt and ice jams (according to historical USGS gage station records). Reports that document the flooding mechanisms on the Bitterroot River are limited. These reports reference floods that occurred in 1948 and 1974. Neither of these floods caused widespread damage (Maki, 2018). There are historical records from 2 USGS stream gages near or within the study area that document flooding history. The gages are listed below:

1. Bitterroot River near Missoula, MT (12352500) 2. Bitterroot River near Florence, MT (12351200)

The USGS stream gage near Missoula, MT (12352500) has a 34-year period of record (1899- 1901,1903-1904,1990-2018). The annual peak flow record for the Missoula gage is shown in Figure 3-2. The gage is located in the Bitterroot River study reach.

The USGS stream gage near Florence MT (12351200) has a 20-year period of record (1958- 1965,1972,1974,1982,2003-2011). The annual peak flow record for the Florence gage is shown in Figure 3-3. The gage is located upstream of the Bitterroot River study reach.

Peak flow recurrence intervals shown in Figure 3-2 and Figure 3-3 are based on previously published flood frequency analysis through Water Year 2011 (SIR 2015-5019-C) (Sando, et al., 2015a).

Figure 3-2 shows that the peak flood of record for the Bitterroot River near Missoula (12352500) gage occurred in 1899 with a flow of 38,300 cfs, exceeding the 1% (100-year) AEP flow of 30,800. The second highest flood on record occurred in 1997 with a flow of 24,800 cfs, exceeding the 10% (10-year) AEP flow of 22,300 cfs. In the 34-year period of record at the Missoula gage, the 10% AEP flow has been equaled or exceeded 2 times. The annual peak flows occurred in April, May, June, and November. The peak flow of record occurred in June.

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Figure 3-2 Bitterroot River near Missoula, MT (12352500)

40000 10 Yr Flood1 50 Yr Flood1 100 Yr Flood1

35000

30000

25000

20000

Peak Flow Flow (cfs) Peak 15000

10000

5000

0 1899 1900 1901 1903 1904 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2006 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 Year 1. Flood Frequency Based on Data through 2011

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Figure 3-3 shows that the peak flood of record for the Bitterroot River near Florence (12351200) gage occurred in 1974 with a flow of 28,400 cfs, exceeding the 1% (100-year) AEP flow of 27,700. The second highest flood on record occurred in 1982 with a flow of 25,000 cfs, exceeding the 10% (10-year) AEP flow of 21,800. The third highest flood on record occurred in 1972 with a flow of 21,400 cfs. In the 20-year period of record at the Florence gage, the 10% AEP flow has been equaled or exceeded 2 times. The annual peak flows occurred in May, June, and November. The peak flow of record occurred in June.

Figure 3-3 Bitterroot River near Florence MT (12351200)

30000 10 Yr Flood1 50 Yr Flood1 100 Yr Flood1

25000

20000

15000 Peak Flow Flow (cfs) Peak

10000

5000

0 1958 1959 1960 1961 1962 1963 1964 1965 1972 1974 1982 2003 2004 2005 2006 2006 2008 2009 2010 2011 Year 1. Flood Frequency Based on Data through 2011

3.2 Past Studies and Existing Flood Data Past studies on the Bitterroot River study reach are limited. A FEMA FIRM and a FIS exist for Missoula County unincorporated areas and the City of Missoula. Studies relevant to this hydrologic study are those that include peak flow frequency analysis. Table 3-1 shows a summary of the Bitterroot River floodplain mapping.

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Table 3-1 Bitterroot River Floodplain Mapping Summary

Map Panel Summary Study Details # of # of FIRM Panel County Approx Detailed Total Community FIRM FBFM Effective FIS Date Stream (mi) (mi) (mi) Panels Panels Date Missoula Co. Unincorporated Bitterroot Missoula 66 7 07/06/2015 03/07/2019 0 23.0 23.0 Areas, City of River Missoula Source: FEMA Map Service Center FIRM: Flood Insurance Rate Map. FIS: Flood Insurance Studies. FBFM: Flood Boundary and Floodway Map. mi: Miles measured along channel alignment

The FIS dates reflect recent revisions in Missoula County in 2019. None of the study reaches analyzed in this report were updated in this revision.

The USGS WRIR 03-4308 (Parret & Johnson, 2004), SIR 2015-5019-C (Sando, et al., 2015b), and USGS SIR 2018-5046 (Sando & McCarthy, 2018) document the flood frequency analysis on several gages along the Bitterroot River. These studies and investigations are discussed in more detail in the following sections.

3.2.1 City of Missoula and Missoula County Flood Insurance Study The City of Missoula and Missoula County FIS was initially issued August 16, 1988 and revised on March 7, 2019 (FEMA, 2019a). As part of this FIS, the Bitterroot mainstem was studied using detailed methods. The study area included the unincorporated areas of Missoula County. The FIS identified a major flood on the Bitterroot in June 1974 with an estimated peak flow of 29,000 cfs.

Table 3-2 provides the City of Missoula and Missoula County FIS peak flow summary for ungaged flow change locations on the Bitterroot River. Peak flow values shown below were not updated in the 2019 revision.

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Table 3-2 Missoula County FIS Summary of Discharges Peak Flow, Return Interval (years) USGS (cfs) USGS Station Drainage 10- 50- 100- 500- Station ID Name Flooding Source and Location Area (mi2) Year Year Year Year Bitterroot River at confluence with NA NA 2,842 20,900 29,700 31,800 42,000 Clark Fork Bitterroot River 4.3 miles below NA NA 2,720 20,400 29,000 31,000 41,000 confluence with Lolo Creek Bitterroot River just upstream of NA NA 2,450 19,200 27,200 29,100 38,500 confluence of Lolo Creek Source: FEMA, 2019a. USGS: U.S. Geological Survey. mi2: square miles. cfs: cubic feet per second. NA: not applicable.

3.2.2 Water Resources Investigations Report 03-4308 The USGS WRIR 03-4308 developed annual peak discharges with recurrence intervals of 2, 5, 10, 25, 50, 100, 200, and 500 years (T-year floods) for 660 gaged sites in Montana and in adjacent areas of Idaho, Wyoming, and Canada, based on data through Water Year 1998 (Parret & Johnson, 2004). The flood-frequency information was used in regression analyses to develop equations relating T-year floods to various basin and climatic characteristics, active-channel width, and bankfull width. The equations can be used to estimate flood frequency at ungaged sites. Flood-frequency data typically were determined by fitting a log-Pearson Type III probability distribution using methods described by the IACWD, Bulletin 17B (IACWD, 1982).

Table 3-3 provides the WRIR 03-4308 peak flow summary for the USGS gages on the Bitterroot within or used in the study area. Table 3-3 shows that, in general, peak flows increase with an increase in drainage area and return interval.

Table 3-3 WRIR 03-4308 Bitterroot River Peak Flow Summary Peak Flow (cfs), Return Interval (years) USGS Drainage Years (cfs) Station Area of 10- 25- 50- 100- 200- 500- Number USGS Station Name (mi2) Record 5-year year year year year year year Bitterroot River near 12352500 2,814 14 20,000 23,400 27,300 30,000 32,500 35,000 38,000 Missoula

Bitterroot River near 12351200 2,354 12 18,600 20,800 23,600 25,700 27,700 29,800 32,500 Florence Based on systematic data through 1998 (Parret & Johnson, 2004). USGS: U.S. Geological Survey. mi2: square miles. cfs: cubic feet per second.

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3.2.3 Scientific Investigations Report 2015-5019 The USGS SIR 2015-5019-C updated annual peak discharges with AEPs of 66.7, 50, 42.9, 20, 10, 4, 2, 1, 0.5, and 0.2% (return intervals of 1.5, 2, 2.33, 5, 10, 25, 50, 100, 200, and 500 years, respectively) for 725 gaged sites in or near Montana, based on data through Water Year 2011 (Sando, et al., 2015a). Flood-frequency data typically were determined by fitting a log-Pearson Type III probability distribution using methods described by the IACWD, Bulletin #17B (IACWD, 1982). The study was part of a larger study to develop an online StreamStats application for Montana, in conjunction with computing streamflow characteristics at gage stations, and estimate peak flow flood frequency at ungaged sites. Table 3-4 provides the SIR 2015-5019-C discharge summary for the Bitterroot River study reach gages.

The USGS SIR 2015-5019-F (Sando, et al., 2015b) selected 537 gaging stations from the gage study. The 537 gaging stations were segregated based on the following criteria: contributing drainage area less than about 2,750 square miles, peak-flow records unaffected by major regulation, small redundancy with nearby stations, and representation of peak-flow frequencies at sites within Montana. The only gaging station used near the study reach was USGS gage station Bitterroot River near Florence, Montana (12351200). The study used regression analyses to develop equations relating AEP flows to various basin and climatic characteristics. The relationships developed for this study resulted in lower mean standard errors of prediction than previous regression analyses.

Table 3-4 SIR 2015-5019-C Bitterroot River Peak Discharge Summary Peak Flow (cfs), Return Interval (years) USGS Drainage Years (cfs) Station USGS Station Area of Number Name (mi2) Record 5-year 10-year 25-year 50-year 100-year 200-year 500-year Bitterroot 12352500 River near 2,814 27 19,200 22,300 25,900 28,400 30,800 33,100 36,000 Missoula Bitterroot 12351200 River near 2,354 20 19,500 21,800 24,400 26,100 27,700 29,100 30,900 Florence Based on systematic data through 2011 (Sando, et al., 2015a). USGS: U.S. Geological Survey. mi2: square miles. cfs: cubic feet per second.

3.2.4 Scientific Investigations Report 2018-5046 The USGS SIR 2018-5046 documents peak flow frequency stream gage analysis methods in Montana using Bulletin 17C guidelines. The report includes updated AEP peak discharges with AEPs of 50, 42.9, 20, 10, 4, 2, 1, 0.5, and 0.2% (return intervals of 2, 2.33, 5, 10, 25, 50, 100, 200, and 500 years, respectively) for 99 gages sites in or near Montana, based on data through Water Year 2015 (Sando & McCarthy, 2018). Flood-frequency data typically were determined by fitting a log-Pearson Type III probability distribution using an Expected Moments Algorithm analysis and other methods described by England and others in Bulletin 17C (England, et al., 2018). The

Page 60 Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

report does include USGS gages on the Bitterroot River; however, no flood frequency analyses were performed on USGS gages within, or used in, the Missoula County Bitterroot River study reach.

3.2.5 Additional Previous Studies Additional related previous studies conducted within Missoula County involve mining impacts, sediment management, fisheries management, watershed management, mining impacts, and watershed characterization reports:

• Bitter Root Water Forum. (2014). Bitterroot Watershed Restoration Plan (Bitter Root Water Forum, 2014). • Clark Fork Coalition. (2017). 2017 Bitterroot Strategy (Clark Fork Coalition, 2017). • Fortman, K., Kron, D., Staten, C., & Starr, B. (2011). Bitterroot Temperature and Tributary Sediment Total Maximum Daily Loads and Framework: Water Quailty Improvement Plan. Montana Department of Enviornmental Quality (Fortman, et al., 2011). • LaFave, J. I. (2006). Ground-Water Quality of the Shallow Basin-Fill, Deep Basin-Fill, and Bedrock Aquifers, Bitterroot Valley, Missoula and Ravalli Counties, Southwest Montana. Montana Bureau of Mines and Geology (LaFave, 2006). • DEQ, 2011. Bitterroot Temperature & Tributary Sediment TMDLs - Appendix B (DEQ, 2011). • Northwest Power and Conservation Council. (2009). Bitterroot River Subbasin Plan. Portland: Columbia River Basin Fish and Wildlife Program (Northwest Power and Conservation Council, 2009). • Smith, L. N., LaFave, J. I., & Patton, T. W. (2013). Montana Ground Water Assessment Atlas No. 4: Groundwater Resources of the Lolo-Bitterroot Area: Mineral, Missoula, and Ravalli Counties, Montana: Part A Descriptive Overview and Water-Quality Data. Montana Bureau of Mines and Geology (Smith, et al., 2013).

3.2.6 Flood Protection Measures The City of Missoula and Missoula County Unincorporated Areas FIS (FEMA, 2019a) described non-levee flood protection measures within Missoula county as dams, jetties, and dikes. The report identified an earthen dike on the Bitterroot River near the Lolo sewage-treatment plant. The dike was partially washed away in a 1975 flood. There are no documented levees on the Bitterroot River within Missoula County.

The USACE maintains the NID, a database that keeps record of all the dams across the country. The NID identifies hazard potential, dam height, ownership, purpose, dam type, and administrative data. The NID identifies two low hazard irrigation dams in Missoula County that are located on tributaries to the Bitterroot River. There are no major flood control structures on the Bitterroot River mainstem study reach (USACE, 2019).

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3.3 Hydrologic Analyses and Results The purpose of the hydrologic analyses conducted as part of this project was to develop peak flow discharge estimates for the 50, 10, 4, 2, 1, and 0.2% AEP and 1%+ AEP events at key flow change locations (such as at significant tributaries confluences, stream gages, and population centers) along the Bitterroot River study reach. The hydrologic analysis was organized into 2 sub- sections:

1. USGS Stream Gage Analysis. 2. Ungaged Flow Node Analysis.

Within the study area, 4 flow nodes on the Bitterroot River study reach were identified as having significant changes in streamflow or being at critical locations. Of the 4 flow nodes on the study reach, 1 is located at an active USGS stream gage. The study extents in this report were based on the Missoula-Granite PMR Study Area Map provided by the DNRC. The river stations used in this report were based on the Bitterroot River S_Wtr_Ln, delineated by Pioneer using 2019 LiDAR and channel bathymetry. The Bitterroot River S_Wtr_Ln Reach alignment begins at the junction with the Clark Fork River. The upstream extent of the study reach ends at the Missoula-Ravalli County line.

3.3.1 USGS Stream Gage Analysis The USGS performed peak flow analysis for selected stream gages in Deer Lodge, Powell, Granite, Missoula, Ravalli, and Mineral counties, using data through 2018, as part of this hydrologic analysis (Sando, et al., 2020). This longer period of record should produce more accurate peak discharge estimates than those based on a shorter period of record. One of those gages analyzed by USGS is located on the Bitterroot River study reach (12352500, Bitterroot River near Missoula, MT) and 1 USGS gage (12351200, Bitterroot River near Florence, MT) is located on the Bitterroot River approximately 2 miles upstream from the county boundary .

The USGS gage flood frequency analysis summary, data, and calculations are provided in Appendix A. Bulletin 17C methods (England, et al., 2018) were used to estimate the 50, 42.9, 20, 10, 4, 2, 1, 0.5, and 0.2% AEPs and the 1%+ AEP. The USGS methodologies used for the gage analyses presented in this report are documented in USGS SIR 2018-5046 (Sando & McCarthy, 2018).

The oldest records used in this study date back to 1899 at USGS gaging station Bitterroot River near Missoula, MT (12352500). This gage remains active. The FEMA guidance document (FEMA, 2019) indicates that gage station records equal or exceeding 10 years in length are applicable to all types of studies. The gages used in this study meet this criterion. Figure 3-4 shows the study reach and the USGS gaging station locations along and near the study reach.

Page 62 W BROADWAY 200 Missoula ¤£ Bonner

BROOKS ST Clark Fork River ¨¦§90

RESERVE ST

12352500

HWY 10 E

HW Y 12 Lolo Bitterroot River ¤£93

MISSOULA COUNTY

RAVALLI COUNTY

Florence 12351200 GRANITE COUNTY E ASTSIDE HWY

LEGEND DISPLAYED AS: ACTIVE USGS GAGE BITTERROOT MAINSTEM STUDY REACH NHS INTERSTATE USGS WATERSHED PROJECTION/ZONE: MONTANA STATE PLANE FIGURE 3-4 BITTERROOT RIVER DATUM: NAD1983 INACTIVE USGS GAGE MISSOULA-GRANITE STUDY REACHES NHS NON-INTERSTATE 12351200 UNITS: INT'L FEET USGS WATERSHEDS SOURCE: DNRC/MSL/PIONEER TOWNS COUNTY BOUNDARIES SECONDARY 12352500 URBAN N 0 1 2 4 HUC 12 BOUNDARIES HUC 8 BOUNDARIES DATE: 7/2/2020 STREAMS Miles Path: Z:\Shared\Active Projects\DNRC\Missoula-Granite\GIS\Hydrology\MSAGNT-FIG-PLN-3-4-UW.mxd Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

The USGS performed flood frequency analysis on gages with a minimum 10 years of record within Deer Lodge, Powell, Granite, Missoula, Ravalli, and Mineral counties. Seven gages were analyzed within Missoula and Granite County. The USGS flood frequency analysis is provided in Appendix A. The USGS flood frequency analysis results based on data through water year 2018 were employed in this hydrologic analysis. Table 3-5 summarizes the USGS stream gages analyzed by the USGS along the Bitterroot River study reach. Figure 3-5 plots the systematic or at-site flood frequency results as a function of drainage area.

Table 3-5 Bitterroot River USGS Gage Summary

Regulation Total Period USGS Station Status as Total Number of Years of Record, in 2018 Number Station Name of 2018 of Peak-Flow Records Water Years Status 1958-1965, 1972, 1974, 12351200* Bitterroot River near Florence, MT U 20 Inactive 1982, 2003-2011 1899-1901, 1903-1904, 12352500 Bitterroot River near Missoula, MT U 34 Active 1990-2018 * Denotes USGS gage outside of study reach. Source: (Sando, et al., 2020). USGS: U.S. Geological Survey. U: unregulated.

3.3.1.1 1%+ Peak Flow Estimates The 1%+ AEP event was calculated by the USGS (Sando, et al., 2020) to provide a confidence range that the 1% flood frequency peak flow estimates are likely to fall within. FEMA guidance defines the 1%+ as “…a flood elevation derived by using discharges that are at the upper 84- percent confidence limit as calculated in the gage analysis for the 1-percent-annual-chance event for the Flood Risk Project. Methods for estimating synthetic statistics outlined in Bulletin 17C Appendix 7 are used to estimate the upper 84 percent confidence limit of the Log Pearson III frequency Curve at the 1-percent-annual-chance event.” (FEMA, 2019) The USGS used the methods for estimating synthetic statistics outlined in Bulletin 17C, Appendix 7.

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Figure 3-5 USGS Flood Frequency Estimates Systematic Record through 2018

45,000 1%+ 500-yr 100-yr 40,000 50-yr 25-yr 10-yr 35,000 2-yr

30,000

25,000

20,000 Peak Flow Flow (cfs) Peak

15,000 12351200 near Florence near near Missoula near 12352500 10,000

5,000

2,300 2,400 2,500 2,600 2,700 2,800 2,900 Drainage Area (mi2)

Figure 3-5 indicates the 2-year peak discharges do not consistently increase with increasing drainage area, as typically expected. These inconsistences can be related to non-congruent periods of record or variability in flood storage. Regional Regression Weighted and Maintenance of Variance Extension, Type III (MOVE.3) are 2 procedures that sometimes can be used to improve at-site frequency analyses. In this Bitterroot River gage analysis, the USGS employed the MOVE.3 method to extend the historical gage record for the Missoula County study reach Bitterroot River USGS gage stations to improve the at-site frequency analysis. Figure 3-6 plots the extended record results for the Bitterroot River extended record analysis.

Page 65 Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

Figure 3-6 USGS Flood Frequency Estimates Extended Record though 2018

45,000 1%+ 500-yr 100-yr 40,000 50-yr 25-yr 10-yr 35,000 2-yr

30,000

25,000

20,000 Peak Flow Flow (cfs) Peak

15,000 near Missoula near 12352500 near Florence near 12351200 10,000

5,000

2,300 2,400 2,500 2,600 2,700 2,800 2,900 Drainage Area (mi2)

Per Bulletin 17C guidance, a minimum correlation coefficient of 0.8 is required when using MOVE.3 extended record analysis. The Pearson correlation coefficient for the study reach gages analyzed by USGS ranged from 0.97 to 0.99 (Appendix A).

Figure 3-6 indicates the extended record data set produces peak flow estimates that increase with increasing drainage areas for all flood frequencies. Using the extended record data set will help minimize the potential error associated with the at-site, non-congruent periods of record. For these reasons, flood flow frequency estimates using the 2020 extended record data set were selected to represent the AEP peak flow estimates at the Bitterroot River study reach gaged locations. The Bitterroot River 1%+ peak flow estimates are listed in Table 3-6. Table 3-6 shows the results for systematic and MOVE.3 flood frequency estimates for gages on the Bitterroot River.

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Table 3-6 Bitterroot River Gage Flood Frequency Peak Flow Estimates Peak Flow (cfs) 50% 10% 4% 2% 1% 0.2% 1% + Annual Annual Annual Annual Annual Annual Annual Peak Flood Chance Chance Chance Chance Chance Chance Chance Node/USGS Frequency Water years of peak flows 10- 25- 50- 100- 500- 100- Station ID Location Description Method used in the analysis 2-year year year year year year year + 1958-1965, 1972, 1974, At-Site 15,100 20,900 23,600 25,600 27,500 31,800 32,300 Bitterroot River near 1982, 2003-2011 12351200* Florence, MT 1899-1901, 1903-1904, MOVE.3 14,100 20,100 23,000 25,100 27,200 32,000 32,400 1930-2018 1899-1901, 1903-1904, At-Site 14,400 23,100 27,200 30,100 32,900 39,200 38,400 Bitterroot River near 1990-2018 12352500 Missoula, MT 1899-1901, 1903-1904, MOVE.3 14,700 22,700 26,400 28,900 31,400 36,900 36,600 1930-2018 * Denotes USGS gage outside of study reach. Source: (Sando, et al., 2020). USGS: U.S. Geological Survey cfs: cubic feet per second. At-site: Peak flow frequency analysis on recorded data. MOVE.3: Peak flow frequency analysis on combined recorded and synthesized data; synthesized data from Maintenance of Variance Extension Type III (MOVE.3) record extension procedure.

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3.3.2 Flow Change Node Locations Future flood studies will use hydraulic models that are composed of geometric data and streamflow data. To accurately model the Bitterroot River, the locations of major tributary confluences and other flow change locations must be identified. The hydrologic analysis results will be used as the streamflow data input at the tributary confluences within the hydraulic model.

Generally, the hydraulic models simulate flood events using steady-state conditions and, therefore, the peak flow rate calculated at a flow node is projected to the next upstream flow node. This method was followed for the hydrologic analysis calculations. Flow nodes were assigned immediately upstream of HUC 12 boundaries; this method of locating the flow nodes was employed so that the additional flow resulting from the tributary confluence was accurately reflected to the reach downstream of the confluence.

A detailed review of the study area was performed to identify all potential flow change locations (flow nodes). The HUC 12-digit watershed boundaries were used to initially locate the flow nodes. The HUC 12-digit watershed boundaries represent the smallest USGS-delineated watershed areas available in GIS format. Using ArcGIS, flow nodes were located just upstream of the HUC 12 boundary intersection with the Bitterroot River.

At each flow node, a drainage basin area was delineated, and streamflow values were calculated for the various recurrence interval floods. Drainage basin areas were calculated with ArcGIS, an ESRI software that produces drainage basin areas.

As an accuracy check, the USGS gaging station watershed areas calculated in ArcGIS were compared to the USGS NWIS (http://waterdata.usgs.goc/nwis) published gaging station areas. Figure 3-4 shows the USGS gaging stations analyzed and the correlating GIS model-generated watershed areas along the study area. Table 3-7 shows the results of this comparison. Based on the 2 gaging stations analyzed, the project GIS model calculates watershed areas that are within 1% of the USGS NWIS published areas (Appendix A).

This study used the nearest GNIS hydrographic feature name for the ungaged flow node names. In some cases, these features (typically tributary streams) flow into the Bitterroot River just downstream of the flow node. Nodes were also named for USGS gage station numbers if placed at gaged locations.

Table 3-8 lists the flow node and gage station Information used in the analysis. A total of 4 flow nodes were identified throughout the study reach, including 1 gaged location and 3 ungaged locations. Figure 3-4 shows the USGS gaging stations analyzed and the correlating watershed areas within the study area. Figure 3-7 maps the flow node locations and corresponding watershed areas from Table 3-8.

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Table 3-7 USGS and GIS Model Watershed Comparison USGS GIS Published Calculated Drainage Basin Relative Station Area1 Area2 Percent Accuracy of Number USGS Station Name (mi2) (mi2) Change Areas 12351200* Bitterroot River near Florence, MT 2,342 2,340 0.09% 99.9% 12352500 Bitterroot River near Missoula, MT 2,824 2,821 0.10% 99.9% 1. Source: (Sando, et al., 2020). 2. Basin areas (watershed areas) used for hydrological analysis. * Denotes USGS gage outside of study reach, not used as a flow node in this analysis. USGS: U.S. Geological Survey. mi2: square miles.

Table 3-8 Flow Node and USGS Gage Station Information Used in Hydrologic Analysis River Calculated Node/USGS Study Station1 Basin Area2 Station ID Location Description County Reach (mi) (mi2) Bitterroot River at junction with Clark Bitterroot 100 Missoula 0.1 2857 Fork River River Bitterroot 12352500 Bitterroot River near Missoula, MT Missoula 5.9 2821 River Bitterroot 300 Lower Lolo Creek Missoula 9.8 2742 River Bitterroot River-North Woodchuck Bitterroot 400 Missoula 14.3 2414 Creek River 12351200* Bitterroot River near Florence, MT Ravalli NA NA 2,340 1. River miles start at the downstream extent of each study reach (mi: miles). 2. Source: Esri ArcGIS. * Denotes USGS gage outside of study reach, not used as a flow node in this analysis. USGS: U.S. Geological Survey. mi: miles. mi2: square miles.

Page 69 Blackfoot River

W BROADWAY 200 Grant Creek Missoula ¤£ Bonner Deep Creek 100

OBrien Creek Grave Creek BROOKS ST Clark Fork River ¨¦§90

East Fork Grave Creek RESERVE ST

Deer Creek 12352500

Bear Creek Hayes Creek

Moose Can Gully

Grave Creek

Sleeman Creek Woodman Creek HWY 10 E 300 Donovan Creek

Miller Creek Allen Creek

HW Y 12 Greenough Creek Lolo Creek Lolo West Fork Butte Creek Bitterroot River Unnamed trib to Greenough Cr 3.3 ¤£93 400 Davis Creek

Mocassin Creek

Mormon Creek

South Fork Lolo Creek

McClain Creek MISSOULA COUNTY Squaw Creek

Swartz Creek

Carlton Creek

One Horse Creek Woodchuck Creek Gilbert Creek

RAVALLI COUNTY Eightmile Creek

Florence GRANITE COUNTY DE H One HorseE A CreekSTSI WY

LEGEND DISPLAYED AS: FLOW NODE MISSOULA-GRANITE STUDY REACHES NHS NON-INTERSTATE FLOW CHANGE NODE PROJECTION/ZONE: MONTANA STATE PLANE FIGURE 3-7 BITTERROOT RIVER DATUM: NAD1983 TOWNS COUNTY BOUNDARIES SECONDARY 100 UNITS: INT'L FEET FLOW NODE LOCATIONS SOURCE: DNRC/MSL/PIONEER HUC 12 BOUNDARIES HUC 8 BOUNDARIES URBAN 12352500 STREAMS 300 BITTERROOT MAINSTEM STUDY REACH NHS INTERSTATE N 0 1 2 4 400 DATE: 7/2/2020 Miles Path: Z:\Shared\Active Projects\DNRC\Missoula-Granite\GIS\Hydrology\MSAGNT-FIG-PLN-3-7-FN.mxd Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

3.3.3 Flood Frequency Estimates at Ungaged Sites To calculate peak flood discharge estimates at the ungaged flow nodes, Pioneer considered methods described in USGS SIR 2015-5109-F (Sando, et al., 2015b). These methods included estimating flood frequency using regional flood-frequency relations (regression analysis) and estimating flood frequency on gaged streams by translating gaged data to ungaged locations (drainage-area ratio adjustment or logarithmic interpolation between 2 gaged sites).

The hydrologic regions defined in SIR 2015-5019-F (Sando, et al., 2015b) indicate the Bitterroot River flows through the west region. All the Bitterroot River flow nodes are classified as unaffected by major regulation. The SIR 2015-5019-F report states that regression equations are possibly not reliable for an ungaged site that is outside the range of gage drainage areas used to develop the equations. The SIR 2015-5019-F developed regional regression equations using stream gages with a drainage area less than 2,465 square miles. Regression analysis on ungaged flow nodes with drainage areas greater than 2,465 square miles is not recommend by the USGS guidance documents. The ungaged Bitterroot River flow node drainage areas exceed or are near 2,465 square miles. Due to the availability of gage data and some flow nodes being outside the recommended basin area, regional regression equations were not used in this analysis.

Two USGS gaging stations were used on the Bitterroot River study reach to calculate peak flows. Nodes 300 and 400 were located between USGS gaging stations Bitterroot River near Missoula (12352500) and Bitterroot River near Florence (12331200). The two-site logarithmic interpolation method was used to estimate peak flows at these ungaged flow nodes on the Bitterroot River study reach.

Node 100 is located downstream of USGS gage station Bitterroot River near Missoula (12352500). Node 100 meets the SIR 2015-5019-F (Sando, et al., 2015b) guidance criteria for the drainage area gage transfer method (i.e., are within the ratio of 1.5 to 0.5 of the gage drainage area [see Table 3-10 in Section 3.3.3.2]) when analyzed with the Bitterroot River near Missoula gage. Since node 100 is not located between 2 Bitterroot River gaging stations, the drainage area gage transfer method was used to estimate peak flows.

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3.3.3.1 Two-Site Logarithmic Interpolation Method Pioneer used the log interpolation method listed in SIR 2015-5019-F (Sando, et al., 2015b) for analysis on ungaged nodes between two gaged sites. Nodes 300 and 400 are between USGS gage station Bitterroot River near Missoula, MT (12352500) and USGS gage station Bitterroot River near Florence, MT (12351200). In this method, the logarithm of the flood-frequency discharge estimates at the ungaged site is linearly interpolated based on discharge estimates and drainage basin areas of the upstream and downstream gaged sites. This method is listed in the equation below from SIR 2015-5019-F:

log Q , log Q , log , = log Q , + (log DA log DA ) log𝐴𝐴𝐴𝐴𝐴𝐴DA𝐺𝐺2 log DA𝐴𝐴𝐴𝐴𝐴𝐴 𝐺𝐺1 𝐴𝐴𝐴𝐴𝐴𝐴 𝑈𝑈 𝐴𝐴𝐴𝐴𝐴𝐴 𝐺𝐺1 − 𝑈𝑈 𝐺𝐺1 𝑄𝑄 𝐺𝐺2 𝐺𝐺1 − Where − log is the base 10 logarithm.

QAEP,U is the AEP-percent peak flow at the ungaged site, in cfs.

QAEP,G1 is the AEP-percent peak flow at the upstream gaged site, in cfs.

QAEP,G2 is the AEP-percent peak flow at the downstream gaged site, in cfs.

DAG2 is the drainage area at the downstream gaged site, in square miles.

DAG1 is the drainage area at the upstream gaged site, in square miles.

DAU is the drainage area at the ungaged site, in square miles.

Table 3-9 shows the calculation results using extended record peak flow estimates. Figure 3-8 plots the relationship between the calculated peak flow estimates and correlating drainage area. Results indicate estimated flow at the ungaged flow nodes increase with increasing drainage area.

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Table 3-9 Two-Site Logarithmic Interpolation Flow Node Peak Flow Estimates

Log Interpolation of Gaged Analysis Peak Flow (cfs) 1% + 50% Annual 10% Annual 4% Annual 2% Annual 1% Annual 0.2% Annual Annual Node/USGS Chance Chance Chance Chance Chance Chance Chance Station ID Location Description 2-year 10-year 25-year 50-year 100-year 500-year 100-year 123512001* Bitterroot River near Florence, MT 14,100 17,800 20,100 23,000 25,100 32,000 32,400 400 Bitterroot River-North Woodchuck Creek 14,200 18,500 21,000 23,900 26,100 32,800 33,100 300 Lower Lolo Creek 14,600 21,900 25,300 27,900 30,400 36,100 35,900 123525001 Bitterroot River near Missoula, MT 14,700 22,700 26,400 28,900 31,400 36,900 36,600 1. Peak flow estimated from extended record analysis (Sando, et al., 2020). * Denotes USGS gage outside of study reach, not used as flow node in this analysis. USGS: U.S. Geological Survey. cfs: cubic feet per second.

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Figure 3-8 Bitterroot River Log Interpolation Gage Analysis Results

45,000 1%+

500-yr 100-yr

40,000

50-yr 12352500 Near Missoula 25-yr Node 300 10-yr 35,000 Node 400 2-yr Near Florence* Near 12351200 30,000

25,000

Peak Flow Flow (cfs) Peak

20,000

15,000

10,000 2,300 2,400 2,500 2,600 2,700 2,800 2,900 Drainage Area (mi2)

* Denotes USGS gage outside of study reach, not used as flow node in this analysis.

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3.3.3.2 Drainage Area Gage Transfer Method The ungaged flow node located downstream of USGS gage Bitterroot River near Missoula, MT (12352500) was considered for applying the drainage area gage transfer method. Limitations for this method include a recommended drainage area ratio between 0.5 and 1.5. This method, listed in SIR 2015-5019-F (Sando, et al., 2015b), uses a drainage area ratio of the ungaged flow node to the gaged station to transfer flow estimates from the gaged site to the ungaged site as shown in the following equation:

DA , = , exp𝑇𝑇 DA𝑈𝑈 𝐴𝐴𝐴𝐴𝐴𝐴 𝑈𝑈 𝐴𝐴𝐴𝐴𝐴𝐴 𝐺𝐺 W𝑄𝑄here 𝑄𝑄 � 𝐺𝐺 � QAEP,U is the AEP-percent peak flow at the ungaged site, in cfs.

QAEP,G is the AEP-percent peak flow at the gaged site, in cfs.

DAG is the drainage area at the gaged site, in square miles.

DAU is the drainage area at the ungaged site, in square miles.

expAEP is the regression coefficient for a simple ordinary least squares (OLS) regression relating to the log of the AEP-percent peak flow to log of drainage area within each region.

Node 100 was considered for gage transfer. Gage transfer was not calculated between the 2 Bitterroot River gages. The drainage area ratio for node 100 was within the recommended range (Table 3-10). Extended record data (MOVE.3) on USGS gage Bitterroot River near Missoula, MT (12352500) was selected to calculate peak flows on node 100 to account for the limited period of record (34 years) at the gage. The longer period of record should produce more accurate peak discharge estimates than those based on a shorter period of record.

The applicable west region regression coefficients provided in Table 3-11 were used in these calculations. The drainage area gage transfer method results are shown in Table 3-12 and Figure 3-9.

Table 3-10 Drainage Basin Ratios

Node/USGS River Station1 Basin Area 2 Station ID Location Description (mi) (mi ) DAU/DAG Bitterroot River at junction with Clark 100 0.1 2857 1.0 Fork River 12352500 Bitterroot River near Missoula, MT 5.9 2821 - 1. River miles start at the downstream extent of each study reach. Source: Esri ArcGIS and USGS. USGS: U.S. Geological Survey. mi: mile. mi2: square miles. DAG: drainage area at the gaged site, in square miles. DAU: drainage area at the ungaged site, in square miles.

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Table 3-11 Drainage Area Gage Transfer Regression Coefficients

AEP Percent Peak Flow expAEP Q50 0.843 Q10 0.794 Q4 0.777 Q2 0.766 Q1 0.755 Q0.2 0.735

Table 3-12 Drainage Area Gage Transfer Method Results for Flow Node Locations near USGS gage 12352500 Bitterroot River near Missoula, MT Drainage Area Gage Transfer Estimated Peak Flow (cfs) 50% 1% + Annual 10% Annual 4% Annual 2% Annual 1% Annual 0.2% Annual Annual Node/USGS Chance Chance Chance Chance Chance Chance Chance Station ID Location Description 2-year 10-year 25-year 50-year 100-year 500-year 100-year 123525001 Bitterroot River near Missoula, MT 14,700 22,700 26,400 28,900 31,400 36,900 36,600 Bitterroot River at junction with Clark 1002 15,200 23,500 27,300 29,800 32,400 38,000 37,800 Fork River 1. Peak flow estimated from extended record analysis. 2. Node within recommended DAU/DAG range of 0.5 to 1.5. DAU: Drainage area at ungaged site. DAG: Drainage area at gaged site. USGS: U.S. Geological Survey. cfs: cubic feet per second.

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Figure 3-9 Bitterroot River Drainage Area Gage Transfer Results

50,000 1%+ 500-yr 100-yr 45,000 50-yr 25-yr 10-yr 40,000 2-yr

35,000

30,000 Peak Flow Flow (cfs) Peak 25,000

12352500

20,000 Node 100 Near Missoula

15,000

10,000 2820 2840 2860 Drainage Area (mi2)

3.3.4 Bitterroot River Discussion Pioneer conducted a peak discharge frequency analysis for the Bitterroot River study reach. The Bitterroot River study reach extends 22 miles upstream from the junction with the Clark Fork River. Information gathered from this analysis will be used to support the Missoula-Granite County hydraulic analyses and floodplain mapping studies.

There are numerous previous flood studies on the Bitterroot River. The most relevant earlier flood studies were the City of Missoula and Missoula County FIS (FEMA, 2019a). The USGS SIR 2015-5019-C, published in 2015 (Sando, et al., 2015a), was also an important study that included flood frequency analysis for multiple Bitterroot River gaging stations. The results of these previous studies were compared with the results of this study.

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data is recommended to perform gage peak flow analysis; the gages used in this analysis surpass that minimum.

The Bitterroot River systematic record data set had variable and discontinuous periods of recorded information between the stream gages. To address these non-congruent periods of record, USGS employed MOVE.3 methods to extend the historical gage records and create an extended record data set (Sando, et al., 2020). The gage records were extended to include water years 1899-1901, 1903-1904, and 1930-2018. Per Bulletin 17C guidance, a minimum correlation of 0.8 is required when using extended record analysis. The Pearson correlation coefficient for USGS gages 12352500 and 12351200 is 0.97 and 0.99 respectively (Sando, et al., 2020).

The flood frequency peak flow estimates based on the gage extended record data set were determined to provide the most accurate peak flow estimates for the Bitterroot River basin, due to the long congruent period of record, which minimized errors associated with non-congruent periods of gage records. Peak flow 1%+ estimates were developed for all gaged locations using standard FEMA methodologies.

The gage-based AEP estimates meet standard hydrologic flood frequency practice guidance including the following:

• A 94-year synthesized period of record that exceeds the minimum FEMA guidance (FEMA, 2019).

Peak flow estimates were calculated at 4 locations (flow nodes) within the watershed (1 gaged site and 3 ungaged sites). The ungaged sites were located at HUC 12 boundaries and at the end of study reaches.

Flood frequency peak flow estimates at ungaged flow nodes were conducted using two-site logarithmic interpolation for flow nodes 300 and 400. The drainage area gage transfer (Sando, et al., 2015b) was used for flow node 100. Regional regression was not used due to the availability of existing gage data.

Higher confidence is typically associated with flood frequency estimates that are based on measured stream flows. For these reasons, the two-site logarithmic interpolation method was selected for the recommended flood discharge estimates on ungaged nodes 300 and 400. The gage transfer method was selected for node 100.

The potential for coincident peaks with the Clark Fork River was investigated. The drainage area at node 100 on the Bitterroot (2,857 square miles) was compared with the drainage area of node 1000 on the Clark Fork River (6,149 square miles). Per FEMA guidance, the assumption of coincident peaks may be appropriate if the ratio of drainage areas lies between 0.6 and 1.4 (FEMA, 2016c). The ratios for drainage areas between these two reaches (0.46) falls outside the guidance range, therefore coincident peaks as a boundary condition is not applicable.

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Table 3-13 summarizes the recommended flood frequency peak flow estimates for the Bitterroot River study reach. Table 3-14 compares the recommended peak flow estimates developed in this study with the existing effective mapping discharges in Missoula County at comparable drainage areas. The comparison shows flood discharges are generally less than or equal to the existing mapping values, except for node 100, which is greater. Figure 3-10 and Figure 3-11 plot the recorded peak flow data and the recommended AEP flow estimates for the Bitterroot River gages updated in this analysis.

Figure 3-12 shows the recommended 1% AEP peak flow for each flow node location. The hydrologic analysis results provided in Table 3-13 represent the recommended discharges at each flow node location throughout the study reach. The methods used for hydrological analysis are industry accepted methods [Bulletin 17C (England, et al., 2018), SIR 2015-5019-C (Sando, et al., 2015a), and SIR 2015-5019-F (Sando, et al., 2015b)] based on the Bitterroot River mainstem basin characteristics. This hydrologic analysis conforms to FEMA standards for enhanced level studies, and the recommended flows of this analysis are deemed reliable and suitable for future floodplain studies and hydraulic analyses.

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Table 3-13 Recommended AEP Peak Flow Estimates

Peak Flow (cfs) 50% Annual 10% Annual 4% Annual 2% Annual 1% Annual 0.2% Annual 1% + Annual Node/USGS Chance Chance Chance Chance Chance Chance Chance Station ID Location Description 2-year 10-year 25-year 50-year 100-year 500-year 100-year + 4002 Bitterroot River-North Woodchuck Creek 14,200 18,500 21,000 23,900 26,100 32,800 33,100 3002 Lower Lolo Creek 14,600 21,900 25,300 27,900 30,400 36,100 35,900 123525003 Bitterroot River near Missoula, MT 14,700 22,700 26,400 28,900 31,400 36,900 36,600 1001 Bitterroot River at junction with Clark Fork River 15,200 23,500 27,300 29,800 32,400 38,000 37,800 1. Analyzed with USGS gage transfer method. 2. Analyzed with USGS two-site logarithmic interpolation Method. 3. Analyzed by USGS MOVE.3 extended record analysis. USGS: U.S. Geological Survey. cfs: cubic feet per second.

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Table 3-14 Recommended Peak Flows vs Existing Missoula County FIS

Estimated Peak Flow (cfs)

10% 2% 1% 0.2% Annual Annual Annual Annual Node/USGS Drainage Chance Chance Chance Chance Station ID Location Description County Peak Flood Frequency Method Area 10-year 50-year 100-year 500-year Bitterroot River-North Woodchuck Creek Missoula Recommended AEP Discharge 2,414 18,500 23,900 26,100 32,800 400 (Just upstream of confluence of Lolo Creek) Missoula Missoula County Existing FIS 2,450 19,200 27,200 29,100 38,500 Lower Lolo Creek (At 4.3 miles below Missoula Recommended AEP Discharge 2,742 21,900 27,900 30,400 36,100 300 confluence of Lolo Creek) Missoula Missoula County Existing FIS 2,720 20,400 29,000 31,000 41,000 Bitterroot River at junction with Clark Fork Missoula Recommended AEP Discharge 2,857 23,500 29,800 32,400 38,000 100 River (At confluence with Clark Fork) Missoula Missoula County Existing FIS 2,842 20,900 29,700 31,800 42,000 Location description in parenthesis is name of flow change node in the existing FIS. Source: Pioneer and Missoula County Flood Insurance Study (FEMA, 2019a). USGS: U.S. Geological Survey. cfs: cubic feet per second. AEP: annual exceedance probability. FIS: Flood Insurance Studies.

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Figure 3-10 Updated Bitterroot River near Missoula, MT (12352500)

40000 10 Yr Flood1 50 Yr Flood1 100 Yr Flood1

35000

30000

25000

20000

Peak Flow Flow (cfs) Peak 15000

10000

5000

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Figure 3-11 Updated Bitterroot River near Florence, MT (12351200)

30000 10 Yr Flood1 50 Yr Flood1 100 Yr Flood1

25000

20000

15000 Peak Flow Flow (cfs) Peak

10000

5000

0 1958 1959 1960 1961 1962 1963 1964 1965 1972 1974 1982 2003 2004 2005 2006 2006 2008 2009 2010 2011 Year 1. Flood Frequency Based on Data through 2018

Page 83 Blackfoot River

W BROADWAY 200 Grant Creek Missoula ¤£ Bonner Deep Creek 100 (32400 CFS)

OBrien Creek Grave Creek BROOKS ST Clark Fork River ¨¦§90

East Fork Grave Creek RESERVE ST

Deer Creek 12352500 (31400 CFS)

Bear Creek Hayes Creek

Moose Can Gully

Grave Creek

Sleeman Creek Woodman Creek 300 (30400 CFS) HWY 10 E

Donovan Creek

Miller Creek Allen Creek

HW Y 12 Greenough Creek Lolo Creek Lolo West Fork Butte Creek Bitterroot River Unnamed trib to Greenough Cr 3.3 ¤£93

400 (26100 CFS) Davis Creek

Mocassin Creek

Mormon Creek

South Fork Lolo Creek

McClain Creek MISSOULA COUNTY Squaw Creek

Swartz Creek

Carlton Creek

One Horse Creek Woodchuck Creek Gilbert Creek

RAVALLI COUNTY Eightmile Creek

Florence GRANITE COUNTY DE H One HorseE A CreekSTSI WY

LEGEND DISPLAYED AS: FLOW NODE MISSOULA-GRANITE STUDY REACHES NHS NON-INTERSTATE FLOW CHANGE NODE PROJECTION/ZONE: MONTANA STATE PLANE FIGURE 3-12 BITTERROOT RIVER DATUM: NAD1983 TOWNS COUNTY BOUNDARIES SECONDARY 100 UNITS: INT'L FEET RECOMMENDED 1-PERCENT SOURCE: DNRC/MSL/PIONEER HUC 12 BOUNDARIES HUC 8 BOUNDARIES URBAN 12352500 ANNUAL DISCHARGE STREAMS 300 BITTERROOT MAINSTEM STUDY REACH NHS INTERSTATE N 0 1 2 4 400 DATE: 7/2/2020 Miles Path: Z:\Shared\Active Projects\DNRC\Missoula-Granite\GIS\Hydrology\MSAGNT-FIG-PLN-3-12-RD.mxd Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

4 ROCK CREEK AND TRIBUTARIES

4.1 Introduction 4.1.1 Basin Description

4.1.1.1 Rock Creek Mainstem The Rock Creek mainstem is a major tributary to the Clark Fork River and is part of the upper headwaters of the Columbia River basin, located west of the continental divide in western Montana. The river is formed by the confluence of West Fork and Middle Fork Rock Creek. The river tributaries originate in Lolo and Beaverhead-Deerlodge National Forests and the Anaconda- Pintler Wilderness. The watershed is formed by the Sapphire Mountains to the west, the Anaconda Range to the South, and the John Long Mountains to the east. Rock Creek mainstem begins southwest of Phillipsburg, Montana, and flows approximately 55 miles to its confluence with the Clark Fork River. The entire Rock Creek watershed encompasses approximately 889 square miles.

Basin elevations within the Rock Creek study reach range from 5,260 feet at its origin to approximately 3,520 feet at the confluence with the Clark Fork River. The overall basin elevations range from over 10,000 feet in the Anaconda Range to 3,520 feet at the confluence with the Clark Fork River. The terrain varies from a high alpine environment in its headwaters to narrow inter-mountain valleys. The hydrology of the basin is primarily snowmelt driven.

Land use in the study reach is primarily recreational, with multiple small guest ranches located along the study reach. Small irrigated farming and ranching operations also exist. The sole community on the study reach is Quigley, Montana, with a population of about 40.

4.1.1.2 Rock Creek Tributaries All 6 Rock Creek tributary study reaches report to Rock Creek, a major tributary to the Clark Fork River. They study watersheds for the Rock Creek tributaries encompass approximately 522 square miles. The tributaries and watersheds are formed by the Sapphire Mountains to the West, the Anaconda Range to the south, and the John Long Mountains to the east.

The Rock Creek tributary study reaches’ elevations range from approximately 5,600 feet in the John Long Mountains to approximately 3,885 feet near Quigley, Montana. The overall basin elevations range from over 10,000 feet in the Anaconda Range to 3,885 feet near Quigley, Montana. The terrain varies from a high alpine environment in its headwaters to narrow inter- mountain valleys. The hydrology of the basin is primarily snowmelt driven.

Land use in the study reach is primarily rural with small irrigated farming and ranching operations. No established communities exist along any Rock Creek tributary study reaches. Figure 4-1 shows the Rock Creek mainstem and tributary study reaches.

Page 85 Clinton «¬141

HWY 12

7 2

Y MISSOULA COUNTY A W

G I 12334510 H

Clark Fork River

90 Florence ¨¦§ Drummond

12334500 12333500 12334000 Ranch Creek

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R D Middle Fork Rock Creek RD DEER LODGE COUNTY O H Maukey Gulch A K L A K S S Ross Fork KAL RD KA N HO E HW L Y A 12332000 G Warm Springs

LEGEND DISPLAYED AS: ACTIVE USGS GAGE ROCK CREEK MAINSTEM STUDY REACH NHS INTERSTATE STREAMS PROJECTION/ZONE: MONTANA STATE PLANE FIGURE 4-1 ROCK CREEK MAINSTEM DATUM: NAD1983 INACTIVE USGS GAGE MISSOULA-GRANITE STUDY REACHES NHS NON-INTERSTATE UNITS: INT'L FEET AND TRIBUTARIES SOURCE: DNRC/MSL/PIONEER TOWNS COUNTY BOUNDARIES PRIMARY STUDY AREA SECONDARY HUC 12 BOUNDARIES HUC 8 BOUNDARIES N 0 2.5 5 10 URBAN DATE: 7/2/2020 Miles Path: Z:\Shared\Active Projects\DNRC\Missoula-Granite\GIS\Hydrology\MSAGNT-FIG-PLN-4-1-SA.mxd Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

4.1.2 Flood History The primary cause of flooding on Rock Creek and its tributaries is spring snowmelt. Reports that document the flooding mechanisms on Rock Creek are limited. There are historical records from two USGS stream gages near or in the study area that document flooding history. The gages are listed below:

1. Rock Creek near Clinton, MT (12334510) 2. Middle Fork Rock Creek near Phillipsburg, MT (12332000)

The USGS stream gage Rock Creek near Clinton, MT (12334510) has a 47-year period of record (1972-2018). The annual peak flow record for the Rock Creek gage is shown in Figure 4-2. The gage is in the Rock Creek mainstem study reach.

Peak flow recurrence intervals shown in Figure 4-2 are based on previously published flood frequency analysis through Water Year 2011 (SIR 2015-5019-C) (Sando, et al., 2015a).

Figure 4-2 shows that the peak flood of record for the Rock Creek near Clinton, MT (12334510) gage occurred in 1972 with a flow of 6,500 cfs, exceeding the 10% (10-year) AEP flow of 5,290 cfs. The second highest flood on record occurred in 2018 with a flow of 5,830 cfs, exceeding the 10-year flow. The third highest flood on record occurred in 2017 with a flow of 5,540 cfs, exceeding the 10-year flow. In the 47-year period of record at the Rock Creek near Clinton, MT gage, the 10% AEP flow has been equaled or exceeded 5 times. All the annual peak flows occurred in May and June. The peak flow of record occurred in June.

The USGS stream gage Middle Fork Rock Creek near Phillipsburg, MT (12332000) has an 81-year period of record (1938-2018). The annual peak flow record for the Middle Fork Rock Creek gage is shown in Figure 4-3. The gage is located upstream of the Rock Creek mainstem and the Middle Fork Rock Creek study reach.

Peak flow recurrence intervals shown in Figure 4-3 are based on previously published flood frequency analysis through Water Year 2015 (SIR 2018-5046) (Sando & McCarthy, 2018)

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Figure 4-2 Rock Creek near Clinton (12334510)

8000 10 Yr Flood1 50 Yr Flood1 100 Yr Flood1

7000

6000

5000

4000

Peak Flow Flow (cfs) Peak 3000

2000

1000

0 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016 2018 Year 1. Flood Frequency Based on Data through 2015

Figure 4-3 shows that the peak flood of record for the Middle Fork Rock Creek near Phillipsburg, MT (12332000) gage occurred in 1974 with a flow of 1,680 cfs, exceeding the 10% (10-year) AEP flow of 1,430 cfs. The second highest flood on record occurred in 2003 with a flow of 1,670 cfs, exceeding the 10-year flow. The third highest flood on record occurred in 1972 with a flow of 1,590 cfs, exceeding the 10-year flow. In the 81-year period of record at the Middle Fork Rock Creek gage, the 10% AEP flow has been equaled or exceeded 8 times. All the annual peak flows occurred in May and June. The peak flow of record occurred in June.

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Figure 4-3 Middle Fork Rock Creek near Phillipsburg (12332000)

10 Yr Flood1 50 Yr Flood1 100 Yr Flood1 2000

1800

1600

1400

1200

1000

Peak Flow Flow (cfs) Peak 800

600

400

200

0 1938 1941 1944 1947 1950 1953 1956 1959 1962 1965 1968 1971 1974 1977 1980 1983 1986 1989 1992 1995 1998 2001 2004 2007 2010 2013 2016

1. Flood Frequency Based on Data through 2011 Year

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4.2 Past Studies and Existing Flood Data Past studies on Rock Creek mainstem and tributary study reaches are limited within Missoula and Granite Counties. FEMA FIRMs and a FIS exist for Missoula County unincorporated areas and the City of Missoula. FEMA FIRMs and a FIS exist for Granite County unincorporated areas, the City of Drummond, and the City of Phillipsburg. Studies relevant to this hydrologic study are those that include peak flow frequency analysis. Table 4-1 shows the floodplain mapping information.

Table 4-1 Rock Creek Mainstem and Tributaries Floodplain Mapping Summary

Map Panel Summary Study Details # of # of FIRM Panel County Approx Detailed Total Community FIRM FBFM Effective FIS Date Stream (mi) (mi) (mi) Panels Panels Date Missoula Co. Unincorporated Missoula 66 24 07/06/2015 03/07/2019 Rock Creek 0 69.6 69.6 Areas, City of Missoula Rock Creek, Ranch Creek, Upper Willow Granite County Creek, Unincorporated Granite 52 4 04/19/2016 04/19/2016 Middle Fork 57.9 9.0 66.9 Areas, City of Rock Creek, Drummond West Fork Rock Creek, Maukey Gulch, Ross Fork Source: FEMA Map Service Center FIRM: Flood Insurance Rate Map. FIS: Flood Insurance Studies. FBFM: Flood Boundary and Floodway Map. mi: Miles measured along channel alignment

The FIS dates reflect recent revisions in both Missoula and Granite Counties in 2019 and 2016, respectively. None of the study reaches analyzed in this report were updated in these revisions.

The USGS WRIR 03-4308 (Parret & Johnson, 2004), SIR 2015-5019-C (Sando, et al., 2015b), and USGS SIR 2018-5046 (Sando & McCarthy, 2018) document the flood frequency analysis on several gages along the mainstem Clark Fork River. These studies and investigations are discussed in more detail in the following sections.

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4.2.1 City of Missoula and Missoula County Flood Insurance Study The City of Missoula and Missoula County FIS was initially issued August 16, 1988 and revised on March 7, 2019 (FEMA, 2019a). As part of the FIS, the Clark Fork mainstem was studied using detailed methods. The study area included the City of Missoula and the unincorporated areas of Missoula County.

Table 4-2 provides the City of Missoula and Missoula County FIS peak discharge summary for ungaged flow change locations and USGS gages on Rock Creek. Peak discharge values shown below were not updated in the 2019 revision.

Table 4-2 Missoula County FIS Summary of Discharges Peak Flow, Return Interval (years) (cfs) USGS USGS Station Drainage 10- 50- 100- 500- Station ID Name Flooding Source and Location Area (mi2) Year Year Year Year Rock Creek at Confluence with Clark NA NA 885 6,200 8,300 9,200 11,200 Fork Source: FEMA, 2019a. mi2: square miles. cfs: cubic feet per second. NA: not applicable.

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4.2.2 Granite County and Incorporated Areas Flood Insurance Study The Granite County FIS was initially issued January 5, 1982 and revised on April 19, 2016 (FEMA, 2016). As part of the FIS, Rock Creek mainstem and tributaries were studied using detailed and approximate methods. The study area included unincorporated areas of Granite County. The FIS identified several major flood events. Rock Creek near Clinton, MT (12334510) had a 0.2% (500-year) AEP event in 1927, although no peak flow value was reported. The FIS also documented 4 smaller floods around the 10% (10-year) AEP event. The first occurred in June 1972 with a peak flow of 6,500 cfs and a stage of 8.52 feet. The second occurred in June 1975 with a peak flow of 5,520 cfs and a stage of 7.49 feet. The third occurred in May 1981 with a peak flow of 5,140 cfs and a stage of 7.53 feet. The fourth occurred in June 1997 with a peak flow of 5,530 cfs and a stage of 8.1 feet. Flood stage on USGS gage Rock Creek near Clinton, MT (12334510) is 8.0 feet.

Table 4-3 provides the Granite County FIS peak discharge summary for flow change locations on Rock Creek mainstem. Peak discharge values shown below were not updated in the 2016 revision.

Table 4-3 Granite County FIS Summary of Peak Flows

USGS Peak Flow, Return Interval (years) Drainage Area USGS Station Flooding Source and (cfs) (mi2) Station ID Name Location 10-Year 50-Year 100-Year 500-Year Rock Creek above Butte NA NA 700 5,200 6,900 7,700 9,300 Cabin Creek Rock Creek above Ranch NA NA 753 5,500 7,400 8,300 10,100 Creek NA NA Rock Creek at mouth 885 6,200 8,300 9,200 11,200

Source: FEMA, 2016. USGS: U.S. Geological Survey. mi2: square miles. cfs: cubic feet per second. NA: not applicable.

4.2.3 Water Resources Investigations Report 03-4308 The USGS WRIR 03-4308 developed annual peak discharges with recurrence intervals of 2, 5, 10, 25, 50, 100, 200, and 500 years (T-year floods) for 660 gaged sites in Montana and in adjacent areas of Idaho, Wyoming, and Canada, based on data through Water Year 1998 (Parret & Johnson, 2004). The flood-frequency information was used in regression analyses to develop equations relating T-year floods to various basin and climatic characteristics, active-channel width, and bankfull width. The equations can be used to estimate flood frequency at ungaged sites. Flood-frequency data typically were determined by fitting a log-Pearson Type III probability distribution using methods described by the IACWD, Bulletin 17B (IACWD, 1982).

Table 4-4 provides the WRIR 03-4308 peak discharge summary for 2 USGS gages in the Rock Creek mainstem and tributaries study area.

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Table 4-4 WRIR 03-4308 Rock Creek Mainstem and Tributaries Peak Flow Summary Peak Flow (cfs), Return Interval (years) USGS Drainage Years (cfs) Station Area of 5- 10- 25- 50- 100- 200- 500- Number USGS Station Name (mi2) Record year year year year year year year Rock Creek near 12334510 885 27 4,480 5,250 6,170 6,800 7,410 7,990 8,720 Clinton Middle Fork Rock 12332000 Creek near 123 61 1,240 1,430 1,640 1,790 1,920 2,040 2,190 Phillipsburg Based on systematic data through 1998. USGS: U.S. Geological Survey. mi2: square mile. cfs: cubic feet per second.

4.2.4 Scientific Investigations Report 2015-5019 The USGS SIR 2015-5019-C updated annual peak discharges with AEPs of 66.7, 50, 42.9, 20, 10, 4, 2, 1, 0.5, and 0.2% (return intervals of 1.5, 2, 2.33, 5, 10, 25, 50, 100, 200, and 500 years, respectively) for 725 gaged sites in or near Montana, based on data through Water Year 2011 (Sando, et al., 2015a). Flood-frequency data typically were determined by fitting a log-Pearson Type III probability distribution using methods described by the IACWD, Bulletin #17B (IACWD, 1982). The study was part of a larger study to develop an online StreamStats application for Montana, in conjunction with computing streamflow characteristics at gage stations, and estimate peak flow flood frequency at ungaged sites. Table 4-5 provides the SIR 2015-5019-C discharge summary for the USGS gages in the Rock Creek mainstem and tributaries study area.

The USGS SIR 2015-5019-F (Sando, et al., 2015b) selected 537 gaging stations from the gage study. The 537 gaging stations were segregated based on the following criteria: contributing drainage area less than about 2,750 square miles, peak-flow records unaffected by major regulation, small redundancy with nearby stations, and representation of peak-flow frequencies at sites within Montana. Both gages in the Rock Creek mainstem and tributaries study area were used in this analysis. The study used regression analyses to develop equations relating AEP flows to various basin and climatic characteristics. The relationships developed for this study resulted in lower mean standard errors of prediction than previous regression analyses.

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Table 4-5 SIR 2015-5019-C Rock Creek Mainstem and Tributaries Peak Flow Summary

Peak Flow (cfs) , Return Interval (years) USGS Drainage Years (cfs) Station Area of 5- 10- 25- 50- 100- 200- 500- Number USGS Station Name (mi2) Record year year year year year year year Rock Creek near 12334510 885 74 1,230 1,430 1,660 1,810 1,950 2,080 2,240 Clinton Middle Fork Rock 12332000 Creek near 123 40 4,550 5,460 6,540 7,270 7,960 8,620 9,430 Phillipsburg Based on systematic data through 2011. USGS: U.S. Geological Survey. mi2: square mile. cfs: cubic feet per second.

4.2.5 Scientific Investigations Report 2018-5046 The USGS SIR 2018-5046 included updated AEP peak discharges with AEPs of 50, 42.9, 20, 10, 4, 2, 1, 0.5, and 0.2% (return intervals of 2, 2.33, 5, 10, 25, 50, 100, 200, and 500 years, respectively) for 99 gages sites in or near Montana, based on Water Year 2015 (Sando & McCarthy, 2018). Flood-frequency data typically were determined by fitting a log-Pearson Type III probability distribution using an Expected Moments Algorithm analysis and other methods described by England and others in Bulletin 17C (England, et al., 2018). The AEP peak discharges for the Rock Creek near Clinton, MT (12334510) gage were updated as part of the report. Table 4-6 provides the SIR 2018-5046 peak discharge summary for the gage.

Table 4-6 SIR 2018-5046 Rock Creek Mainstem Peak Flow Summary Peak Flow (cfs), Return Interval (years) USGS Drainage Years (cfs) Station Area of 5- 10- 25- 50- 100- 200- 500- Number USGS Station Name (mi2) Record year year year year year year year Rock Creek near 12334510 889 44 4,470 5,290 6,260 6,930 7,560 8,160 8,910 Clinton, MT Based on systematic data through 2015. USGS: U.S. Geological Survey. mi2: square mile. cfs: cubic feet per second.

4.2.6 Additional Previous Studies Additional related previous studies conducted in the Rock Creek mainstem and tributaries study area involve mining impacts, sediment management, fisheries management, and watershed management. Specific studies for Rock Creek, Middle Fork Rock Creek, Ross Fork, Upper Willow Creek, and West Fork Rock Creek were conducted and are listed below.

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Rock Creek

• Marvin, R., Metesh, J., Lonn, J., Madison, J., & Wintergerst, R., 1995. Abandoned-Inactive Mines Program Deerlodge National Forest Volume III: Flint and Rock Creek Drainages. Montana Bureau of Mines and Geology (Marvin, et al., 1995). • State Engineer, 1959. Water Resources Survey Part I: History of Land and Water Use on Irrigated Areas and Part II: Maps Showing Irrigated Areas in Colors Designating the Sources of Supply. Helena: State Engineer's Office (State Engineer, 1959). • Trout Unlimited, 2018. Rock Creek Waterhsed Resoration Plan (Trout Unlimited, 2018).

Middle Fork Rock Creek & Ross Fork

• Lindstrom, J., Liermann, B., & Kreiner, R., 2018. An Assessment of Fish Populations and Riparian Habitat in Tributaries of the Upper Clark Fork River Basin. Montana Fish, Wildlife and Parks (Lindstrom, et al., 2018).

West Fork Rock Creek

• U.S. Forest Service Pintlar Ranger District, 2007. West Fork Rock Creek Watershed Assessment. United States Department of Agriculture (USDA). (U.S. Forest Service Pintlar Ranger District, 2007).

Upper Willow Creek

• Sanctuary, M., 2002. Pre-Restoration Characteristics of upper Willow Creek, Granite County, Montana. University of Montana (Sanctuary, 2002).

4.2.7 Flood Protection Measures The USACE maintains the NID, a database that keeps record of all the dams across the country. The NID identifies hazard potential, dam height, ownership, purpose, dam type, and administrative data. One dam exists upstream of a Rock Creek tributary study reach (USACE, 2019).

The East Fork Rock Creek Dam is a high hazard dam designed primarily for hydroelectric use located on the East Fork Rock Creek, a tributary to the Middle Fork Rock Creek study extents.

There are no major flood control structures on Rock Creek, Ranch Creek, Upper Willow Creek, Middle Fork Rock Creek, West Fork Rock Creek, Ross Fork, or Maukey Gulch. No minor flood control structures exist on any of the study reaches (USACE, 2019).

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4.3 Hydrologic Analysis and Results The purpose of the hydrologic analyses conducted as part of this project was to develop peak flow discharge estimates for the 50, 10, 4, 2, 1, and 0.2% AEP and 1%+ AEP events at key flow change locations (such as at significant tributaries confluences, stream gages, and population centers) along the study reach. The analysis was split into 2 parts: Rock Creek mainstem and Rock Creek tributaries hydrologic analysis, organized into 2 sub-sections:

1. USGS Stream Gage Analysis. 2. Ungaged Flow Node Analysis.

4.3.1 Rock Creek Mainstem Throughout the study area, 12 flow nodes on the Rock Creek mainstem study reach were identified as having significant changes in streamflow or being at critical locations. Of the 12 flow nodes on the study reach, 1 is located at an active USGS stream gage and 1 is located at in inactive USGS stream gage. The study extents in this report were based on the Missoula-Granite County Modernization Study Area Map provided by the DNRC. The river stations used in this report were based on the Rock Creek mainstem S_Wtr_Ln, delineated by Pioneer using 2019 LiDAR and channel bathymetry. The Rock Creek mainstem S_Wtr_Ln alignment begins at the junction with the Clark Fork River. The upstream extent of the study reach ends at the headwaters of Rock Creek, at the junction of Middle Fork Rock Creek and West Fork Rock Creek.

4.3.1.1 USGS Stream Gage Analysis The USGS performed peak flow analysis for selected stream gages in Deer Lodge, Powell, Granite, Missoula, Ravalli, and Mineral counties using gage data through 2018 as part of this hydrologic analysis (Sando, et al., 2020). Seven gages were analyzed by USGS within Missoula and Granite Counties. One USGS gage falls within the Rock Creek mainstem study reach and 1 USGS gage is located upstream of the study reach. The USGS gage flood frequency analysis summary, data, and calculations are provided in Appendix A. Bulletin 17C methods were used to estimate the 50, 42.9, 20, 10, 4, 2, 1, 0.5, and 0.2% AEPs and the 1%+ AEP. The USGS methodologies used for the gage analyses presented in this report are documented in USGS SIR 2018-5046 (Sando & McCarthy, 2018). This longer period of record should produce more accurate peak discharge estimates than those based on a shorter period of record.

The oldest records used in this study date back to 1938 at USGS gaging station Middle Fork Rock Creek near Phillipsburg, MT (12332000). This gage remains active. The FEMA guidance document (FEMA, 2019) indicates that gage station records equal or exceeding 10 years in length are applicable to all types of studies. The gages used in this study meet this criterion. Figure 4-4 shows the study reach and the USGS gaging station locations along and near the study reach.

Page 96 Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

The USGS flood frequency analysis is provided in Appendix A. Table 4-7 summarizes the USGS stream gages analyzed by the USGS within the Rock Creek watershed. Figure 4-5 plots the systematic or at-site flood frequency results as a function of drainage area.

Page 97 Lolo Clinton «¬141

HWY 12

7 2

Y MISSOULA COUNTY A W

G 12334510 I H

Clark Fork River 90 Florence ¨¦§ Drummond

Ranch Creek

POWELL COUNTY

Stevensville

Rock Creek ¤£12

GRANITE COUNTY

Victor EASTSIDE HWY

Upper Willow Creek

¤£93 Pinesdale RAVALLI COUNTY LC RE E K RD AL SH AR M «¬1 Philipsburg

S FRONTAGE RD

Hamilton

W E West Fork Rock Creek S

T S

I D

R Middle Fork Rock Creek D RD DEER LODGE COUNTY O H A Maukey Gulch LK A K S S KAL Ross Fork RD KA N HO E HW L Y A 12332000 G Warm Springs

LEGEND DISPLAYED AS: ACTIVE USGS GAGE MISSOULA-GRANITE STUDY REACHES NHS NON-INTERSTATE USGS WATERSHED PROJECTION/ZONE: MONTANA STATE PLANE FIGURE 4-4 ROCK CREEK MAINSTEM DATUM: NAD1983 TOWNS COUNTY BOUNDARIES PRIMARY 12332000 UNITS: INT'L FEET USGS WATERSHEDS SOURCE: DNRC/MSL/PIONEER HUC 12 BOUNDARIES HUC 8 BOUNDARIES SECONDARY 12334510 URBAN N 0 2.5 5 10 ROCK CREEK MAINSTEM STUDY REACH NHS INTERSTATE DATE: 7/2/2020 STREAMS Miles Path: Z:\Shared\Active Projects\DNRC\Missoula-Granite\GIS\Hydrology\MSAGNT-FIG-PLN-4-4-UW.mxd Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

Table 4-7 Rock Creek Mainstem USGS Gage Summary Total Total Number Period of Regulation of Years of Record, USGS Station Status as Peak-Flow in Water Number Station Name of 2018 Records Years 2018 Status 12334510 Rock Creek near Clinton, MT U 1972-2018 47 Active 12332000 Middle Fork Rock Creek near Phillipsburg, MT U 1938-2018 81 Active Source: (Sando, et al., 2020). USGS: U.S. Geological Survey. U: unregulated.

4.3.1.2 1%+ Peak Flow Analysis The 1%+ AEP event was calculated by the USGS (Sando, et al., 2020) to provide a confidence range that the 1% flood frequency peak flow estimates are likely to fall within. FEMA guidance defines the 1%+ as “…a flood elevation derived by using discharges that are at the upper 84- percent confidence limit as calculated in the gage analysis for the 1-percent-annual-chance event for the Flood Risk Project. Methods for estimating synthetic statistics outlined in Bulletin 17C Appendix 7 are used to estimate the upper 84 percent confidence limit of the Log Pearson III frequency Curve at the 1-percent-annual-chance event” (FEMA, 2019). The USGS used the methods for estimating synthetic statistics outlined in Bulletin 17C, Appendix 7.

Page 99 Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

Figure 4-5 USGS Flood Frequency Estimates Systematic Record through 2018

12,000 1%+ 500-yr 12334510 100-yr Rock Creek Near Clinton 10,000 50-yr 25-yr 10-yr 2-yr 8,000

6,000

Peak Flow Flow (cfs) Peak

4,000 12332000 Middle Fork Rock Creek PhilipsburgNear

2,000

0 100 200 300 400 500 600 700 800 900 Drainage Area (mi2)

Figure 4-5 indicates peak flows increase with increasing drainage area as typically expected.

To address the non-congruent periods of record, USGS employed the MOVE.3 (Maintenance of Variance Extension, Type 3) method to extend the historical gage record used in this analysis. Flood frequency peak flow estimates using the extended record data set established a congruent period of record for USGS gage Rock Creek near Clinton, MT (12334510) and the Middle Fork Rock Creek near Phillipsburg, MT (12332000). Using the extended record data set will minimize the potential error associated with non-congruent periods of record. Per Bulletin 17C guidance, a minimum correlation of 0.8 is required when using extended record analysis. The weighted average correlation coefficient for the Rock Creek gages used in this study is 0.86 to 0.95 (Appendix A). Figure 4-6 plots the extended record peak flows with drainage area for the study reach gages. Table 4-8 shows the results for systematic and MOVE.3 flood frequency estimates for gages on Rock Creek mainstem. The Rock Creek mainstem flood frequency peak flow estimates are also listed in Table 4-8 along with the Rock Creek mainstem 1%+ flood frequency peak flow estimates.

Page 100 Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

Figure 4-6 USGS Flood Frequency Estimates Extended Record though 2018

12,000 1%+ 500-yr 12334510 100-yr Rock Creek near Clinton 50-yr 10,000 25-yr 10-yr 2-yr 8,000

6,000

Peak Flow Flow (cfs) Peak 0

4,000 200 iddle Fork Rock Creek 1233 M Philipsburg near

2,000

0 100 200 300 400 500 600 700 800 900 Drainage Area (mi2)

Figure 4-6 indicates the extended record data set peak flows increase with increasing drainage area as expected. Table 4-8 indicates the extended data record set produces slightly higher AEP peak flow estimates. Using the extended record data set will help minimize the potential error associated with the at-site, non-congruent periods of record. For these reasons, flood flow frequency estimates using the 2020 extended record data set were selected to represent the annual chance flood potential at the Rock Creek mainstem gaged locations.

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Table 4-8 Rock Creek Mainstem Gage Flood Frequency Estimates Estimated Peak Flow (cfs)

10% 4% 2% 1% 0.2% 1% + Peak 50% Annual Annual Annual Annual Annual Annual Annual Flood Water years of peak Chance Chance Chance Chance Chance Chance Chance Node/USGS Frequency flows used in the 2- 10- 25- 50- 100- 500- 100- Station ID Location Description Method analysis year year year year year year year + 10,100 Rock Creek near At-Site 1972-2018 3,130 5,520 6,580 7,320 8,010 9,460 12334510 Clinton, MT MOVE.3 1899-1908, 1930-2018 3,340 5,780 6,900 7,690 8,440 10,100 10,000 Middle Fork Rock At-Site 1938-2018 901 1,440 1,650 1,800 1,930 2,190 2,210 12332000* Creek near 2,370 Philipsburg, MT MOVE.3 1899-1908, 1930-2018 902 1,500 1,750 1,920 2,070 2,390 * Denotes USGS gage outside of study reach. Source: (Sando, et al., 2020). USGS: U.S. Geological Survey. cfs: cubic feet per second. At-site: Peak flow frequency analysis on recorded data. MOVE.3: Peak flow frequency analysis on combined recorded and synthesized data; synthesized data from Maintenance of Variance Extension Type III (MOVE.3) record extension procedure.

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4.3.1.1 Flow Change Node Locations Future flood studies will use hydraulic models that are composed of geometric data and streamflow data. To accurately model Rock Creek mainstem, the locations of major tributary confluences and other flow change locations must be identified. The hydrologic analysis results will be used as the streamflow data input at the tributary confluences within the hydraulic model.

As an accuracy check, the USGS gaging station watershed area calculated in ArcGIS was compared to the USGS NWIS (http://waterdata.usgs.goc/nwis) published gaging station area. Figure 4-4 shows the USGS gaging stations analyzed and the correlating GIS model-generated watershed area along the study area. Table 4-9 shows the results of this comparison. Based on the gaging station analyzed, the project GIS model calculates watershed areas that are within 1% of the USGS NWIS published areas (Appendix A).

This study used the nearest GNIS hydrographic feature name for the ungaged flow node names. In some cases, these features (typically tributary streams) flow into Rock Creek just downstream of the flow node. Nodes were also named for USGS gage station numbers if placed at gaged locations.

Table 4-10 lists the flow node and gage station information used in the analysis. A total of 12 flow nodes were identified throughout the study reach, including 2 gaged locations. Figure 4-4 shows the USGS gaging stations analyzed and the correlating watershed areas within the study area. Figure 4-7 maps the flow node locations and corresponding watershed areas from Table 4-10.

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Table 4-9 USGS and GIS Model Watershed Comparison USGS GIS Published Calculated Drainage Basin Relative Station Area1 Area2 Percent Accuracy of Number USGS Station Name (mi2) (mi2) Change Areas Middle Fork Rock Creek near 12332000* Phillipsburg, MT 121 121 0.06% 99.9% 12334510 Rock Creek near Clinton, MT 889 889 0.05% 99.9% 1. Source: (Sando, et al., 2020). 2. Basin areas (watershed areas) used for hydrological analysis. * Denotes USGS gage outside of study reach. USGS: U.S. Geological Survey. mi2: square miles.

Table 4-10 Flow Node and USGS Gage Station Information Used in this Hydrologic Analysis River Calculated Node/USGS Study Station1 Basin Area2 Station ID Location Description County Reach (mi) (mi2) 12334510 Rock Creek near Clinton, MT Missoula Rock Creek 0.3 888 200 Gilbert Creek Missoula Rock Creek 4.3 854 300 Brewster Creek Granite Rock Creek 9.8 811 12333500 Rock Creek above Quigley, MT Granite Rock Creek 12.4 756 500 Welcome Creek Granite Rock Creek 14.7 731 600 Rock Creek-Wahlquist Creek Granite Rock Creek 19.7 702 700 Rock Creek-Hutsinpilar Creek Granite Rock Creek 25.5 662 800 Hogback Creek Granite Rock Creek 31.3 619 900 Wyman Gulch Granite Rock Creek 33.3 601 1000 Rock Creek-Flat Gulch Granite Rock Creek 39.7 543 1100 Lower Upper Willow Creek Granite Rock Creek 43.8 431 1200 Mallard Creek Granite Rock Creek 49.3 390 Middle Fork Rock Creek near 12332000* Granite Rock Creek NA 121 Phillipsburg, MT 1. River miles start at the downstream extent of each study reach (mi: miles). 2. Source: Esri ArcGIS. * Denotes USGS gage outside of study reach. USGS: U.S. Geological Survey. mi: miles. mi2: square miles.

Page 104 Lolo Cottonwood Creek

Miller Creek Lolo Creek Clinton Tenmile Creek Douglas Creek 141 Cramer Creek «¬ HWY 12 Bear Creek

West Fork Butte Creek 1

Davis Creek 7 2 Mormon Creek Y

South Fork Lolo Creek MISSOULA COUNTY East Fork Lolo Creek McClain Creek W H

G I

12334510 H Swartz Creek

Gillispie Creek

Carlton Creek Dry Gulch

Antelope Creek Woodchuck Creek One Horse Creek Gilbert Creek 200

Squaw Creek Squaw Eightmile Creek Clark Fork River Tigh Creek 90 Florence Spring Creek ¨¦§ Drummond Tyler Creek

Sawmill Creek Brewster Creek Bert Creek Welcome Creek 300

Cinnabar Creek Harvey Creek Cow Creek 12333500 North Fork Lower Willow Creek Cinnamon Bear Creek Ranch Creek 500 Lower Willow Creek Ranch Creek POWELL COUNTY Wahlquist Creek Stevensville 600 Cottonwood Creek Big Creek Rock Creek ¤£12

Cougar Creek GRANITE COUNTY Burnt Fork Bitterroot River 700

Alder Creek

Victor EASTSIDE HWY Smart Creek Hogback Creek

Upper Willow Creek 800 900 Upper Willow Creek Boulder Creek

Fred Burr Creek Bitterroot River Marshall Creek ¤£93 Flint Creek Pinesdale RAVALLI COUNTY Wyman Gulch LC RE E K RD South Boulder Creek AL Williams Gulch SH 1000 AR 1100 M «¬1 Philipsburg

Blodgett Creek

S FRONTAGE RD

Stony Creek

Hamilton 1200

W Racetrack Creek E West Fork Rock Creek S North Fork Rock Creek

T S

I D

E R Middle Fork Rock Creek

Daly Creek D RD DEER LODGE COUNTY O Skalkaho Creek H A Maukey Gulch LK A K Trout CreekS Lost Creek Warm Springs Creek S KAL Ross Fork East Fork Rock Creek RD KA N Sleeping Child Creek HO E HW L Y A 12332000 G Lost Creek Lost Horse Creek Warm Springs Storm Lake Creek West Fork Rock Creek Ross Fork Lost Horse Creek Warm Springs Creek

LEGEND DISPLAYED AS: FLOW NODE ROCK CREEK MAINSTEM STUDY REACH NHS INTERSTATE STREAMS 12333500 900 PROJECTION/ZONE: MONTANA STATE PLANE FIGURE 4-7 ROCK CREEK MAINSTEM DATUM: NAD1983 ACTIVE USGS GAGE MISSOULA-GRANITE STUDY REACHES NHS NON-INTERSTATE FLOW CHANGE NODE 500 1000 UNITS: INT'L FEET FLOW NODE LOCATIONS SOURCE: DNRC/MSL/PIONEER TOWNS COUNTY BOUNDARIES PRIMARY 12334510 600 1100 SECONDARY 200 700 1200 HUC 12 BOUNDARIES HUC 8 BOUNDARIES N 0 2.5 5 10 URBAN 800 DATE: 7/2/2020 300 Miles Path: Z:\Shared\Active Projects\DNRC\Missoula-Granite\GIS\Hydrology\MSAGNT-FIG-PLN-4-7-FN.mxd Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

4.3.1.2 Flood Frequency Estimates at Ungaged Sites To calculate peak flood discharge estimates at the ungaged flow nodes, Pioneer considered methods described in USGS SIR 2015-5109-F (Sando, et al., 2015b). These methods included estimating flood frequency using regional flood-frequency relations (regression analysis) and estimating flood frequency on gaged streams by translating gaged data to ungaged locations (drainage-area ratio adjustment or logarithmic interpolation between 2 gaged sites).

All the ungaged flow nodes on the study reach are located between 2 USGS gaging stations, Rock Creek near Clinton, MT (12334510) and the Middle Fork Rock Creek near Phillipsburg, MT (12332000). These 2 gages have 47 years of overlapping records, which were extended to a 99-year congruent period of record using MOVE.3 techniques. The gages share similar climatic conditions, land use, and meet the USGS guidance for two-site logarithmic interpolation, as described in SIR 2015-5019-F (Sando, et al., 2015b).

For these reasons, the two-site logarithmic interpolation method was used to estimate peak flows at ungaged flow nodes on the Rock Creek mainstem study reach.

The drainage area gage transfer method can also be used to calculate peak flow frequency estimates at ungaged flow nodes near a single gaging station. Since the two-site logarithmic interpolation method is generally considered the more accurate peak flow method, the drainage area gage transfer method was not used to calculate peak flow frequency estimates at ungaged flow nodes on the Rock Creek mainstem study reach.

4.3.1.3 Two-Site Logarithmic Interpolation Method Pioneer used the two-site logarithmic interpolation method listed in SIR 2015-5019-F (Sando, et al., 2015b) for analysis on ungaged nodes between 2 gaged sites. Nodes 200 through 1200 are between USGS gage station Rock Creek near Clinton, MT (12334510) and USGS gage station Middle Fork Rock Creek near Phillipsburg, MT (12332000). In this method, the logarithm of the flood-frequency discharge estimates at the ungaged site is linearly interpolated based on discharge estimates and drainage basin areas of the upstream and downstream gaged sites. This method is listed in the equation below from SIR 2015-5019-F:

log Q , log Q , log = log Q + (log DA log DA ) , , log DA log DA 𝐴𝐴𝐴𝐴𝐴𝐴 𝐺𝐺2 𝐴𝐴𝐴𝐴𝐴𝐴 𝐺𝐺1 𝐴𝐴𝐴𝐴𝐴𝐴 𝑈𝑈 𝐴𝐴𝐴𝐴𝐴𝐴 𝐺𝐺1 − 𝑈𝑈 𝐺𝐺1 Where𝑄𝑄 𝐺𝐺2 𝐺𝐺1 − log is the base 10 logarithm. −

QAEP,U is the AEP-percent peak flow at the ungaged site, in cfs.

QAEP,G1 is the AEP-percent peak flow at the upstream gaged site, in cfs.

QAEP,G2 is the AEP-percent peak flow at the downstream gaged site, in cfs.

DAG2 is the drainage area at the downstream gaged site, in square miles.

DAG1 is the drainage area at the upstream gaged site, in square miles.

DAU is the drainage area at the ungaged site, in square miles.

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Table 4-11 shows the calculation results using extended record peak flow estimates. Figure 4-8 plots the relationship between the calculated peak flow estimates and correlating drainage area. Results indicate estimated flow at the ungaged flow nodes increase with increasing drainage area.

Page 107 Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

Table 4-11 Rock Creek Mainstem Two-Site Logarithmic Interpolation Flow Node Peak Flow Estimates Drainage Area Gage Transfer Estimated Discharge (cfs) Estimated Discharge (cfs) 50% 1% + Annual 10% Annual 4% Annual 2% Annual 1% Annual 0.2% Annual Annual Node/USGS Chance Chance Chance Chance Chance Chance Chance Station ID Location Description 2-year 10-year 25-year 50-year 100-year 500-year 100-year Middle Fork Rock Creek near 902 1,500 1,750 1,920 2,070 2,390 2,370 123320001* Philipsburg, MT 1200 Mallard Creek 1,940 3,310 3,920 4,340 4,720 5,570 5,520 1100 Lower Upper Willow Creek 2,080 3,540 4,190 4,650 5,070 5,990 5,930 1000 Rock Creek-Flat Gulch 2,420 4,140 4,920 5,460 5,960 7,070 7,010 900 Wyman Gulch 2,580 4,430 5,270 5,860 6,400 7,610 7,540 800 Hogback Creek 2,640 4,530 5,380 5,980 6,540 7,780 7,710 700 Rock Creek-Hutsinpilar Creek 2,750 4,740 5,640 6,270 6,860 8,170 8,090 600 Rock Creek-Wahlquist Creek 2,860 4,930 5,870 6,530 7,150 8,520 8,440 500 Welcome Creek 2,940 5,060 6,030 6,710 7,350 8,770 8,680 12333500 Rock Creek near Quigley, MT 3,000 5,180 6,170 6,870 7,530 8,990 8,900 300 Brewster Creek 3,150 5,440 6,480 7,220 7,920 9,460 9,370 200 Gilbert Creek 3,250 5,630 6,710 7,480 8,210 9,810 9,720 123345101 Rock Creek near Clinton, MT 3,340 5,780 6,900 7,690 8,440 10,100 10,000 1. Peak flow estimated from extended record analysis. * Denotes USGS gage outside of study reach. USGS: U.S. Geological Survey. cfs: cubic feet per second.

Page 108 Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

Figure 4-8 Rock Creek Mainstem Two-Site Logarithmic Interpolation Flow Node Results

Rock Creek Near Near Creek Rock Node 200 Node

12,000 300 Node 1%+

1233

500-yr Qu

100-yr 500 Node ley

50-yr Node 10,000 Node 700

Node 800

25-yr Node 900 10-yr 2-yr Node 8,000

6,000 Peak Flow Flow (cfs) Peak MF near Philipsburg near MF 12332000 4,000 Near Clinton Near Creek Rock 12334510

2,000 500 Node

0 100 200 300 400 500 600 700 800 900 Drainage Area (mi2)

Page 109 Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

4.3.2 Rock Creek Mainstem Discussion Peak flow frequency analysis was performed on the Rock Creek mainstem study reach. The Rock Creek mainstem study reach extends 53 miles upstream from the junction with the Clark Fork River. Information gathered from this analysis will be used to support the Missoula-Granite County hydraulic analyses and floodplain mapping studies.

Previous flood studies on Rock Creek mainstem are limited. The most relevant earlier flood study was the Granite County FIS (FEMA, 2016). Another important study was the USGS SIR 2015- 5019-F, published in 2015 (Sando, et al., 2015b), that included flood frequency analysis for USGS gage Rock Creek near Clinton, MT (12334510).

This hydrologic analysis included performing flood frequency estimates for both gaged and ungaged sites. Flood frequency estimates at the gaged sites with more than 10 years of record were conducted by USGS (Sando, et al., 2020) using Bulletin 17C methodologies (England, et al., 2018). Both systematic analysis and extended record analysis, using MOVE.3 methods, were performed. Peak flow 1%+ estimates were developed for all gaged locations using standard FEMA methodologies. The flood flow frequency estimates based on the gage extended-record data set were determined to provide the most accurate peak flow estimates for the Rock Creek basin, due to the long congruent period of record, which minimized errors associated with non- congruent periods of gage records. Per Bulletin 17C guidance, a minimum correlation of 0.8 is required when using extended record analysis. The weighted average correlation coefficient for the Rock Creek gages used in this study is 0.86 to 0.95 (Appendix A).

Pioneer used the two-station logarithmic interpolation method based on the USGS gage flood frequency analysis to calculate flood frequency at ungaged locations. Peak flow estimates were calculated at 12 locations (flow nodes) within the watershed (2 gage sites and 10 ungaged sites). The ungaged sites were located at HUC 12 boundaries and at the end of study reaches.

Higher confidence is typically associated with flood frequency estimates that are based on measured stream flows, such as in the gage transfer and two-site logarithmic interpolation method. For these reasons, the two-site logarithmic interpolation method was used to estimate peak flows at ungaged flow nodes on the Rock Creek mainstem study reach.

Coincident peaks with the Clark Fork River was investigated. The drainage area at node 12334510 on Rock Creek mainstem (888 square miles) was compared with the drainage area of

Page 110 Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

node 1700 on the Clark Fork River (2,682 square miles). Per FEMA guidance, the consideration of coincident peaks may be appropriate if the ratio of drainage areas lies between 0.6 and 1.4 (FEMA, 2016c). The ratios for drainage areas between these two reaches (0.33) falls outside of the FEMA guidance range, therefore coincident peaks were not further considered.

Table 4-12 summarizes the recommended flood frequency peak flows for the Rock Creek mainstem study reach. Table 4-13 compares the recommended discharges in this study with the existing effective mapping discharges in Missoula and Granite Counties at comparable drainage areas. The comparison shows the recommended AEP peak flows are generally less than the existing mapping values. Figure 4-9 plots the recorded peak flow data and the recommended AEP flow estimates for the Rock Creek mainstem gage updated in this analysis.

Figure 4-10 shows the recommended 1% AEP discharge for each flow node location. The hydrologic analysis results provided in Table 4-13 represent the recommended discharges at each flow node location throughout the study reach. The methods used for hydrological analysis are industry accepted methods [Bulletin 17C (England, et al., 2018), SIR 2015-5019-C (Sando, et al., 2015a), and SIR 2015-5019-F (Sando, et al., 2015b)] based on the Rock Creek mainstem basin characteristics. This hydrologic analysis conforms to FEMA standards for enhanced level studies, and the recommended flows of this analysis are deemed reliable and suitable for future floodplain studies and hydraulic analyses.

Page 111 Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

Table 4-12 Recommended AEP Peak Flow Estimates

Page 112 Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

Table 4-13 Recommended Rock Creek Mainstem AEP Peak Flows vs Missoula and Granite Existing FIS Reports

Estimated Peak Flow (cfs)

10% 2% 1% 0.2% Annual Annual Annual Annual Node/USGS Drainage Chance Chance Chance Chance Station ID Location Description County Peak Flood Frequency Method Area 10-year 50-year 100-year 500-year Rock Creek-Wahlquist Creek Granite Recommended AEP Discharge 702 4,930 6,530 7,150 8,520 600 (Rock Creek above Butte Cabin Creek Granite Granite County Existing FIS 700 5,200 6,900 7,700 9,300 Rock Creek near Quigley, MT Granite Recommended AEP Discharge 756 5,180 6,870 7,530 8,990 12333500 (Rock Creek above Ranch Creek) Granite Granite County Existing FIS 753 5,500 7,400 8,300 10,100 Rock Creek near Clinton, MT Missoula Recommended AEP Discharge 888 5,780 7,690 8,440 10,100 12334510 (Rock Creek at Mouth) Missoula Missoula and Granite County Existing FIS 885 6,200 8,300 9,200 11,200 Location description in parenthesis is name of flow change node in the existing FIS. Source: Pioneer, Granite County Flood Insurance Study (FEMA, 2016), Missoula County Flood Insurance Study (FEMA, 2019a). USGS: U.S. Geological Survey. cfs: cubic feet per second.

Page 113 Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

Figure 4-9 Updated Rock Creek near Clinton (12334510)

9000 10 Yr Flood1 50 Yr Flood1 100 Yr Flood1

8000

7000

6000

5000

4000 Peak Flow Flow (cfs) Peak

3000

2000

1000

0 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016 2018 Year 1. Flood Frequency Based on Data through 2018

Page 114 Lolo Cottonwood Creek

Miller Creek Lolo Creek Clinton Tenmile Creek Douglas Creek 141 Cramer Creek «¬ HWY 12 Bear Creek

West Fork Butte Creek 1

Davis Creek 7 2 Mormon Creek Swartz Creek Y A

South Fork Lolo Creek MISSOULA COUNTY East Fork Lolo Creek McClain Creek W H

G 12334510 (8440 CFS) I H

Gillispie Creek

Carlton Creek Dry Gulch

Antelope Creek Woodchuck Creek One Horse Creek Gilbert Creek 200 (8210 CFS)

Squaw Creek Squaw Eightmile Creek Clark Fork River Tigh Creek 90 Florence Spring Creek ¨¦§ Drummond Tyler Creek

Sawmill Creek

Brewster Creek Bert Creek

Welcome Creek 300 (7920 CFS)

Cinnabar Creek Harvey Creek Cow Creek 12333500 (7530 CFS) North Fork Lower Willow Creek Cinnamon Bear Creek Ranch Creek

500 (7350 CFS) Lower Willow Creek

Ranch Creek POWELL COUNTY Wahlquist Creek Stevensville 600 (7150 CFS) Cottonwood Creek Big Creek Rock Creek ¤£12

700 (6860 CFS) GRANITE COUNTY Burnt Fork Bitterroot River

Alder Creek

Victor EASTSIDE HWY Hogback Creek Smart Creek

800 (6540 CFS) Upper Willow Creek

900 (6400 CFS) Upper Willow Creek Boulder Creek

Fred Burr Creek Bitterroot River Marshall Creek ¤£93 Flint Creek Pinesdale RAVALLI COUNTY Wyman Gulch LC RE E K RD South Boulder Creek 1000 (5960 CFS) AL Williams Gulch SH AR M «¬1 Philipsburg 1100 (5070 CFS) Blodgett Creek

S FRONTAGE RD

Stony Creek

Hamilton

W 1200 (4720 CFS) Racetrack Creek E West Fork Rock Creek S North Fork Rock Creek

T S

I D

E R Middle Fork Rock Creek

Daly Creek D RD DEER LODGE COUNTY O Skalkaho Creek H A Maukey Gulch LK A K Trout CreekS Lost Creek Warm Springs Creek S KAL Ross Fork East Fork Rock Creek RD KA N Sleeping Child Creek HO E HW L Y A G Lost Creek Lost Horse Creek Ross Fork Warm Springs Storm Lake Creek West Fork Rock Creek

Lost Horse Creek Warm Springs Creek

LEGEND DISPLAYED AS: FLOW NODE MISSOULA-GRANITE STUDY REACHES NHS NON-INTERSTATE FLOW CHANGE NODE 500 1000 PROJECTION/ZONE: MONTANA STATE PLANE FIGURE 4-10 ROCK CREEK MAINSTEM DATUM: NAD1983 TOWNS COUNTY BOUNDARIES PRIMARY 12334510 600 1100 UNITS: INT'L FEET RECOMMENDED 1-PERCENT SOURCE: DNRC/MSL/PIONEER HUC 12 BOUNDARIES HUC 8 BOUNDARIES SECONDARY 200 700 1200 ANNUAL DISCHARGE URBAN 300 800 ROCK CREEK MAINSTEM STUDY REACH NHS INTERSTATE N 0 2.5 5 10 12333500 900 DATE: 7/2/2020 STREAMS Miles Path: Z:\Shared\Active Projects\DNRC\Missoula-Granite\GIS\Hydrology\MSAGNT-FIG-PLN-4-10-RD.mxd Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

4.3.3 Rock Creek Tributaries The purpose of the hydrologic analyses conducted as part of this project was to develop peak flow discharge estimates for the 50, 10, 4, 2, 1, and 0.2% AEP and 1%+ AEP events at key flow change locations (such as at significant tributaries confluences, stream gages, and population centers) along the study reach. The analysis was organized into 2 sections:

1. USGS Stream Gage Analysis. 2. Ungaged Flow Node Analysis.

Throughout the study area, 9 flow change nodes (flow nodes) within the Rock Creek tributary study area were identified as having significant changes in streamflow or being at a critical location. All 9 flow change nodes are at ungaged locations. The study tributaries in this report were based on the Missoula-Granite Country Modernization Study Area Map provided by the DNRC. The river stations used in this report were based on the Rock Creek tributaries S_Wtr_Ln, delineated by Pioneer using 2019 LiDAR and channel bathymetry. The Rock Creek Tributaries S_Wtr_Ln alignments begin at the junctions of Ranch Creek, Willow Creek, Middle Fork Rock Creek, and West Fork Rock Creek with Rock Creek. Figure 4-1 shows the study reach and tributary locations.

4.3.3.1 USGS Stream Gage Analysis The USGS performed peak flow analysis for selected stream gages in Deer Lodge, Powell, Granite, Missoula, Ravalli, and Mineral counties, using data through 2018, as part of this hydrologic analysis (Sando, et al., 2020). This longer period of record should produce more accurate peak discharge estimates than those based on a shorter period of record. Seven gages were analyzed by USGS within Missoula and Granite Counties. One USGS gage is located just upstream of the Rock Creek tributaries study area station [Middle Fork Rock Creek near Phillipsburg, Montana (12332000)] and 1 gage is located downstream from the Rock Creek tributaries study area on the Rock Creek mainstem [Rock Creek near Clinton, MT (12334510)]. The USGS gage flood frequency analysis summary, data, and calculations are provided in Appendix A. Bulletin 17C methods were used to estimate the 50, 42.9, 20, 10, 4, 2, 1, 0.5, and 0.2% AEPs and the 1%+ AEP for gages with more than 10 years of record. This gage analysis is documented in the USGS data release (Sando, et al., 2020). The USGS methodologies used for the gage analyses in this report are documented in USGS SIR 2018-5046 (Sando & McCarthy, 2018).

The oldest records used in this study date back to 1938 at USGS gaging station Middle Fork Rock Creek near Phillipsburg, Montana (12332000). This gage remains active. The FEMA guidance document (FEMA, 2019) indicates that gage station records equal or exceeding 10 years in length are applicable to all types of studies. The gages used in this study meet this criterion.

As discussed previously, the AEP peak discharges were updated for 725 gaged sites in or near Montana, based on data through Water Year 2011 (Sando, et al., 2015a). Flood-frequency data were determined using methods described by the IACWD Bulletin 17B (IACWD, 1982). The study

Page 116 Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

was part of a larger study to develop an online StreamStats application for Montana. The USGS gages in and near the study reach were included in this analysis.

Table 4-14 summarizes the USGS stream gages analyzed by the USGS along the Rock Creek Tributaries study reach. Figure 4-4 and Figure 4-11 show the study reach and the USGS gaging station locations along and near the study reach.

Table 4-14 Rock Creek Tributaries USGS Gage Summary Total Total Number Period of USGS Regulation of Years of Record, Station Status as of Peak-Flow in Water Number USGS Station Name 2018 Records Years 2018 Status 12332000 Middle Fork Rock Creek near Phillipsburg, MT U 1938–2018 81 Active 12334510 Rock Creek near Clinton, MT U 1972-2018 47 Active Source: (Sando, et al., 2020). USGS: U.S. Geological Survey. U: unregulated.

To address the non-congruent periods of record, USGS employed the MOVE.3 (Maintenance of Variance Extension, Type 3) method to extend the historical gage record used in this analysis. Flood frequency peak flow estimates using the extended record data set established a congruent period of record for USGS gage Middle Fork Rock Creek near Philipsburg, MT (12332000) and the Rock Creek near Clinton, MT gage (12334510). Using the extended record data set will minimize the potential error associated with non-congruent periods of record. Per Bulletin 17C guidance, a minimum correlation of 0.8 is required when using extended record analysis. The weighted average Pearson correlation coefficient for the Middle Fork Rock Creek near Philipsburg, MT gage and the Rock Creek near Clinton, MT gage, MT used in this study is 0.86 and 0.95 respectively (Appendix A). Table 4-15 shows the results for systematic and MOVE.3 flood frequency estimates for gages located on Rock Creek tributaries study reaches.

4.3.3.2 1%+ Peak Flow Analysis The 1%+ AEP event was calculated by the USGS (Sando, et al., 2020) to provide a confidence range that the 1% flood frequency peak flow estimates are likely to fall within. FEMA guidance defines the 1%+ as “…a flood elevation derived by using discharges that are at the upper 84- percent confidence limit as calculated in the gage analysis for the 1-percent-annual-chance event for the Flood Risk Project. Methods for estimating synthetic statistics outlined in Bulletin 17C Appendix 7 are used to estimate the upper 84 percent confidence limit of the Log Pearson III frequency Curve at the 1-percent-annual-chance event” (FEMA, 2019). The USGS used the methods for estimating synthetic statistics outlined in Bulletin 17C, Appendix 7. Rock Creek Tributaries 1%+ flood frequency peak flow estimates are listed in Table 4-15.

Page 117 O RD SKALKAH West Fork Rock Creek Rock Creek

Middle Fork Rock Creek Maukey Gulch

Ross Fork

Y W H HO 12332000 SKALKA

«¬1

GRANITE COUNTY

RAVALLI COUNTY

DEER LODGE COUNTY

LEGEND DISPLAYED AS: ACTIVE USGS GAGE COUNTY BOUNDARIES STREAMS PROJECTION/ZONE: MONTANA STATE PLANE FIGURE 4-11 ROCK CREEK TRIBUTARIES DATUM: NAD1983 HUC 12 BOUNDARIES HUC 8 BOUNDARIES USGS WATERSHED UNITS: INT'L FEET USGS WATERSHED SOURCE: DNRC/MSL/PIONEER ROCK CREEK TRIBUTARIES STUDY REACH PRIMARY 12332000 SECONDARY N 0 1.25 2.5 5 MISSOULA_GRANITE STUDY REACHES DATE: 7/2/2020 Miles Path: Z:\Shared\Active Projects\DNRC\Missoula-Granite\GIS\Hydrology\MSAGNT-FIG-PLN-4-11-UW.mxd Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

Table 4-15 Rock Creek Tributaries Gage Flood Frequency Peak Flow Estimates Estimated Peak Flow (cfs)

4% 2% 1% 0.2% 1% + Peak Water years of 50% Annual 10% Annual Annual Annual Annual Annual Annual Flood peak flows Chance Chance Chance Chance Chance Chance Chance Node/USGS Location Frequency used in the 1%+ Station ID Description Method analysis 2-year 10-year 25-year 50-year 100-year 500-year Middle Fork At-Site 1938-2018 901 1,440 1,650 1,800 1,930 2,190 2,210 12332000 Rock Creek near 1899-1908 MOVE.3 Phillipsburg, MT 1930-2018 902 1,500 1,750 1,920 2,070 2,390 2,370 At-Site 1972-2018 Rock Creek near 3,130 5,520 6,580 7,320 8,010 9,460 10,100 1234510 1899-1908 Clinton, MT MOVE.3 1930-2018 3,340 5,780 6,900 7,690 8,440 10,100 10,000 Source: (Sando, et al., 2020). USGS: U.S. Geological Survey cfs: cubic feet per second. At-site: Peak flow frequency analysis on recorded data. MOVE.3: Peak flow frequency analysis on combined recorded and synthesized data; synthesized data from Maintenance of Variance Extension Type III (MOVE.3) record extension procedure.

Page 119 Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

4.3.3.3 Flow Change Node Locations Future flood studies will use hydraulic models that are composed of geometric data and streamflow data. To accurately model Rock Creek tributaries, the locations of major tributary confluences and other flow change locations must be identified. The hydrologic analysis results will be used as the streamflow data input at the tributary confluences within the hydraulic model.

A detailed review of the study area was performed to identify all potential flow change locations (flow nodes). The end of each tributary study reach was initially used to locate flow nodes. The HUC 12-digit watershed boundaries were used to locate the flow nodes on the Upper Willow Creek study reach. The HUC 12-digit watershed boundaries represent the smallest USGS- delineated watershed areas available in GIS format. Using ArcGIS, flow nodes were located just upstream of the HUC 12 boundary intersection with Upper Willow Creek. Using ArcGIS, 4 flow nodes (Ranch Creek, Upper Willow Creek, Middle Fork Rock Creek, and West Fork Rock Creek) were located just upstream of each tributary’s confluence with Rock Creek mainstem. Two additional flow nodes were added on the Upper Willow Creek study reach. One node was added on the West Fork Rock Creek study reach. One node was located at the Ross Fork confluence with West Fork Rock Creek. One node was located at the Maukey Gulch confluence with West Fork Rock Creek. In total, there were 9 flow nodes.

At each flow node, a drainage basin area was delineated, and streamflow values were calculated for the various recurrence interval floods. Two methods were used to generate flow node drainage basins. The primary method, used for most flow nodes, involved calculating drainage basins with ArcGIS, an ESRI software that produces drainage basin areas. Complex, low-gradient topographic conditions in the lower study reach required additional methods to calculate drainage basins for nodes 700 and 900. These drainage basin areas were calculated with StreamStats, a USGS software that produces peak flow data and drainage basin area.

As an accuracy check, the watershed areas for USGS gaging station Middle Fork Rock Creek near Phillipsburg, MT (12332000) and Rock Creek near Clinton (12334510) were calculated in ArcGIS and compared to the USGS NWIS (http://waterdata.usgs.goc/nwis) published gaging station areas. Table 4-16 shows the results of this comparison. Based on the gaging station analyzed, the project GIS model calculates watershed areas that are within 1% of the USGS NWIS published areas (Appendix A).

Page 120 Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

Table 4-16 USGS and GIS Model Watershed Comparison USGS GIS Published Calculated Relative Station Drainage Basin Percent Accuracy Number Station Name Area1 (mi2) Area2 (mi2) Change of Areas 12332000 Middle Fork Rock Creek near Phillipsburg, MT 121 121 0.06% 99.9% 12334510 Rock Creek near Clinton, MT 889 889 0.05% 99.9% 1. Source: (Sando, et al., 2020). 2. Basin areas (watershed areas) used for hydrological analysis. USGS: U.S. Geological Survey. GIS: Geographical Interface System. mi2: square miles.

This study used the nearest GNIS hydrographic feature name for the ungaged flow node names. In some cases, these features (typically tributary streams) flow into Rock Creek just downstream of the flow node. Due to their drainage area size, some ungaged flow nodes do not have a GNIS hydrographic feature name. Therefore, a location description was developed for these ungaged flow nodes.

Table 4-17 lists the flow node and gage station Information used in the analysis. A total of 9, ungaged flow nodes were identified throughout the study reach. Figure 4-12 and Figure 4-13 map the flow node locations and corresponding watershed areas.

Page 121 Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

Table 4-17 Flow Node and USGS Gage Station Information Used in this Analysis Tributary River Calculated Node/USGS Station1 Basin Area2 Station ID Location Description County Study Reach (mi) (mi2) Middle Fork 1234510 Rock Creek near Clinton, MT Granite NA 888 Rock Creek 100 Ranch Creek at junction with Rock Creek Granite Ranch Creek 0.4 43 Upper Willow Creek at junction with Rock Upper Willow 200 Granite 0.0 95 Creek Creek Upper Willow 300 Middle Upper Willow Creek Granite 5.6 76 Creek Upper Willow 400 Upper Upper Willow Creek Granite 11.8 46 Creek Middle Fork Rock Creek at junction with Middle Fork 500 Granite 0.1 203 Rock Creek Rock Creek Middle Fork Rock Creek near Middle Fork 12332000* Granite NA 121 Phillipsburg, MT Rock Creek West Fork Rock Creek at junction with West Fork 600 Granite 0.0 178 Rock Creek Rock Creek West Fork 7003 Lower West Fork Rock Creek Granite 0.0 93 Rock Creek Ross Fork at junction with West Fork Rock 800 Granite Ross Fork 0.0 85 Creek Maukey Gulch at junction with West Fork 9003 Granite Maukey Gulch 0.1 3 Rock Creek 1. River miles start at the downstream extent of each study reach (mi: miles). 2. Source: Esri ArcGIS and USGS StreamStats. 3. Watershed area generated with USGS StreamStats. *Denotes USGS gage outside of study reach, not used as a flow node in this analysis. USGS: U.S. Geological Survey. mi2: square miles.

Page 122 Brewster Creek

Tyler Creek

Welcome Creek Brewster Creek Cow Creek

Cinnabar Creek 100 Harvey Creek Ranch Creek North Fork Lower Willow Creek FARM TO MARKET RD W

Lower Willow Creek Cinnamon Bear Creek

Ranch Creek

Wahlquist Creek Butte Cabin Creek

Upper Willow Creek Cottonwood Creek

Rock Creek RAVALLI COUNTY South Fork Lower Willow Creek

Cougar Creek

Alder Creek

GRANITE COUNTY Boulder Creek Smart Creek «¬1 Hogback Creek Burnt Fork Bitterroot River 400

Flint Creek

Upper Willow Creek

Marshall Creek

Wyman Gulch South Boulder Creek MA Williams Gulch RSHA 300 LL CR E EK RD

200

Stony Creek Philipsburg

Rock Creek

LEGEND DISPLAYED AS: FLOW NODE MISSOULA_GRANITE STUDY REACHES SECONDARY 300 PROJECTION/ZONE: MONTANA STATE PLANE FIGURE 4-12 ROCK CREEK TRIBUTARIES DATUM: NAD1983 TOWNS COUNTY BOUNDARIES STREAMS 400 UNITS: INT'L FEET FLOW NODE 100 to 400 SOURCE: DNRC/MSL/PIONEER HUC 12 BOUNDARIES HUC 8 BOUNDARIES FLOW CHANGE NODE 600 700 100 N 0 1.25 2.5 5 ROCK CREEK TRIBUTARIES STUDY REACH PRIMARY DATE: 7/9/2020 200 Miles Path: Z:\Shared\Active Projects\DNRC\Missoula-Granite\GIS\Hydrology\MSAGNT-FIG-PLN-4-12-FN.mxd West Fork Rock Creek Rock Creek West Fork Rock Creek

SKALK AHO RD

900 700 Maukey Gulch 800 600

500

Ross Fork

Middle Fork Rock Creek

Maukey Gulch

Middle Fork Rock Creek

Ross Fork

LEGEND DISPLAYED AS: FLOW NODE SECONDARY 700 PROJECTION/ZONE: MONTANA STATE PLANE FIGURE 4-13 ROCK CREEK TRIBUTARIES DATUM: NAD1983 HUC 12 BOUNDARIES STREAMS 800 UNITS: INT'L FEET FLOW NODE 500 to 900 SOURCE: DNRC/MSL/PIONEER ROCK CREEK TRIBUTARIES STUDY REACH FLOW CHANGE NODE 900 500 N 0 0.125 0.25 0.5 MISSOULA-GRANITE STUDY REACHES DATE: 7/9/2020 600 Miles Path: Z:\Shared\Active Projects\DNRC\Missoula-Granite\GIS\Hydrology\MSAGNT-FIG-PLN-4-13-FN.mxd Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

4.3.3.4 Flood Frequency Estimates at Ungaged Sites To calculate peak flood discharge estimates at the ungaged flow nodes, Pioneer considered methods described in USGS SIR 2015-5109-F (Sando, et al., 2015b). These methods included estimating flood frequency using regional flood-frequency relations (regression analysis) and estimating flood frequency on gaged streams by translating gaged data to ungaged locations (drainage-area ratio adjustment or logarithmic interpolation between 2 gaged sites).

4.3.3.5 Two Site Logarithmic Interpolation Pioneer used the two-site logarithmic interpolation method listed in SIR 2015-5019-F (Sando, et al., 2015b) for analysis on ungaged nodes between 2 gaged sites. Node 500 is between USGS gage station Rock Creek near Clinton, MT (12334510) and USGS gage station Middle Fork Rock Creek near Phillipsburg, MT (12332000). In this method, the logarithm of the flood-frequency discharge estimates at the ungaged site is linearly interpolated based on discharge estimates and drainage basin areas of the upstream and downstream gaged sites. This method is listed in the equation below from SIR 2015-5019-F:

QAEP,U is the AEP-percent peak flow at the ungaged site, in cfs.

QAEP,G1 is the AEP-percent peak flow at the upstream gaged site, in cfs.

QAEP,G2 is the AEP-percent peak flow at the downstream gaged site, in cfs.

DAG2 is the drainage area at the downstream gaged site, in square miles.

DAG1 is the drainage area at the upstream gaged site, in square miles.

DAU is the drainage area at the ungaged site, in square miles.

Table 4-18 shows the calculation results using extended record peak flow estimates. Figure 4-14 plots the relationship between the calculated peak flow estimates and correlating drainage area. Results indicate estimated flow at the ungaged flow nodes increase with increasing drainage area.

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Table 4-18 Two-Site Logarithmic Interpolation Method Results for Flow Node 500 Two-Site Logarithmic Interpolation Estimated Peak Flow (cfs) 50% 10% 4% 2% 1% 0.2% 1% + Annual Annual Annual Annual Annual Annual Annual Node/USGS Chance Chance Chance Chance Chance Chance Chance Station ID Location Description 2-year 10-year 25-year 50-year 100-year 500-year 100-year 12332000 Middle Fork Rock Creek near Philipsburg, MT 902 1,500 1,750 1,920 2,070 2,390 2,370 500 Middle Fork Rock Creek at junction with Rock Creek 1,270 2,130 2,500 2,750 2,980 3,480 3,450 12334510 Rock Creek near Clinton, MT 3,340 5,780 6,900 7,690 8,440 10,100 10,000 1. Peak flow estimated from extended record analysis. USGS: U.S. Geological Survey. cfs: cubic feet per second.

Page 126 Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

Figure 4-14 Middle Fork Rock Creek Two-Site Logarithmic Interpolation Flood Frequency Analysis

12,000 1%+ 500-yr 12334510 100-yr Rock Creek near Clinton 10,000 50-yr 25-yr 10-yr 2-yr 8,000

6,000 Node 500 Peak Flow Flow (cfs) Peak

4,000 Middle Fork Rock Creek 12332000 Near Philipsburg

2,000

0 100 200 300 400 500 600 700 800 900 Drainage Area (mi2)

Page 127 Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

4.3.3.6 Regional Regression Equations Method The hydrologic regions defined in SIR 2015-5019-F (Sando, et al., 2015b) indicate Rock Creek tributaries flow through the west region.

The East Fork Reservoir is located on the East Fork Rock Creek a tributary to Middle Fork Rock Creek which is in the Missoula-Granite study area. The most upstream flow node on the Middle Fork of Rock Creek is flow node 500, with a drainage area of 203 mi2. The East Fork Rock Creek Dam is classified as a high hazard dam (USACE, 2019) and has a drainage area of 30.6 mi2. USGS guidance states that a flow node is considered regulated by a dam if the drainage area of the upstream dam exceeds 20 percent of the drainage area of the given node (Sando, et al., 2018). The East Fork Rock Creek Reservoir drainage area equals approximately 15 percent of the Middle Fork flow node 500 contributing drainage area. By USGS definition, node 500 is not considered regulated by the dam.

The remaining Rock Creek tributaries flow nodes are not located downstream of dams and therefor all 9 flow nodes are considered unaffected by major regulation.

All 9 flow nodes have watershed areas that are within the watershed area limits as defined by SIR 2015-5019-F. Therefore, it was determined that regional regression equations were applicable to the Rock Creek tributaries study reaches at all 9 flow nodes and regional regression equations were used to develop peak flow estimates.

The regression equations, presented in SIR 2015-5019-F, use drainage area (A), percentage of basin with forest land cover (F), and mean annual precipitation (P) as shown below in the following set of equations (Sando, et al., 2015b):

= 0.131 . . ( + 1) . . . . = 2.44 0 920 2 24( + 1) −0 845 50 . . . 𝑄𝑄 = 6.61 𝐴𝐴0 853 𝑃𝑃1 71( +𝐹𝐹 1) −0 875 10 . . . 𝑄𝑄 = 12.2 𝐴𝐴0 831 𝑃𝑃1 53( 𝐹𝐹+ 1)−0 890 4 . . . 𝑄𝑄 = 21.5𝐴𝐴0 818𝑃𝑃1 42(𝐹𝐹 + 1)−0 896 2 . . . 𝑄𝑄 . = 63.5𝐴𝐴0 806𝑃𝑃1 32 𝐹𝐹( + 1−0) 904 1 𝑄𝑄 𝐴𝐴 0 783𝑃𝑃 1 12𝐹𝐹 −0 915 0 2 where𝑄𝑄 𝐴𝐴 𝑃𝑃 𝐹𝐹 Qx is the X AEP peak flow magnitude, in cfs. A is the contributing drainage area, in square miles. F is the percent of basin with forest land cover. P is the mean annual precipitation in inches.

Percentage of basin with forest land cover (F) and mean annual precipitation (P) were calculated using StreamStats, a USGS software that produces peak flow data and regression equation variables. Drainage area was calculated using ArcGIS and ESRI mapping software. Peak flows

Page 128 Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

were calculated by Pioneer using the above regression equations (calculations are in Appendix B).

The 1%+ AEP event for regional regression estimates was calculated using FEMA guidance methodologies. For regional regression analyses, the 1%+ flood elevation is defined as a flood elevation derived by using discharges that include the predictive error for the regression equation discharge calculation for the Flood Risk Project. This error is then added to the 1% annual chance discharge to calculate the new 1%+ discharge (FEMA, 2019).

The west hydrologic regional regression equation input variables are shown in Table 4-19. Regional regression flood frequency peak flow estimates are listed in Table 4-20. Figure 4-15 and Figure 4-16 plot the calculated peak discharges and correlating drainage areas. Results indicate increasing flow magnitude with increasing drainage area.

Page 129 Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

Table 4-19 Regional Regression Variables Node/USGS Basin Area Station ID Location Description (mi2)1 F (%)2 P (in)3 Maukey Gulch at junction with West Fork Rock 900 3 47.6 20.4 Creek 800 Ross Fork at junction with West Fork Rock Creek 85 80.2 28.9 700 Lower West Fork Ross Creek 93 87.0 29.4 600 West Fork Rock Creek at junction with Rock Creek 178 83.8 29.1 Middle Fork Rock Creek at junction with Rock 500 203 78.5 28.5 Creek 400 Upper Upper Willow Creek 46 89.4 27.1 300 Middle Upper Willow Creek 76 81.0 25.0 200 Willow Creek at junction with Rock Creek 95 67.8 23.9 100 Ranch Creek at junction with Rock Creek 43 91.4 31.6 1. Basin Area calculated by ArcGIS. 2. Percent Forest (F) calculated by StreamStats. 3. Annual Precipitation in inches (P) calculated by StreamStats. USGS: U.S. Geological Survey. mi2: square miles.

Page 130 Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

Table 4-20 Regional Regression Flood Frequency Peak Flow Estimates

Southwest Region Regression Estimated Discharge (cfs)1 50% 10% 4% 2% 1% 0.2% 1% + Annual Annual Annual Annual Annual Annual Annual Node/USGS Chance Chance Chance Chance Chance Chance Chance Station ID Location Description 2-year 10-year 25-year 50-year 100-year 500-year 100-year 900 Maukey Gulch at junction with West Fork Rock Creek 13 39 56 72 89 134 139 800 Ross Fork at junction with West Fork Rock Creek 355 723 908 1,060 1,230 1,590 1,920 700 Lower West Fork Ross Creek 374 749 935 1,090 1,250 1,620 1,950 600 West Fork Rock Creek at junction with Rock Creek 688 1,330 1,640 1,900 2,160 2,750 3,370 500 Middle Fork Rock Creek at junction with Rock Creek 786 1,520 1,880 2,180 2,490 3,170 3,880 400 Upper Upper Willow Creek 159 348 448 533 622 828 970 300 Middle Upper Willow Creek 229 509 657 784 917 1,230 1,430 200 Willow Creek at junction with Rock Creek 297 668 868 1,040 1,220 1,640 1,900 100 Ranch Creek at junction with Rock Creek 206 418 524 613 705 911 1,100 1. Peak flow estimated by Pioneer using west regional regression equations. USGS: U.S. Geological Survey. cfs: cubic feet per second.

Page 131 Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

Figure 4-15 Upper Willow Creek Regional Regression Flood Frequency Analysis

2,000 1%+ 500-yr 100-yr 50-yr

25-yr Node 300 10-yr 2-yr Node 200 Node 400

1,000 Peak Flow Flow (cfs) Peak

0 40 60 80 100 Drainage Area (mi2)

Page 132 Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

Figure 4-16 West Fork Rock Creek Regional Regression Flood Frequency Analysis

4,000 1%+ 500-yr 100-yr 50-yr 25-yr 10-yr 3,000 2-yr 600 Node Node 700

2,000 Peak Flow Flow (cfs) Peak

1,000

0 90 110 130 150 170 190 Drainage Area (mi2)

Page 133 Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

4.3.4 Rock Creek Tributaries Discussion Pioneer conducted a peak discharge frequency analysis for 6 tributaries that report to Rock Creek mainstem. The Rock Creek tributaries study reach includes 41 tributary miles within Rock Creek mainstem watershed. Information gathered from this analysis will be used to support the Missoula-Granite County hydraulic analyses and floodplain mapping studies.

Previous flood studies on Rock Creek tributary study reaches are limited. Relevant earlier flood studies include the Granite County FIS (FEMA, 2016), SIR 2015-5019-C (Sando, et al., 2015a) and SIR 2015-5019-F (Sando, et al., 2015b), which included flood frequency analysis for USGS gage Middle Fork Rock Creek near Philipsburg, MT (12332000).

Flood frequency estimates at USGS stream gages used in this analysis were performed by USGS (Sando, et al., 2020) using Bulletin 17C methodologies (England, et al., 2018). The analysis included systematic (at-site) and extended record analysis using MOVE.3 methods (Appendix A). Peak flow 1%+ estimates were developed for each gage location using standard FEMA methodologies.

This hydrologic analysis included conducting flood frequency estimates for both gaged and ungaged sites. Peak flow estimates were calculated at 9 ungaged locations (flow nodes) within the Rock Creek watershed. The flow nodes were located at HUC 12 boundaries and at the end of study reaches. Peak flow 1%+ estimates were developed for all gaged locations using standard FEMA methodologies.

Three methods were considered for estimating peak flood discharges at ungaged flow nodes: 1) two-site logarithmic interpolation method; 2) drainage area gage transfer method; and 3) the regional regression method.

Two stream gages, the USGS gage Middle Fork Rock Creek near Phillipsburg, MT (12332000) and Rock Creek near Clinton, MT (12334510), were used to estimate AEP peak flow values for the Middle Fork Rock Creek study reach (flow node 500). The drainage area transfer method was not used since flow node 500 was located between 2 USGS gaging stations.

Regional regression methods were also used to perform flood frequency analysis on ungaged flow nodes in this study reach. The Rock Creek tributaries watershed areas in this study are considered unregulated. The basin parameters for the flow nodes evaluated in this study fall within the range of basin and climatic characteristics used to develop the regional regression equations. Therefore, regional regression method peak flow estimates are applicable to this study.

Higher confidence is typically associated with flood frequency estimates that are based on measured stream flows, such as in the gage transfer and two-site logarithmic interpolation method. For these reasons, the two-site logarithmic interpolation method was selected for the recommended flood discharge estimates for node 500. Since no gage data exists on the

Page 134 Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

remaining tributary study reaches, the regional regression method was selected for the recommended flood discharge estimates for nodes 100-400 and nodes 600-900.

Table 4-21 summarizes the recommended flood frequency discharge rates for the Rock Creek Tributaries study reach. Figure 4-17 plots the recorded peak flow data and the recommended AEP flow estimates for the Rock Creek tributaries gage updated in this analysis.

Figure 4-18 and Figure 4-19 show the recommended 1% AEP discharge for each flow node location. The hydrologic analysis results provided in Table 4-21 represent the recommended discharges at each flow node location throughout the study reach. The methods used for hydrological analysis are industry accepted methods [Bulletin 17C (England, et al., 2018), SIR 2015-5019-C (Sando, et al., 2015a) and SIR 2015-5019-F (Sando, et al., 2015b)]. This hydrologic analysis conforms to FEMA standards for enhanced level studies, and the recommended flows of this analysis are deemed reliable and suitable for future floodplain studies and hydraulic analyses.

Page 135 Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

Table 4-21 Recommended Flood Discharge Estimates Using Regional Regression and Gage Transfer Analysis

Page 136 Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

Figure 4-17 Updated Middle Fork Rock Creek near Phillipsburg (12332000)

2500 10 Yr Flood1 50 Yr Flood1 100 Yr Flood1

2000

1500

Peak Flow Flow (cfs) Peak 1000

500

0 1938 1941 1944 1947 1950 1953 1956 1959 1962 1965 1968 1971 1974 1977 1980 1983 1986 1989 1992 1995 1998 2001 2004 2007 2010 2013 2016 Year 1. Flood Frequency Based on Data through 2018

Page 137 Brewster Creek

Tyler Creek

Welcome Creek Brewster Creek Cow Creek

Cinnabar Creek 100 (705 CFS) Harvey Creek Ranch Creek North Fork Lower Willow Creek FARM TO MARKET RD W

Lower Willow Creek Cinnamon Bear Creek

Ranch Creek

Wahlquist Creek Butte Cabin Creek

Upper Willow Creek Cottonwood Creek

Rock Creek RAVALLI COUNTY South Fork Lower Willow Creek

Cougar Creek

Alder Creek

GRANITE COUNTY Boulder Creek Smart Creek «¬1 Hogback Creek Burnt Fork Bitterroot River 400 (622 CFS)

Flint Creek

Upper Willow Creek

Marshall Creek

Wyman Gulch 300 (917 CFS) South Boulder Creek MA Williams Gulch RSHA LL CR E EK RD

200 (1220 CFS) Stony Creek Philipsburg

Rock Creek

LEGEND DISPLAYED AS: ROCK CREEK TRIBUTARIES FLOW NODE MISSOULA_GRANITE STUDY REACHES SECONDARY 300 PROJECTION/ZONE: MONTANA STATE PLANE FIGURE 4-18 DATUM: NAD1983 RECOMMENDED 1-PERCENT TOWNS COUNTY BOUNDARIES STREAMS 400 UNITS: INT'L FEET SOURCE: DNRC/MSL/PIONEER ANNUAL DISCHARGE HUC 12 BOUNDARIES HUC 8 BOUNDARIES FLOW CHANGE NODE 600 700 FLOW NODE 100 to 400 100 N 0 1.25 2.5 5 ROCK CREEK TRIBUTARIES STUDY REACH PRIMARY DATE: 7/9/2020 200 Miles Path: Z:\Shared\Active Projects\DNRC\Missoula-Granite\GIS\Hydrology\MSAGNT-FIG-PLN-4-18-RD.mxd West Fork Rock Creek Rock Creek West Fork Rock Creek

SKALK AHO RD

900 (89 CFS) Maukey Gulch 700 (1250 CFS) 800 (1230 CFS) 600 (2160 CFS)

500 (2980 CFS)

Ross Fork

Middle Fork Rock Creek

Maukey Gulch

Middle Fork Rock Creek

Ross Fork

LEGEND DISPLAYED AS: ROCK CREEK TRIBUTARIES FLOW NODE SECONDARY 700 PROJECTION/ZONE: MONTANA STATE PLANE FIGURE 4-19 DATUM: NAD1983 RECOMMENDED 1-PERCENT HUC 12 BOUNDARIES STREAMS 800 UNITS: INT'L FEET SOURCE: DNRC/MSL/PIONEER ANNUAL DISCHARGE ROCK CREEK TRIBUTARIES STUDY REACH FLOW CHANGE NODE 900 FLOW NODE 500 to 900 500 N 0 0.125 0.25 0.5 MISSOULA-GRANITE STUDY REACHES DATE: 7/9/2020 600 Miles Path: Z:\Shared\Active Projects\DNRC\Missoula-Granite\GIS\Hydrology\MSAGNT-FIG-PLN-4-19-RD.mxd Missoula-Granite PMR Missoula-Granite Hydrologic Analysis

5 REFERENCES Anon., 2019. National Inventory of Dams. [Online] Available at: https://nid.sec.usace.army.mil/ords/f?p=105:1:::::: Applied Geomorphology, Inc., 2009. Technical Memorandum: Clark Fork River CMZ Pilot, s.l.: Missoula County. Applied Geomorphology, Inc., 2012. Technical Memorandum: Phase 1 Clark Fork River Channel Migration Zone Map Development, s.l.: US Fish and Wildlife Service. Bitter Root Water Forum, 2014. Bitterroot Watershed Restoration Plan, s.l.: s.n. Bonner Milltown History Center, n.d. The Great Flood of 1908. [Online] Available at: https://www.bonnermilltownhistory.org/the-great-flood-of-1908 Breslin, S., 2018. Montana's Clark Fork River Rises to Major Flood Levels; Man Rescued from Floodwaters in Missoula. [Online] Available at: https://weather.com/safety/floods/news/2018-05-07-western-montana- flooding-impacts CDM & Applied Geomorphology, Inc., 2010. Part:2 Geomorphic, Hydrologic, and Hydraulic Investigation for Phase 1 Remidial Design/Remidial Action, Helena: Montana Department of Enviornmental Quality. Clark Fork Coalition, 2011. Aquatic Restoration Strategy for the Upper Clark Fork Basin, s.l.: Clark Fork Coalition. Clark Fork Coalition, 2017. 2017 Bitterroot Strategy, s.l.: s.n. Clark Fork Watershed Education Program, 2015. 1908 Flood influenced what we are now.. [Online] Available at: http://cfwep.org/1908-flood-influenced-what-we-are-now/ Clark, K. W., 1986. Interactions between the Clark Fork River and Missoula Aquifer, Missoula County, Montana, s.l.: Student Thesis. DEQ and EPA, 2015. Clark Fork River Operable Unit (OU #3) Milltown Reservoir/Clark Fork River Superfund Site: Explanation of Significant Differences, s.l.: Montana Department of Environmental Quality & U.S. Environmental Protection Agency Region 8. DEQ, 2011. Bitterroot Temperature & Tributary Sediment TMDLs - Appendix B, s.l.: Montana Department of Environmental Quality. EMC2, 2004. 2004 Flood Frequency Analysis Calc Brief for the Milltown Reservoir Sediments Operable Unit (MRSOU) of the Milltown Reservoir/Clark Fork Rover Superfund Site, s.l.: s.n. England, J. F. et al., 2018. Guidelines for Determining Flood Flow Frequency - Bulletin #17C: U.S. Geological Survey Techniques and Methods, s.l.: s.n. EPA and DEQ, 2003. Superfund Program Clean-up Proposal: Milltown Reservoir Sediments Operable Unit of the Milltown Reservoir/Clark Fork River Superfund Site, s.l.: U.S. Environmental Protection Agency and Montana Department of Enviornmental Quality. EPA, 2004. Millotwn Reservoir Sediments Operable Unit: Record of Decision, Helena: U.S. Enviornmental Protection Agency Region 8.

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EPA, 2011. Integrating the "3 Rs": Remediation, Restoration and Redevelopment, The Milltown Reservoir Sediments Site and Missoula County, Montana, Helena: U.S. Environmental Protection Agency. Evans, E. & Wilcox, A. C., 2013. Fine Sediment Infiltration Dynamics in a Gravel-Bed River Following A Sediment Pulse, s.l.: Wiley Online Library. FEMA, 2016a. Guidance for Flood Risk Analysis and Mapping - General Hydraulics Considerations, s.l.: s.n. FEMA, 2016b. Guidance for Flood Risk Analysis and Mapping - Automated Engineering., s.l.: Federal Emergency Management Agency. FEMA, 2016c. Guidance for Flod Risk Analysis and Mapping - Hydraulics: One Dimensional Analysis, s.l.: s.n. FEMA, 2016. Flood Insurance Study, Granite County, Montana and Incorporated Areas, s.l.: FEMA. FEMA, 2019a. Flood Insurance Study, Missoula County, Montana and Incorporated Areas, s.l.: FEMA. FEMA, 2019. Guidance for Flood Risk Analysis and Mapping - General Hydrologic Considerations, s.l.: Federal Emegency Management Agency, February 2019. Fortman, K., Kron, D., Staten, C. & Starr, B., 2011. Bitterroot Temperature and Tributary Sediment Total Maximum Daily Loads and Framework: Water Quailty Improvement Plan, s.l.: Montana Department of Enviornmental Quality. Geldon, A. L., 1979. Hydrogeology and water resources of the Missoula basin Montana, Ann Arbor: ProQuest LLC. Hydrosolutions, Inc, 2012. Clark Fork - Pend Oreille Watershed Water Quality Monitoring Program, Sandpoint: Tri-State Water Quality Council. IACWD, 1982. Guidelines for Determining Flood Flow Frequency Bulletin #17B of the Hydrology Subcommittee. , s.l.: U.S. Department of the Interior Office of Water Data Coordination, Interagency Advisory committee on Water Data. KAPX, 2018. KAPX Archive: The great Missoula flood of 1908. [Online] Available at: https://www.kpax.com/news/2018/05/14/kpax-archive-the-great-missoula- flood-of-1908/ Kendy, E. & Tresch, R. E., 1996. Geographic, Geologic, and Hydrologic Summaries of Intermontane Basins of the Northern Rocky Mountains, Helena: USGS. KirK Engineering & Natural Resources, Inc., 2015. Water Supply Report Series I: Water Availability and Mitigation Options in the Clark Fork Basin, s.l.: DNRC. LaFave, J. I., 2006. Ground-Water Quality of the Shallow Basin-Fill, Deep Basin-Fill, and Bedrock Aquifers, Bitterroot Valley, Missoula and Ravalli Counties, Southwest Montana, s.l.: Montana Bureau of Mines and Geology. Lambing, J. H. & Sando, S. K., 2009. Estimated Loads of Suspended Sediment and Selected Trace Elements Transported through the Milltown Reservoir Project Area Before and After the Breaching of Milltown Dam in the Upper Clark Fork Basin, Montana, Water Year 2008: USGS SIR 2009-5095., Helena: s.n.

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Lindstrom, J., Liermann, B. & Kreiner, R., 2018. An Assessment of Fish Populations and Riparian Habitat in Tributaries of the Upper Clark Fork River Basin, s.l.: Montana Fish, Wildlife and Parks. Maki, K., 2018. High water history in Bitterroot is well documented. [Online] Available at: https://nbcmontana.com/news/local/high-water-history-in-bitterroot-is- well-documented Marvin, R. et al., 1995. Abandoned-Inactive Mines Program Deerlodge National Forest Volume III: Flint and Rock Creek Drainages, s.l.: Montana Bureau of Mines and Geology. McCarthy, P., Sando, S. & Chase, K., 2018. Results of Peak Flow Frequency Analysis for Selected Streamgages in the Beaverhead River and Clark Fork Basins, Montana, Based on Data through Water Year 2016, s.l.: USGS. Northwest Power and Conservation Council, 2009. Bitterroot River Subbasin Plan, Portland: Columbia River Basin Fish and Wildlife Program. Parret, C. & Johnson, D. R., 2004. Methods for Estimating Magnitude and Frequency of Floods in Montana Based on Data through Water Year 1998: U.S. Geological Survey (USGS) Water- Resources Investigations Report (WRIR) 03-4308, s.l.: s.n. River Design Group, Inc.; Westwater Consulants, Inc., 2005. Restoration Plan for the Clark Fork RIver and Blackfoot River near Milltown Dam - October 2005: Appendix A - Hydrology and Flood Series Analysis, s.l.: s.n. River Design Group, Inc., 2013. Letter of Map Revision (LOMAR): Clark Fork River and Blackfoot River Near Millown, Montana, Helena, MT: s.n. Sanctuary, M., 2002. Pre-Restoration Characteristics of upper Willow Creek, Granite County, Montana, s.l.: University of Montana. Sando, R., Sando, S. K., McCarthy, P. M. & Dutton, D. M., 2015b. Methods for Estimating Peak- Flow Frequencies at Ungaged Sites in Montana Based on Data through Water Year 2011: U.S. Geological Survey (USGS) Scientific Investigations Report (SIR) 2015-5019-F, s.l.: s.n. Sando, S. C. K. M. P., 2019. Peak-flow frequency analyses for 11 selected streamgages in Jefferson County, Montana, based on data through water year 2017, s.l.: USGS. Sando, S. K., Chase, K. J. & McCarthy, P. M., 2019. Peak-flow frequency analyses for 11 selected stream gages in Jefferson County, Montana, based on data through water year 2017: U.S. Geological Survey data release, https://doi.org/10.5066/P9TK3KFE, s.l.: USGS. Sando, S. K., Chase, K. J. & McCarthy, P. M., 2020. Peak-Flow frequency analyses for selected stream gages in Missoula and Granite Counties, Montana based on data through water year 2018. The results were from a courtesy data release while the information is being processed by USGS., Helena, MT: U.S. Geological Survey. Sando, S. K. & McCarthy, P. M., 2018. Methods for Peak-Flow Frequency Analysis and Reporting for Stream gages in or near Montana Based on Data though Water Year 2015: U.S. Geological Survey (USGS) Scientific Investigations Report (SIR) 2018-5046, s.l.: s.n. Sando, S. K., McCarthy, P. M. & Dutton, D. M., 2015a. Peak-Flow Frequency Analyses and Results Based on Data through Water Year 2011 for Selected Streamflow-Gaging Stations in or Near Montana: USGS Geological Survey (USGS) Scientific Investigations Report (SIR) 2015- 5019-C., s.l.: s.n.

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Sando, S. K., Sando, R., McCarthy, P. M. & Dutton, D. M., 2015c. Adjusted Peak-Flow Frequency Estimates for Selected Steramflow-Gaging Stations in or near Montana Based on Data through Water Year 2011: U.S. Geological Survey (USGS) Scientific Investigations Report (SIR) 2015-5019-D, s.l.: s.n. Smith, C. A., 1992. Hydrogeology of the central and northwestern Missoula Valley, s.l.: Student Thesis. Smith, J. D. et al., 1998. Geomorphology, Flood Plain Tailings, and Metal Transport in the Upper Clark Fork Valley, Montana: Water Resources Investigations Report 98-4170, s.l.: USGS. Smith, L. N., LaFave, J. I. & Patton, T. W., 2013. Montana Ground Water Assessment Atlas No. 4: Groundwater Resources of the Lolo-Bitterroot Area: Mineral, Missoula, and Ravalli Counties, Montana: Part A Descriptive Overview and Water-Quality Data, s.l.: Montana Bureau of Mines and Geology. State Engineer, 1959. Water Resources Survey Part I: History of Land and Water Use on Irrigated Areas and Part II: Maps Showing Irrigated Areas in Colors Designating the Sources of Supply, Helena: State Engineer's Office. Tetra Tech, 2013. State of Montana Multi-Hazard Mitigation Plan & Statewide Hazard Assessment, Helena: Tetra Tech. Trout Unlimited, 2018. Rock Creek Waterhsed Resoration Plan, s.l.: s.n. U.S. Forest Service Pintlar Ranger District, 2007. West Fork Rock Creek Watershed Assessment, s.l.: United States Department of Agriculture. USACE, 1967. Floodplain Information: Clark Fork, Missoula, Montana. Prepared for Montana State Resources Board., s.l.: U.S. Army Corps of Engineers. USACE, 2019. National Inventory of Dams. U.S. Army Corps of Engineers. [Online] Available at: https://nid.sec.usace.army.mil/ords/f?p=105:113:10845977845872::NO::: USFWS, 2002. Chapter 3: Clark Fork River Recovery Unit, Montana and Idaho, Portland: U.S. Fish and Wildlife Service. USGS, 1949. Floods of May-June 1948 in Columbia River Basin: Geological Survey Water-Supply Paper 1080, Washington: United States Government Printing Office. USGS, 1967. Floods of June 1964 in Northwestern Montana: Geological Survey Water-Supply Paper 1840-B, Washington: United States Government Printing Office. USGS, 1982. Floods of May 1981 in West-Central Montana: Water-Resources Investigations 82- 33, Helena: USGS.

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Appendix A. USGS Gage Flood Frequency Analysis

TableX0AT 1–1. Information on streamgages for which peak-flow frequency analyses are reported. [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. NAD 83, North American Datum of 1983; --, not applicable; U, unregulated; ND, not determined; R, regulate Number of Number of Streamgage Latitude, in Longitude, in Contributing Data 4 Number of Percentage of Regulation status for Map number Type of 3 Regulation status as unregulated regulated identification Streamgage name decimal degrees decimal degrees 1 drainage area, in Data correction recorded peak Water years of recorded peak flows Water years of unregulated peak-flow records Water years of regulated peak-flow records drainage basin reported at-site peak-flow (fig. 1) streamgage 2 of 2014 peak-flow peak-flow number (NAD 83) (NAD 83) square miles combination flows regulated by dams frequency analyses records records 632 12323800 Clark Fork near Galen, Montana 46.2082 -112.7674 CONT 656 -- -- U 30 1989–2018 30 1989–2018 0-- 20 U 634 12324200 Clark Fork at Deer Lodge, Montana 46.3977 -112.7425 CONT 1,001 -- -- U 40 1979–2018 40 1979–2018 0-- 18 U -- 12324400 Clark Fork above Little Blackfoot River near Garrison, Montana 46.5109 -112.7897 CONT 1,130 -- -- U 10 2009–2018 10 2009–2018 0-- 16 U 637 12324680 Clark Fork at Goldcreek, Montana 46.5900 -112.9287 CONT 1,774 -- -- U 41 1978–2018 41 1978–2018 0-- 15 U 646 12331800 Clark Fork near Drummond, Montana 46.7119 -113.3308 CONT 2,516 Yes Yes U 46 1967, 1973–90, 1992–2018 46 1967, 1973–90, 1992–2018 0 -- 17 U 648 12332000 Middle Fork Rock Creek near Philipsburg, Montana 46.1846 -113.5016 CONT 121 -- -- U 81 1938–2018 81 1938–2018 0-- 0 U 649 12334510 Rock Creek near Clinton, Montana 46.7223 -113.6831 CONT 889 -- -- U 47 1972–2018 47 1972–2018 0-- 6 U 650 12334550 Clark Fork at Turah Bridge, near Bonner, Montana 46.8259 -113.8140 CONT 3,657 -- -- U 33 1986–2018 33 1986–2018 0-- 13 U 665 12340500 Clark Fork above Missoula, Montana 46.8772 -113.9319 CONT 6,021 Yes -- U 99 1899–1908, 1930–2018 99 1899–1908, 1930–2018 0 -- 12 U 666 12342500 West Fork Bitterroot River near Conner, Montana 45.7248 -114.2823 CONT 317 -- -- R (MIN–dams)5 78 1941–2018 0-- 78 1941–2018 100 Total 669 12343400 East Fork Bitterroot River near Conner, Montana 45.8830 -114.0663 CONT 380 Yes -- U 40 1937–72, 2001–04 40 1937–72, 2001–04 0 -- 2 U 671 12344000 Bitterroot River near Darby, Montana 45.9721 -114.1412 CONT 1,050 -- -- R (MIN–dams)5 81 1938–2018 2 1938–39 79 1940–2018 31 Total 680 12350250 Bitterroot River at Bell Crossing, near Victor, Montana 46.4432 -114.1238 CONT 1,944 -- -- R (MIN–dams) 32 1987–2018 0-- 32 1987–2018 21 Total 683 12351200 Bitterroot River near Florence, Montana 46.6331 -114.0510 CONT 2,342 -- -- U 20 1958–65, 1972, 1974, 1982, 2003–11 20 1958–65, 1972, 1974, 1982, 2003–11 0 -- 18 U 687 12352500 Bitterroot River near Missoula, Montana 46.8317 -114.0549 CONT 2,824 -- -- U 34 1899–1901, 1903–04, 1990–2018 34 1899–1901, 1903–4, 1990–2018 0 -- 15 U 688 12353000 Clark Fork below Missoula, Montana 46.8686 -114.1277 CONT 9,017 -- -- U 89 1930–2018 89 1930–2018 0-- 13 U 697 12354500 Clark Fork at St. Regis, Montana 47.3016 -115.0869 CONT 10,728 -- -- U 103 1911–23, 1929–2018 103 1911–23, 1929–2018 0 -- 11 U 1Abbreviations for type of streamgage are defined as follows: CONT: continuous streamflow operations. CSG: crest-stage gage operations. In cases where both CONT and CSG are indicated for an individual streamgage, the historic operations of the streamgage have included periods of continuous streamflow operations and periods of crest-stage gage operations. 2Data combination refers to combining peak-flow records of two or more closely located streamgages on the same channel. Information on combining records of multiple streamgages is presented in table 1–2. 3Data correction refers to manual adjustment of specific peak-flow records to provide reliable frequency analyses. Information on manual correction of peak-flow records is presented in table 1–3.

4Abbreviations for regulation status are defined as follows: U, unregulated, where the cumulative drainage area upstream from all dams is less than 20 percent of the drainage area of the streamgage. R (MAJ–dam): major dam regulation, where a single upstream dam has a drainage area that exceeds 20 percent of the drainage area of the streamgage. R (MAJ–canal): major diversion canal regulation, where a large diversion canal is known to be located on the channel upstream from the streamgage. R (MIN–dams): minor dam regulation, where the cumulative drainage area of all upstream dams exceeds 20 percent of the drainage area of the streamgage, but no single upstream dam has a drainage area that exceeds 20 percent of the drainage area of the streamgage. Total: the combined unregulated and regulated peak-flow records for streamgages with peak-flow records before and after the start of regulation, . The "Total" peak-flow frequency analysis is provided in cases where major regulation affects less than 50 percent of the drainage area of the streamgage and there is uncertainty in the effects of regulation on specific peak-flow characteristics. Also, the "Total" peak-flow frequency analysis is the only peak-flow frequency analysis provided in cases of minor dam regulation.

5Although a single dam accounts for more than 20 percent of the streamgage drainage area, detailed investigations determined that regulation does not have a substantial effect on peak flows. Thus, the streamgage regulation status is designated as R (MIN–dams) and the at-site frequency analysis is conducted on the total period of record. TableX1A0T 1–2. Information on data combination by combining records of multiple streamgages. [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends.] Primary streamgage Secondary streamgage(s) combined with primary streamgage Combined characteristics Combined Streamgage Contributing Number of Number of Map number Streamgage Drainage area, in Water years of recorded number of Water years of combined peak-flow identification Streamgage name drainage area, in recorded peak Water years of recorded peak flows Streamgage name recorded peak (fig. 1) identification number square miles peak flows recorded records number square miles flows flows peak flows

646 12331800 Clark Fork near Drummond, Montana 2,516 26 1993–2018 12331600 Clark Fork at Drummond, Montana 2,386 12 1967, 1973–83 46 1967, 1973–90, 1992–2018 12331900 Clark Fork near Clinton, Montana 2,650 14 1980–90, 1992–94 665 12340500 Clark Fork above Missoula, Montana 6,021 90 1908, 1930–2018 12341500 Clark Fork at Missoula, Montana 6,108 9 1899–1907 99 1899–1908, 1930–2018 669 12343400 East Fork Bitterroot River near Conner, Montana 380 21 1956–72, 2001–4 12343500 East Fork Bitterroot River at Conner, Montana 404 21 1937–57 40 1937–72, 2001–4 TableX2A0T 1–3. Information on data correction of specific peak-flow records [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends.]

Map Streamgage Contributing Number of Recorded peak Substituted peak Water year of data number identification Streamgage name drainage area, in recorded Water years of recorded peak flows flow, in cubic feet Type of data correction flow, in cubic feet Comments correction (fig. 1) number square miles peak-flows per second per second

646 12331800 Clark Fork near Drummond, Montana 2,516 46 1967; 1973–90; 1992–2018 1967 4,350 Exclusion (opportunistic) -- This peak was recorded at streamgage 12331600; data from 12331600 were combined with 12331800 for analysis of 12331800. The indicated peak flow is outside of the systematic record and was of insufficient magnitude to determine nonexceedance during an ungaged period. Table 1–4. Documentation regarding analytical procedures for peak-flow frequency analyses. [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. PILF, potentially influential low flow; U, unregulated; --, not applicable; R, regulated; MGBT, multiple Grubbs-Beck test; BP, base period used in the Maintenance of Variance Extension Type III record extension] Log-distribution information for peak-flow data PILF information

X3A0T Frequency analysis Streamgage Primary reason Contributing Type of peak-flow Number of peak Source of Analysis skew Number of non-zero incorporates historical Map number identification number Regulation status for for deviation from Station skew of Number of Streamgage name drainage area, in 2 frequency flows used in the Water years of peak flows used in the analysis Standard Skew type used Generalized generalized skew used for the PILF threshold, cubic Type of PILF systematic peak information? (if Yes, see (fig. 1) and analysis analysis 3 standard Bulletin Mean the peak-flow systematic peak 1 square miles analysis analysis 4 deviation in analysis skew used in weighted frequency feet per second threshold flows less than PILF Table 1-5 for additional designation 17C procedures data flows equal to zero skew determinations analysis threshold information)

X3A1T 632 12323800.00 Clark Fork near Galen, Montana 656 U At-site 30 1989–2018 -- 2.7520 0.2618 Weighted ‐0.707 ‐0.070 Bulletin 17B5 ‐0.476 ‐‐ MGBT 0 0 -- X3A2T 632 12323800.01 Clark Fork near Galen, Montana 656 U MOVE.3 99 BP 1899–1908; 1930–2018 -- 2.8155 0.2626 Weighted ‐0.529 ‐0.070 Bulletin 17B5 ‐0.348 417 MGBT 0 23 Yes X3A3T 634 12324200.00 Clark Fork at Deer Lodge, Montana 1,001 U At-site 40 1979–2018 -- 2.9443 0.2737 Weighted ‐0.226 0.000 Bulletin 17B5 ‐0.168 ‐‐ MGBT 0 0 -- X3A4T 634 12324200.01 Clark Fork at Deer Lodge, Montana 1,001 U MOVE.3 99 BP 1899–1908; 1930–2018 -- 2.9719 0.2727 Weighted ‐0.037 0.000 Bulletin 17B5 ‐0.032 ‐‐ MGBT 0 0 Yes X3A5T -- 12324400.00 Clark Fork above Little Blackfoot River near Garrison, Montana 1,130 U At-site 10 2009–18 -- 3.0966 0.2664 Weighted ‐0.391 0.031 Bulletin 17B5 ‐0.159 ‐‐ MGBT 0 0 -- X3A6T -- 12324400.01 Clark Fork above Little Blackfoot River near Garrison, Montana 1,130 U MOVE.3 81 BP 1938–2018 -- 3.0837 0.2748 Weighted ‐0.834 0.031 Bulletin 17B5 ‐0.408 708 MGBT 0 17 Yes X3A7T 637 12324680.00 Clark Fork at Goldcreek, Montana 1,774 U At-site 41 1978–2018 upper tail 3.3833 0.2838 Station 0.149 0.036 -- 0.149 941 Manual 0 2 -- X3A8T 637 12324680.01 Clark Fork at Goldcreek, Montana 1,774 U MOVE.3 99 BP 1899–1908; 1930–2018 upper tail 3.4237 0.2978 Station ‐0.244 0.036 -- ‐0.244 1,850 Manual 0 29 Yes X3A9T 646 12331800.00 Clark Fork near Drummond, Montana 2,516 U At-site 46 1967; 1973–90; 1992–2018 upper tail 3.4733 0.3221 Station ‐0.168 ‐0.028 -- ‐0.168 1,130 Manual 0 4 -- X3A10T 646 12331800.01 Clark Fork near Drummond, Montana 2,516 U MOVE.3 113 BP 1899–1908; 1911–23; 1929–2018 upper tail 3.5237 0.2666 Station 0.192 ‐0.028 -- 0.192 2,570 Manual 0 38 Yes X3A11T 648 12332000.00 Middle Fork Rock Creek near Philipsburg, Montana 121 U At-site 81 1938–2018 -- 2.9379 0.1815 Weighted ‐1.031 ‐0.206 Bulletin 17B5 ‐0.555 670 MGBT 0 21 -- X3A12T 648 12332000.01 Middle Fork Rock Creek near Philipsburg, Montana 121 U MOVE.3 99 BP 1899–1908; 1930–2018 -- 2.9380 0.1952 Weighted ‐0.856 ‐0.206 Bulletin 17B5 ‐0.525 670 MGBT 0 27 Yes X3A13T 649 12334510.00 Rock Creek near Clinton, Montana 889 U At-site 47 1972–2018 -- 3.4788 0.2159 Weighted ‐0.618 ‐0.132 Bulletin 17B5 ‐0.483 ‐‐ MGBT 0 0 -- X3A14T 649 12334510.01 Rock Creek near Clinton, Montana 889 U MOVE.3 99 BP 1899–1908; 1930–2018 -- 3.5114 0.2027 Weighted ‐0.549 ‐0.132 Bulletin 17B5 ‐0.377 2,390 MGBT 0 25 Yes 650 12334550.00 Clark Fork at Turah Bridge, near Bonner, Montana 3,657 U At-site 33 1986–2018 -- 3.7108 0.2513 Weighted ‐0.396 ‐0.156 Bulletin 17B5 ‐0.321 ‐‐ MGBT 0 0 -- X3A15T 650 12334550.01 Clark Fork at Turah Bridge, near Bonner, Montana 3,657 U MOVE.3 113 BP 1899–1908, 1911–23, 1929–2018 -- 3.7970 0.2190 Weighted 0.219 ‐0.156 Bulletin 17B 0.021 4,570 Manual 0 29 Yes X3A16T 665 12340500.00 Clark Fork above Missoula, Montana 6,021 U At-site 99 1899–1908; 1930–2018 -- 4.1558 0.2127 Weighted ‐0.322 ‐0.189 Bulletin 17B5 ‐0.302 ‐‐ MGBT 0 0 Yes X3A17T 665 12340500.01 Clark Fork above Missoula, Montana 6,021 U MOVE.3 113 BP 1899–1908; 1911–23; 1929–2018 -- 4.1643 0.2097 Weighted ‐0.352 ‐0.189 Bulletin 17B5 ‐0.330 ‐‐ MGBT 0 0 Yes X3A18T 666 12342500.20 West Fork Bitterroot River near Conner, Montana 317 Total At-site 78 1941–2018 -- 3.2593 0.1959 Weighted ‐1.050 ‐0.300 Bulletin 17B5 ‐0.556 1,530 MGBT 0 26 -- X3A19T 666 12342500.21 West Fork Bitterroot River near Conner, Montana 317 Total MOVE.3 94 BP 1899–1901; 1903–4; 1930–2018 -- 3.2565 0.1985 Weighted ‐0.310 ‐0.300 Bulletin 17B5 ‐0.306 1,450 MGBT 0 29 Yes X3A20T 669 12343400.00 East Fork Bitterroot River near Conner, Montana 380 U At-site 40 1937–72; 2001–4 -- 3.2671 0.2085 Weighted ‐0.442 ‐0.300 Bulletin 17B5 ‐0.385 ‐‐ MGBT 0 0 -- X3A21T 669 12343400.01 East Fork Bitterroot River near Conner, Montana 380 U MOVE.3 89 BP 1930–2018 -- 3.2395 0.2188 Weighted ‐0.132 ‐0.300 Bulletin 17B5 ‐0.156 ‐‐ MGBT 0 0 -- X3A22T 671 12344000.20 Bitterroot River near Darby, Montana 1,050 Total At-site 81 1938–2018 -- 3.7830 0.1646 Weighted ‐0.829 ‐0.300 Bulletin 17B5 ‐0.507 5,480 MGBT 0 30 -- 671 12344000.21 Bitterroot River near Darby, Montana 1,050 Total MOVE.3 94 BP 1899–1901; 1903–4; 1930–2018 -- 3.7716 0.1885 Weighted ‐0.521 ‐0.300 Bulletin 17B5 ‐0.409 4,870 MGBT 0 30 Yes 680 12350250.20 Bitterroot River at Bell Crossing, near Victor, Montana 1,944 Total At-site 32 1987–2018 -- 4.0400 0.1450 Weighted ‐0.272 ‐0.261 Bulletin 17B ‐0.267 ‐‐ MGBT 0 0 -- X3A23T 680 12350250.21 Bitterroot River at Bell Crossing, near Victor, Montana 1,944 Total MOVE.3 94 BP 1899–1901, 1903–04, 1930–2018 -- 4.0660 0.1360 Weighted ‐0.318 ‐0.261 Bulletin 17B ‐0.294 9,870 MGBT 0 28 Yes X3A24T 683 12351200.00 Bitterroot River near Florence, Montana 2,342 U At-site 20 1958–65; 1972; 1974; 1982; 2003–11 -- 4.1809 0.1086 Weighted 0.349 ‐0.240 Bulletin 17B5 0.070 11,400 MGBT 0 2 Yes X3A25T 683 12351200.01 Bitterroot River near Florence, Montana 2,342 U MOVE.3 94 BP 1899–1901; 1903–4; 1930–2018 -- 4.1517 0.1175 Weighted 0.305 ‐0.240 Bulletin 17B5 0.110 12,200 MGBT 0 28 Yes X3A26T 687 12352500.00 Bitterroot River near Missoula, Montana 2,824 U At-site 34 1899–1901; 1903–4; 1990–2018 -- 4.1539 0.1665 Weighted ‐0.178 ‐0.221 Bulletin 17B5 ‐0.195 ‐‐ MGBT 0 0 Yes X3A27T 687 12352500.01 Bitterroot River near Missoula, Montana 2,824 U MOVE.3 94 BP 1899–1901; 1903–4; 1930–2018 -- 4.1606 0.1557 Weighted ‐0.222 ‐0.221 Bulletin 17B5 ‐0.222 11,800 MGBT 0 27 Yes X3A28T 688 12353000.00 Clark Fork below Missoula, Montana 9,017 U At-site 89 1930–2018 -- 4.4354 0.1708 Weighted ‐0.808 ‐0.224 Bulletin 17B5 ‐0.506 22,000 MGBT 0 25 Yes X3A29T 688 12353000.01 Clark Fork below Missoula, Montana 9,017 U MOVE.3 113 BP 1899–1908; 1911–23; 1929–2018 -- 4.4536 0.1799 Weighted ‐0.578 ‐0.224 Bulletin 17B5 ‐0.442 21,000 MGBT 0 26 Yes X3A30T 697 12354500.00 Clark Fork at St. Regis, Montana 10,728 U At-site 103 1911–23; 1929–2018 -- 4.5453 0.1624 Weighted ‐0.659 ‐0.272 Bulletin 17B5 ‐0.497 27,400 MGBT 0 25 Yes 697 12354500.01 Clark Fork at St. Regis, Montana 10,728 U MOVE.3 113 BP 1899–1908; 1911–23; 1929–2018 -- 4.5494 0.1825 Weighted ‐0.686 ‐0.272 Bulletin 17B5 ‐0.514 26,100 MGBT 0 26 Yes

1The streamgage identification number and analysis designation is defined by XXXXXXXX.AB, where, XXXXXXXX is the streamgage identification number; A is the regulation status for the analysis period; and B is the type of peak-flow frequency analysis.

Values of A (regulation status) are defined as: A = 0, unregulated; A = 1, regulated by major regulation; and A =2, total; that is, the combined unregulated and regulated peak-flow records for streamgages with peak-flow records before and after the start of regulation (see footnote 2). Also, the "Total" peak-flow frequency analysis is the only peak-flow frequency analysis provided in cases of minor dam regulation.

Values of B (type of peak-flow frequency analysis) are defined as: B = 0, at-site peak-flow frequency analysis conducted on recorded data; B = 1, peak-flow frequency analysis conducted on combined recorded and synthesized data; synthesized data from Maintenance of Variance Extension Type III (MOVE.3) record extension procedure; B =2, peak-flow frequency analysis determined from regional regression equations (RREs); RRE frequency results not presented in this report; and B = 3, at-site peak-flow frequency analysis weighted with results from RREs; distributional parameters not available for RRE weighted frequency analyses.

2Abbreviations for regulation status are defined as follows: U, unregulated, where the cumulative drainage area upstream from all dams is less than 20 percent of the drainage area of the streamgage. R (MAJ–dam): major dam regulation, where a single upstream dam has a drainage area that exceeds 20 percent of the drainage area of the streamgage. R (MAJ–canal): major diversion canal regulation, where a large diversion canal is known to be located on the channel upstream from the streamgage. R (MIN–dams): minor dam regulation, where the cumulative drainage area of all upstream dams exceeds 20 percent of the drainage area of the streamgage, but no single upstream dam has a drainage area that exceeds 20 percent of the drainage area of the streamgage. Total: the combined unregulated and regulated peak-flow records for streamgages with peak-flow records before and after the start of regulation, . The "Total" peak-flow frequency analysis is provided in cases where major regulation affects less than 50 percent of the drainage area of the streamgage and there is uncertainty in the effects of regulation on specific peak-flow characteristics. Also, the "Total" peak-flow frequency analysis is the only peak-flow frequency analysis provided in cases of minor dam regulation.

4Standard Bulletin 17C (England and others, 2017) procedures are considered to be the use of the weighted skew and the use of the multiple Grubbs-Beck low-outlier test (MGBT) for identifying PILFs. In cases where either the station skew or a manual (analyst-selected) PILF threshold was used, the peak-flow frequency analysis was considered to deviate from standard Bulletin 17C procedures. The abbreviations for the reasons for deviation from standard Bulletin 17C procedures are defined as follows: reg: the peak-flow records are affected by major dam or canal regulation; upper tail: the probability plots of the peak-flow records deviate from typical patterns in the upper tail of the frequency curve, generally because of mixed population characteristics; and lower tail: the probability plots of the peak-flow records deviate from typical patterns in the lower tail of the frequency curve at high annual exceedance probabilities (greater than about 50.0 percent). 5U.S. Interagency Advisory Council on Water Data, 1982, Guidelines for determining flood flow frequency: Hydrology Subcommittee, Bulletin 17B, appendixes 1–14, 28 p. TableX4A0T 1–5. Documentation regarding incorporating historical information in applicable at-site peak-flow frequency analyses. [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. U, unregulated; R, regulated; --, not applicable; BP, base period used in the Maintenance of Variance Type III record extension] Information on peak flows used to define perception thresholds (and associated flow intervals) in ungaged historical periods

Map Streamgage identification Contributing Number of peak Regulation status for Number of peak Water year of Peak flow used for Number of years number number and analysis Streamgage name drainage area, in 2 flows used in the Water years of peak flows used in the analysis flows used for peak flow used for perception threshold, in the associated 1 analysis Associated ungaged historical period, in water years (fig. 1) designation square miles analysis perception perception in cubic feet per ungaged thresholds threshold second historical period

632 12323800.01 Clark Fork near Galen, Montana 656 U 99 BP 1899–1908, 1930–2018 1 1908 2,810 21 1909–1929 634 12324200.01 Clark Fork at Deer Lodge, Montana 1,001 U 99 BP 1899–1908, 1930–2018 1 1908 4,670 21 1909–1929 -- 12324400.01 Clark Fork above Little Blackfoot River near Garrison, Mo 1,130 U 81 BP 1938–2018 1 1981 5,590 29 1909–1937 637 12324680.01 Clark Fork at Goldcreek, Montana 1,774 U 99 BP 1899–1908, 1930–2018 1 1908 14,700 21 1909–1929 646 12331800.01 Clark Fork near Drummond, Montana 2,516 U 113 BP 1899–1908, 1911–23, 1929–2018 1 1908 17,200 7 1909–10, 1924–1928 648 12332000.01 Middle Fork Rock Creek near Philipsburg, Montana 121 U 99 BP 1899–1908; 1930–2018 1 1908 2,340 21 1909–1929 649 12334510.01 Rock Creek near Clinton, Montana 889 U 99 BP 1899–1908, 1930–2018 1 1908 10,400 21 1909–1929 650 12334550.01 Clark Fork at Turah Bridge, near Bonner, Montana 3,657 U 113 BP 1899–1908, 1911–23, 1929–2018 1 1908 26,300 7 1909–10, 1924–1928 665 12340500.00 Clark Fork above Missoula, Montana 6,021 U 99 1899–1908, 1930–2018 1 1948 31,500 21 1909–1929 665 12340500.01 Clark Fork above Missoula, Montana 6,021 U 113 BP 1899–1908, 1911–23, 1929–2018 1 1948 31,500 7 1909–10, 1924–1928 666 12342500.21 West Fork Bitterroot River near Conner, Montana 317 Total 94 BP 1899–1901, 1903–04, 1930–2018 1 1899 6,640 25 1902, 1905–07, 1909–29 671 12344000.21 Bitterroot River near Darby, Montana 1,050 Total 94 BP 1899–1901, 1903–04, 1930–2018 1 1899 20,000 25 1902, 1905–07, 1909–29 680 12350250.21 Bitterroot River at Bell Crossing, near Victor, Montana 1,944 Total 94 BP 1899–1901, 1903–04, 1930–2018 1 1899 27,100 25 1902, 1905–07, 1909–29 683 12351200.00 Bitterroot River near Florence, Montana 2,342 U 20 1958–65, 1972, 1974, 1982, 2003–11 3 1964 20,300 6 1966–1971 1972 21,400 9 1973–1981 1974 28,400 27 1983–2002, 2012–18 683 12351200.01 Bitterroot River near Florence, Montana 2,342 U 94 BP 1899–1901, 1903–04, 1930–2018 1 1899 32,500 25 1902, 1905–07, 1909–29 687 12352500.00 Bitterroot River near Missoula, Montana 2,824 U 34 1899–1901, 1903–04, 1990–2018 1 1899 38,300 85 1902, 1905–07, 1905–89 687 12352500.01 Bitterroot River near Missoula, Montana 2,824 U 94 BP 1899–1901, 1903–04, 1930–2018 1 1899 38,300 25 1902, 1905–07, 1909–29 688 12353000.00 Clark Fork below Missoula, Montana 9,017 U 89 1930–2018 1 1948 52,800 21 1909–1929 688 12353000.01 Clark Fork below Missoula, Montana 9,017 U 113 BP 1899–1908, 1911–23, 1929–2018 1 1948 52,800 7 1909–10, 1924–1928 697 12354500.00 Clark Fork at St. Regis, Montana 10,728 U 103 1911–23; 1929–2018 1 1948 68,900 7 1909–10, 1924–1928 697 12354500.01 Clark Fork at St. Regis, Montana 10,728 U 113 BP 1899–1908, 1911–23, 1929–2018 1 1948 68,900 7 1909–10, 1924–1928 1The streamgage identification number and analysis designation is defined by XXXXXXXX.AB, 2Abbreviations for regulation status are defined as follows: X5A0T Table 1–6. Documentation regarding the Maintenance of Variance Type III (MOVE.3) record extension procedure for selected streamgages. [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. SEP, standard error of prediction, in percent; --, not applicable] Target streamgage for which peak streamflows were synthesized Index streamgage(s) used for synthesis of peak streamflows Pearson correlation Number of concurrent Effective record Map Streamgage Contributing Number of Contributing Number of peak flows coefficient for Weighted average Estimated standard Water years of recorded Number of years requiring Water years requiring Percentage of record Streamgage identification recorded peak flows for length for the number identification Streamgage Name drainage area, in recorded peak Streamgage Name drainage area, in synthesized based on this concurrent data for Pearson correlation error of MOVE.3 peak flows synthesis of peak flows synthesis of peak flows synthesized number target and index synthesized peak (fig. 1) number square miles flows square miles streamgage target and index coefficient1 analysis, in percent2 streamgages flows streamgage Streamgages on the Clark Fork between Galen and Deer Lodge, Montana—base period 1899–1908, 1930–2018 (99 years) 632 12323800 Clark Fork near Galen, Montana 656 30 1989–2018 69 1899–1908, 1930–88 69.7 12323750 Silver Bow Creek at Warm 473 8 26 0.98 0.87 32.8 30.7 Springs, Montana 12323770 Warm Springs Creek at 160 5 29 0.96 Warm Springs, Montana 12324200 Clark Fork at Deer Lodge, 1,001 4 30 0.93 Montana 12332000 Middle Fork Rock Creek near 121 34 30 0.86 Philipsburg, Montana

12340500 Clark Fork above Missoula, 6,021 10 30 0.79 Montana 12353000 Clark Fork below Missoula, 9,017 8 30 0.79 Montana 634 12324200 Clark Fork at Deer Lodge, 1,001 40 1979–2018 59 1899–1908, 1930–78 59.6 12323750 Silver Bow Creek at Warm 473 7 27 0.91 0.81 47.7 18.4 Montana Springs, Montana 12330000 Boulder Creek at Maxville, 70.5 32 39 0.81 Montana 12332000 Middle Fork Rock Creek near 121 2 40 0.81 Philipsburg, Montana

12340500 Clark Fork above Missoula, 6,021 18 40 0.78 Montana Clark Fork above Little Blackfoot River near Garrison, Montana (streamgage 12324400)—base period 1938–2018 (81 years) -- 12324400 Clark Fork above Little Blackfoot 1,130 10 2009–2018 71 1938–2008 87.7 12324680 Clark Fork at Goldcreek, 1,774 31 10 0.99 0.93 20.6 35.7 River near Garrison, Montana Montana 12331800 Clark Fork near Drummond, 2,516 6 10 0.97 Montana 12332000 Middle Fork Rock Creek near 121 34 10 0.88 Philipsburg, Montana

Clark Fork at Goldcreek, Montana (streamgage 12324680)—base period 1899–1908, 1930–2018 (99 years) 637 12324680 Clark Fork at Goldcreek, Montana 1,774 41 1978–2018 58 1899–1908, 1930–77 58.6 12331800 Clark Fork near Drummond, 2,516 6 40 0.98 0.81 46.9 18.5 Montana 12332000 Middle Fork Rock Creek near 121 34 41 0.80 Philipsburg, Montana

12340500 Clark Fork above Missoula, 6,021 18 41 0.76 Montana Clark Fork near Drummond, Montana (streamgage 12331800)—base period 1899–1908, 1911–23, 1929–2018 (113 years) 646 12331800 Clark Fork near Drummond, 2,516 46 1967, 1973–90, 67 1899–1908, 1911–23, 59.3 12330000 Boulder Creek at Maxville, 71 33 45 0.84 0.81 57.7 20.3 Montana 1992–2018 1929–66, 1968–72, 1991 Montana 12340500 Clark Fork above Missoula, 6,021 20 46 0.80 Montana 12354500 Clark Fork at St. Regis, 10,728 14 46 0.73 Montana Streamgages in the Rock Creek drainage basin, Montana—base period 1899–1908, 1930–2018 (99 years) 648 12332000 Middle Fork Rock Creek near 121 81 1938–2018 18 1899–1908, 1930–37 18.2 12340500 Clark Fork above Missoula, 6,021 10 81 0.83 0.86 23.0 9.2 Philipsburg, Montana Montana 12353000 Clark Fork below Missoula, 9,017 8 81 0.89 Montana 649 12334510 Rock Creek near Clinton, 889 47 1972–2018 52 1899–1908, 1930–71 52.5 12340500 Clark Fork above Missoula, 6,021 10 47 0.93 0.95 15.9 36.9 Montana Montana 12353000 Clark Fork below Missoula, 9,017 42 47 0.95 Montana Streamgages on the Clark Fork between Turah Bridge and above Missoula, Montana—base period 1899–1908, 1911–23, 1929–2018 (113 years) 650 12334550 Clark Fork at Turah Bridge, near 3,657 33 1986–2018 80 1899–1908, 1911–23, 70.8 12331800 Clark Fork near Drummond, 2,516 14 32 0.93 0.88 29.6 38.3 Bonner, Montana 1929–85 Montana 12332000 Middle Fork Rock Creek near 121 34 33 0.91 Philipsburg, Montana

12340500 Clark Fork above Missoula, 6,021 10 33 0.87 Montana 12353000 Clark Fork below Missoula, 9,017 8 33 0.87 Montana 12354500 Clark Fork at St. Regis, 10,728 14 33 0.81 Montana 665 12340500 Clark Fork above Missoula, 6,021 99 1899–1908, 1930–2018 14 1911–23, 1929 12.4 12354500 Clark Fork at St. Regis, 10,728 14 89 0.95 0.95 15.8 10.9 Montana Montana West Fork Bitterroot River near Conner, Montana (streamgage 12342500)—base period 1899–1901, 1903–04, 1930–2018 (94 years) 666 12342500 West Fork Bitterroot River near 317 78 1941–2018 16 1899–1901, 1903–04, 17.0 12344000 Bitterroot River near Darby, 1,050 3 78 0.94 0.91 23.5 10.3 Conner, Montana 1930–40 Montana 12352500 Bitterroot River near 2,824 5 29 0.94 Missoula, Montana 12353000 Clark Fork below Missoula, 9,017 8 78 0.87 Montana East Fork Bitterroot River near Conner, Montana (streamgage 12343400)—base period 1930–2018 (89 years) 669 12343400 East Fork Bitterroot River near 380 40 1937–72, 2001–04 49 1930–36, 1973–2000, 55.1 12344000 Bitterroot River near Darby, 1,050 33 39 0.95 0.96 13.8 35.7 Conner, Montana 2005–18 Montana 12351200 Bitterroot River near 2,342 9 11 0.97 Florence, Montana 12353000 Clark Fork below Missoula, 9,017 7 40 0.95 Montana Streamgages on the Bitterroot River between Darby and Missoula, Montana —base period 1899–1901, 1903–04, 1930–2018 (94 years) 671 12344000 Bitterroot River near Darby, 1,050 81 1938–2018 13 1899–1901, 1903–04, 13.8 12352500 Bitterroot River near 2,824 5 29 0.97 0.93 15.8 9.6 Montana 1930–37 Missoula, Montana 12353000 Clark Fork below Missoula, 9,017 8 81 0.91 Montana 680 12350250 Bitterroot River at Bell Crossing, 1,944 21 31987–97, 2002–03, 73 1899–1901, 1903–04, 77.7 12344000 Bitterroot River near Darby, 1,050 49 21 0.92 0.92 13.9 32.2 near Victor, Montana 2010–18 1930–86, 1998–2001, Montana 2004–09 12352500 Bitterroot River near 2,824 16 18 0.94 Missoula, Montana 12353000 Clark Fork below Missoula, 9,017 8 21 0.87 Montana 683 12351200 Bitterroot River near Florence, 2,342 20 1958–65, 1972, 1974, 74 1899–1901, 1903–1904, 78.7 12342500 West Fork Bitterroot River 317 15 20 0.93 0.96 7.8 49.6 Montana 1982, 2003–11 1930–57, 1966–71, 1973, near Conner, Montana 1975–81, 1983–2002, 2012–18 12343400 East Fork Bitterroot River 380 27 11 0.97 near Conner, Montana

12352500 Bitterroot River near 2,824 25 9 0.99 Missoula, Montana 12353000 Clark Fork below Missoula, 9,017 7 20 0.90 Montana 687 12352500 Bitterroot River near Missoula, 2,824 34 1899–1901, 1903–04, 60 1930–89 63.8 12344000 Bitterroot River near Darby, 1,050 41 29 0.97 0.97 8.9 45.2 Montana 1990–2018 Montana 12351200 Bitterroot River near 2,342 11 9 0.99 Florence, Montana 12353000 Clark Fork below Missoula, 9,017 8 29 0.96 Montana Streamgages on the Clark Fork between below Missoula and St. Regis, Montana—base period 1899–1908, 1911–23, 1929–2018 (113 years) 688 12353000 Clark Fork below Missoula, 9,017 89 1930–2018 24 1899–1908, 1911–23, 1929 21.2 12340500 Clark Fork above Missoula, 6,021 10 89 0.96 0.98 8.7 21.6 Montana Montana 12354500 Clark Fork at St. Regis, 10,728 14 89 0.99 Montana 697 12354500 Clark Fork at St. Regis, Montana 10,728 103 1911–23, 1929–2018 10 1899–1908 8.8 12340500 Clark Fork above Missoula, 6,021 10 89 0.95 0.95 13.4 7.9 Montana 1The weighted average Pearson correlation coefficient was determined by multiplying the number of peak flows synthesized based on an index streamgage times the Pearson correlation coefficent for the index streamgage for each index streamgage. The resultant products then were summed and divided by the total number of synthesized peak flows.

2A standard error was calculated based on an ordinary least squares (OLS) formulation of the analysis. That OLS standard error was adjusted to an estimated MOVE.3 formulation by multiplying times the following adjustment factor (Wilbert O. Thomas, Michael Baker International, written commun., November 2016): AF = 2/(1+), where, AF is the adjustment factor; and  is the weighted average Pearson correlation coefficient. 3 For streamgage 12350250, 11 years of recorded peak flows (1997–2001, 2004–09) were excluded from the MOVE.3 analysis because the actual peak flows were known to be greater than the recorded values. Table 1–7. Peak-flow frequency results [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. U, unregulated; R, regulated; --, not applicable; BP, base period used in the Maintenance of Variance Type III record extension] Annual peak flow, in cubic feet per second, for indicated annual exceedance probability, in percent

X6A0T Type of peak- Frequency analysis Map Streamgage identification Contributing Number of peak Analyses considered by U.S. Regulation status for flow incorporates historical 84 percent confidence number number and analysis Streamgage name drainage area, in flows used in the Water years of peak flows used in the analysis level for the 1 percent Geological Survey to be most analysis 2 frequency information? (if Yes, see Table 1- 50 42.9 20 10 4 2 1 0.5 0.2 (fig. 1) designation 1 square miles analysis annual exceedance appropriate for flood-plain analysis 3 5 for additional information) probability peak flow mapping purposes 5

X6A1T 632 12323800.00 Clark Fork near Galen, Montana 656 U At-site 30 1989–2018 -- 593 658 947 1,180 1,460 1,660 1,860 2,040 2,280 2,670 --- X6A2T 632 12323800.01 Clark Fork near Galen, Montana 656 U MOVE.3 99 BP 1899–1908, 1930–2018 Yes 677 753 1,100 1,380 1,750 2,020 2,280 2,550 2,890 3,080 Yes X6A3T 634 12324200.00 Clark Fork at Deer Lodge, Montana 1,001 U At-site 40 1979–2018 -- 895 1,000 1,500 1,950 2,550 3,030 3,520 4,040 4,750 5,240 -- X6A4T 634 12324200.01 Clark Fork at Deer Lodge, Montana 1,001 U MOVE.3 99 BP 1899–1908, 1930–2018 Yes 940 1,050 1,590 2,090 2,790 3,370 3,980 4,630 5,570 5,890 Yes X6A5T -- 12324400.00 Clark Fork above Little Blackfoot River near Garrison, Montana 1,130 U At-site 10 2009–2018 -- 1,270 1,420 2,100 2,710 3,530 4,180 4,840 5,530 6,490 11,100 -- X6A6T -- 12324400.01 Clark Fork above Little Blackfoot River near Garrison, Montana 1,130 U MOVE.3 81 BP 1938–2018 Yes 1,270 1,410 2,080 2,640 3,340 3,860 4,360 4,860 5,500 6,130 Yes X6A7T 637 12324680.00 Clark Fork at Goldcreek, Montana 1,774 U At-site 41 1978–2018 -- 2,380 2,670 4,170 5,640 7,840 9,740 11,900 14,300 17,800 22,900 -- X6A8T 637 12324680.01 Clark Fork at Goldcreek, Montana 1,774 U MOVE.3 99 BP 1899–1908, 1930–2018 Yes 2,730 3,080 4,760 6,260 8,300 9,900 11,600 13,300 15,600 17,200 Yes X6A9T 646 12331800.00 Clark Fork near Drummond, Montana 2,516 U At-site 46 1967, 1973–90, 1992–2018 -- 3,040 3,460 5,580 7,580 10,400 12,800 15,200 17,900 21,600 26,200 -- X6A10T 646 12331800.01 Clark Fork near Drummond, Montana 2,516 U MOVE.3 113 BP 1899–1908, 1911–23, 1929–2018 Yes 3,280 3,660 5,560 7,420 10,200 12,500 15,200 18,100 22,600 26,100 Yes X6A11T 648 12332000.00 Middle Fork Rock Creek near Philipsburg, Montana 121 U At-site 81 1938–2018 -- 901 968 1,240 1,440 1,650 1,800 1,930 2,050 2,190 2,210 -- X6A12T 648 12332000.01 Middle Fork Rock Creek near Philipsburg, Montana 121 U MOVE.3 99 BP 1899–1908, 1930–2018 Yes 902 975 1,270 1,500 1,750 1,920 2,070 2,210 2,390 2,370 Yes X6A13T 649 12334510.00 Rock Creek near Clinton, Montana 889 U At-site 47 1972–2018 -- 3,130 3,420 4,610 5,520 6,580 7,320 8,010 8,650 9,460 10,100 -- X6A14T 649 12334510.01 Rock Creek near Clinton, Montana 889 U MOVE.3 99 BP 1899–1908, 1930–2018 Yes 3,340 3,630 4,840 5,780 6,900 7,690 8,440 9,160 10,100 10,000 Yes 650 12334550.00 Clark Fork at Turah Bridge, near Bonner, Montana 3,657 U At-site 33 1986–2018 -- 5,300 5,870 8,420 10,500 13,200 15,200 17,200 19,200 21,700 24,700 -- X6A15T 650 12334550.01 Clark Fork at Turah Bridge, near Bonner, Montana 3,657 U MOVE.3 113 BP 1899–1908, 1911–23, 1929–2018 Yes 6,260 6,840 9,570 12,000 15,200 17,700 20,400 23,200 27,000 28,500 Yes X6A16T 665 12340500.00 Clark Fork above Missoula, Montana 6,021 U At-site 99 1899–1908, 1930–2018 Yes 14,700 16,000 21,700 26,300 32,000 36,100 40,100 44,000 49,100 46,900 -- X6A17T 665 12340500.01 Clark Fork above Missoula, Montana 6,021 U MOVE.3 113 BP 1899–1908, 1911–23, 1929–2018 Yes 15,000 16,300 22,000 26,600 32,100 36,100 39,900 43,600 48,400 46,600 Yes X6A18T 666 12342500.20 West Fork Bitterroot River near Conner, Montana 317 Total At-site 78 1941–2018 -- 1,890 2,050 2,670 3,130 3,650 3,990 4,310 4,600 4,940 5,020 -- X6A19T 666 12342500.21 West Fork Bitterroot River near Conner, Montana 317 Total MOVE.3 94 BP 1899–1901, 1903–04, 1930–2018 Yes 1,850 2,000 2,660 3,190 3,820 4,270 4,710 5,140 5,680 5,570 Yes X6A20T 669 12343400.00 East Fork Bitterroot River near Conner, Montana 380 U At-site 40 1937–72, 2001–04 -- 1,910 2,070 2,790 3,340 4,010 4,480 4,920 5,360 5,900 6,380 -- X6A21T 669 12343400.01 East Fork Bitterroot River near Conner, Montana 380 U MOVE.3 89 BP 1930–2018 -- 1,760 1,920 2,660 3,280 4,080 4,680 5,290 5,900 6,730 6,730 Yes X6A22T 671 12344000.20 Bitterroot River near Darby, Montana 1,050 Total At-site 81 1938–2018 -- 6,260 6,690 8,390 9,620 11,000 11,900 12,700 13,500 14,400 14,500 -- 671 12344000.21 Bitterroot River near Darby, Montana 1,050 Total MOVE.3 94 BP 1899–1901, 1903–04, 1930–2018 Yes 6,090 6,570 8,570 10,100 11,800 13,100 14,200 15,300 16,700 16,400 Yes 680 12350250.20 Bitterroot River at Bell Crossing, near Victor, Montana 1,944 Total At-site 32 1987–2018 -- 11,100 11,800 14,600 16,600 19,000 20,700 22,300 23,800 25,700 29,400 -- X6A23T 680 12350250.21 Bitterroot River at Bell Crossing, near Victor, Montana 1,944 Total MOVE.3 94 BP 1899–1901, 1903–04, 1930–2018 Yes 11,800 12,500 15,200 17,200 19,500 21,000 22,500 23,900 25,600 27,000 Yes X6A24T 683 12351200.00 Bitterroot River near Florence, Montana 2,342 U At-site 20 1958–65, 1972, 1974, 1982, 2003–11 Yes 15,100 15,800 18,700 20,900 23,600 25,600 27,500 29,400 31,800 32,300 -- X6A25T 683 12351200.01 Bitterroot River near Florence, Montana 2,342 U MOVE.3 94 BP 1899–1901, 1903–04, 1930–2018 Yes 14,100 14,800 17,800 20,100 23,000 25,100 27,200 29,300 32,000 32,400 Yes X6A26T 687 12352500.00 Bitterroot River near Missoula, Montana 2,824 U At-site 34 1899–1901, 1903–04, 1990–2018 Yes 14,400 15,400 19,700 23,100 27,200 30,100 32,900 35,700 39,200 38,400 -- X6A27T 687 12352500.01 Bitterroot River near Missoula, Montana 2,824 U MOVE.3 94 BP 1899–1901, 1903–04, 1930–2018 Yes 14,700 15,600 19,600 22,700 26,400 28,900 31,400 33,800 36,900 36,600 Yes X6A28T 688 12353000.00 Clark Fork below Missoula, Montana 9,017 U At-site 89 1930–2018 Yes 28,200 30,200 38,200 44,000 50,400 54,800 58,700 62,300 66,700 65,600 -- X6A29T 688 12353000.01 Clark Fork below Missoula, Montana 9,017 U MOVE.3 113 BP 1899–1908, 1911–23, 1929–2018 Yes 29,300 31,500 40,500 47,200 54,900 60,200 65,000 69,600 75,200 73,500 Yes X6A30T 697 12354500.00 Clark Fork at St. Regis, Montana 10,728 U At-site 103 1911–23, 1929–2018 Yes 36,200 38,600 48,400 55,300 63,100 68,200 73,000 77,300 82,500 81,200 -- 697 12354500.01 Clark Fork at St. Regis, Montana 10,728 U MOVE.3 113 BP 1899–1908, 1911–23, 1929–2018 Yes 36,700 39,500 50,800 59,000 68,300 74,500 80,200 85,500 91,900 90,100 Yes 1The streamgage identification number and analysis designation is defined by XXXXXXXX.AB, 2Abbreviations for regulation status are defined as follows: 3Abbreviations for type of frequency analysis are defined as follows: Table 1–8. Variance of peak-flow frequency estimates. [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. U, unregulated; R, regulated; --, not applicable; BP, base period used in the Maintenance of Variance Type III record extension] Variance, in base 10 logarithm, for indicated annual exceedance probability, in percent

X7A0T Type of peak- Map Streamgage identification Contributing Number of peak Frequency analysis incorporates Regulation status for flow number number and analysis Streamgage name drainage area, in flows used in the Water years of peak flows used in the analysis historical information? (if Yes, see analysis2 frequency (fig. 1) designation1 square miles analysis Table 1-5 for additional information) 50 42.9 20 10 4 2 1 0.5 0.2 analysis3

X7A1T 632 12323800.00 Clark Fork near Galen, Montana 656 U At-site 30 1989–2018 -- 0.0025 0.0024 0.0023 0.0027 0.0041 0.0057 0.0078 0.0104 0.0145

X7A2T 632 12323800.01 Clark Fork near Galen, Montana 656 U MOVE.3 99 BP 1899–1908, 1930–2018 Yes 0.0009 0.0009 0.0009 0.0011 0.0015 0.0020 0.0028 0.0037 0.0055

X7A3T 634 12324200.00 Clark Fork at Deer Lodge, Montana 1,001 U At-site 40 1979–2018 -- 0.0021 0.0021 0.0023 0.0030 0.0046 0.0064 0.0087 0.0116 0.0163

X7A4T 634 12324200.01 Clark Fork at Deer Lodge, Montana 1,001 U MOVE.3 99 BP 1899–1908, 1930–2018 Yes 0.0010 0.0010 0.0011 0.0015 0.0025 0.0036 0.0052 0.0070 0.0100

X7A5T -- 12324400.00 Clark Fork above Little Blackfoot River near Garrison, Montana 1,130 U At-site 10 2009–2018 -- 0.0075 0.0075 0.0085 0.0108 0.0154 0.0202 0.0260 0.0328 0.0433

X7A6T -- 12324400.01 Clark Fork above Little Blackfoot River near Garrison, Montana 1,130 U MOVE.3 81 BP 1938–2018 Yes 0.0011 0.0010 0.0011 0.0012 0.0018 0.0023 0.0031 0.0041 0.0060

X7A7T 637 12324680.00 Clark Fork at Goldcreek, Montana 1,774 U At-site 41 1978–2018 -- 0.0023 0.0024 0.0029 0.0041 0.0071 0.0107 0.0156 0.0219 0.0323

X7A8T 637 12324680.01 Clark Fork at Goldcreek, Montana 1,774 U MOVE.3 99 BP 1899–1908, 1930–2018 Yes 0.0011 0.0011 0.0012 0.0015 0.0022 0.0031 0.0043 0.0061 0.0090

X7A9T 646 12331800.00 Clark Fork near Drummond, Montana 2,516 U At-site 46 1967, 1973–90, 1992–2018 -- 0.0026 0.0026 0.0029 0.0037 0.0059 0.0085 0.0122 0.0169 0.0247

X7A10T 646 12331800.01 Clark Fork near Drummond, Montana 2,516 U MOVE.3 113 BP 1899–1908, 1911–23, 1929–2018 Yes 0.0008 0.0008 0.0011 0.0016 0.0028 0.0042 0.0063 0.0092 0.0143

X7A11T 648 12332000.00 Middle Fork Rock Creek near Philipsburg, Montana 121 U At-site 81 1938–2018 -- 0.0005 0.0004 0.0004 0.0005 0.0007 0.0009 0.0012 0.0016 0.0023

X7A12T 648 12332000.01 Middle Fork Rock Creek near Philipsburg, Montana 121 U MOVE.3 99 BP 1899–1908, 1930–2018 Yes 0.0004 0.0004 0.0004 0.0005 0.0006 0.0009 0.0012 0.0016 0.0023

X7A13T 649 12334510.00 Rock Creek near Clinton, Montana 889 U At-site 47 1972–2018 -- 0.0011 0.0011 0.0010 0.0012 0.0018 0.0026 0.0036 0.0049 0.0070

X7A14T 649 12334510.01 Rock Creek near Clinton, Montana 889 U MOVE.3 99 BP 1899–1908, 1930–2018 Yes 0.0005 0.0005 0.0005 0.0006 0.0008 0.0011 0.0015 0.0021 0.0030 650 12334550.00 Clark Fork at Turah Bridge, near Bonner, Montana 3,657 U At-site 33 1986–2018 -- 0.0021 0.0021 0.0021 0.0026 0.0040 0.0055 0.0075 0.0100 0.0139

X7A15T 650 12334550.01 Clark Fork at Turah Bridge, near Bonner, Montana 3,657 U MOVE.3 113 BP 1899–1908, 1911–23, 1929–2018 Yes 0.0005 0.0005 0.0006 0.0010 0.0016 0.0023 0.0033 0.0047 0.0069

X7A16T 665 12340500.00 Clark Fork above Missoula, Montana 6,021 U At-site 99 1899–1908, 1930–2018 Yes 0.0005 0.0005 0.0005 0.0006 0.0009 0.0013 0.0018 0.0025 0.0036

X7A17T 665 12340500.01 Clark Fork above Missoula, Montana 6,021 U MOVE.3 113 BP 1899–1908, 1911–23, 1929–2018 Yes 0.0004 0.0004 0.0004 0.0005 0.0008 0.0012 0.0017 0.0024 0.0035

X7A18T 666 12342500.20 West Fork Bitterroot River near Conner, Montana 317 Total At-site 78 1941–2018 -- 0.0006 0.0005 0.0005 0.0006 0.0009 0.0012 0.0015 0.0020 0.0027

X7A19T 666 12342500.21 West Fork Bitterroot River near Conner, Montana 317 Total MOVE.3 94 BP 1899–1901, 1903–04, 1930–2018 Yes 0.0005 0.0005 0.0005 0.0006 0.0009 0.0013 0.0018 0.0023 0.0034

X7A20T 669 12343400.00 East Fork Bitterroot River near Conner, Montana 380 U At-site 40 1937–72, 2001–04 -- 0.0012 0.0012 0.0012 0.0014 0.0021 0.0030 0.0042 0.0056 0.0079

X7A21T 669 12343400.01 East Fork Bitterroot River near Conner, Montana 380 U MOVE.3 89 BP 1930–2018 -- 0.0006 0.0006 0.0007 0.0009 0.0014 0.0020 0.0029 0.0039 0.0055

X7A22T 671 12344000.20 Bitterroot River near Darby, Montana 1,050 Total At-site 81 1938–2018 -- 0.0005 0.0004 0.0004 0.0005 0.0007 0.0009 0.0011 0.0014 0.0019 671 12344000.21 Bitterroot River near Darby, Montana 1,050 Total MOVE.3 94 BP 1899–1901, 1903–04, 1930–2018 Yes 0.0005 0.0004 0.0004 0.0005 0.0007 0.0010 0.0013 0.0018 0.0025 680 12350250.20 Bitterroot River at Bell Crossing, near Victor, Montana 1,944 Total At-site 32 1987–2018 -- 0.0010 0.0010 0.0011 0.0014 0.0021 0.0028 0.0037 0.0049 0.0067

X7A23T 680 12350250.21 Bitterroot River at Bell Crossing, near Victor, Montana 1,944 Total MOVE.3 94 BP 1899–1901, 1903–04, 1930–2018 Yes 0.0002 0.0002 0.0002 0.0003 0.0004 0.0006 0.0008 0.0010 0.0016

X7A24T 683 12351200.00 Bitterroot River near Florence, Montana 2,342 U At-site 20 1958–65, 1972, 1974, 1982, 2003–11 Yes 0.0006 0.0006 0.0006 0.0007 0.0009 0.0012 0.0016 0.0021 0.0030

X7A25T 683 12351200.01 Bitterroot River near Florence, Montana 2,342 U MOVE.3 94 BP 1899–1901, 1903–04, 1930–2018 Yes 0.0002 0.0002 0.0002 0.0003 0.0005 0.0007 0.0010 0.0014 0.0022

X7A26T 687 12352500.00 Bitterroot River near Missoula, Montana 2,824 U At-site 34 1899–1901, 1903–04, 1990–2018 Yes 0.0009 0.0009 0.0009 0.0010 0.0013 0.0017 0.0022 0.0028 0.0038

X7A27T 687 12352500.01 Bitterroot River near Missoula, Montana 2,824 U MOVE.3 94 BP 1899–1901, 1903–04, 1930–2018 Yes 0.0003 0.0003 0.0003 0.0004 0.0006 0.0008 0.0011 0.0015 0.0022

X7A28T 688 12353000.00 Clark Fork below Missoula, Montana 9,017 U At-site 89 1930–2018 Yes 0.0004 0.0003 0.0003 0.0004 0.0005 0.0007 0.0009 0.0012 0.0016

X7A29T 688 12353000.01 Clark Fork below Missoula, Montana 9,017 U MOVE.3 113 BP 1899–1908, 1911–23, 1929–2018 Yes 0.0003 0.0003 0.0003 0.0004 0.0005 0.0007 0.0010 0.0013 0.0020

X7A30T 697 12354500.00 Clark Fork at St. Regis, Montana 10,728 U At-site 103 1911–23, 1929–2018 Yes 0.0003 0.0003 0.0003 0.0003 0.0004 0.0006 0.0008 0.0011 0.0016 697 12354500.01 Clark Fork at St. Regis, Montana 10,728 U MOVE.3 113 BP 1899–1908, 1911–23, 1929–2018 Yes 0.0003 0.0003 0.0003 0.0004 0.0005 0.0007 0.0009 0.0013 0.0018

3Abbreviations for type of frequency analysis are defined as follows: At-site: peak-flow frequency analysis on recorded data. RRE wtd: the at-site peak-flow frequency analysis was weighted with results from regional regression equations (RREs). MOVE.3: peak-flow frequency analysis on combined recorded and synthesized data; synthesized data from Maintenance of Variance Extension Type III (MOVE.3) record extension procedure. 12323800 Clark Fork near Galen, Montana 12323800 Clark Fork near Galen, Montana Analysis for unregulated period of record Analysis for unregulated period of record Analysis period of record, water years: 1989–2018 Analysis period of record, water years: 1989–2018 At-site peak-flow frequency analysis conducted on recorded data At-site peak-flow frequency analysis conducted on recorded data Table 1‐1 Table 1‐2 Table 1‐3 Table 1‐4 Table 1‐5 Table 1‐6 Table 1‐7 Table 1‐8 Table 1‐1 Table 1‐2 Table 1‐3 Table 1‐4 Table 1‐5 Table 1‐6 Table 1‐7 Table 1‐8 [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. PILF; potentially influential low flow; MGBT, multiple Grubbs-Beck test] [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends; --, not applicable or not available.] Peak-flow data Ranked (largest to smallest) peak-flow data Number of Number of Total number Contributing PILF Type of peak‐ recorded Skew type peak flows of years in Type of PILF flow Peak flow, in Gage Peak-flow Peak flow, in Gage Peak-flow drainage area, threshold, in Water 2 Water 2 peak flows used in 1 used for ungaged cubic feet per height, designation in cubic feet per height, designation in in square threshold cubic feet per frequency Date Date used in the analysis perception historical year 4 year 4 miles second second in feet analysis second in feet analysis analysis thresholds period(s) analysis 1989 03/10/1989 737 3.78 2011 06/13/2011 1,410 5.11 656 30 Weighted MGBT ------At-site 1990 05/30/1990 374 3.18 2018 06/19/2018 1,250 4.59 Peak flow, in cubic feet per second, for indicated annual exceedance probability (bold values), in percent 1991 06/07/1991 795 4.18 1997 06/07/1997 1,240 5.07 66.7 50 42.9 20 10 4 2 1.0 0.5 0.2 1992 06/18/1992 150 2.45 1995 06/07/1995 1,120 4.85 454 593 658 947 1,180 1,460 1,660 1,860 2,040 2,280 1993 05/18/1993 581 3.68 2010 06/17/2010 1,020 4.48 Upper and lower 90-percent confidence intervals, in cubic feet per second, for indicated annual exceedance probability, in percent 1994 05/13/1994 428 3.28 2009 06/01/2009 955 4.27 66.7 50 42.9 20 10 4 2 1.0 0.5 0.2 1995 06/07/1995 1,120 4.85 1996 06/09/1996 926 4.50 577 744 824 1,210 1,600 2,210 2,720 3,290 3,940 4,950 1996 06/09/1996 926 4.50 2003 06/01/2003 912 4.11 340 462 518 759 949 1,150 1,270 1,350 1,420 1,480 1997 06/07/1997 1,240 5.07 2017 06/14/2017 883 4.17 1998 07/04/1998 545 3.59 1991 06/07/1991 795 4.18 1999 05/30/1999 610 3.63 2008 06/05/2008 787 3.90 2000 05/31/2000 145 2.33 1989 03/10/1989 737 3.78 2001 05/16/2001 224 2.63 1999 05/30/1999 610 3.63 2002 06/02/2002 324 2.98 2007 06/06/2007 605 3.63 2003 06/01/2003 912 4.11 1993 05/18/1993 581 3.68 2004 06/11/2004 213 2.56 2012 06/06/2012 581 3.47 2005 06/18/2005 571 3.49 2005 06/18/2005 571 3.49 2006 05/21/2006 528 3.40 2014 05/27/2014 568 3.40 2007 06/06/2007 605 3.63 1998 07/04/1998 545 3.59 2008 06/05/2008 787 3.90 2006 05/21/2006 528 3.40 2009 06/01/2009 955 4.27 2015 05/17/2015 515 3.34 2010 06/17/2010 1,020 4.48 1994 05/13/1994 428 3.28 2011 06/13/2011 1,410 5.11 2016 06/09/2016 426 3.08 2012 06/06/2012 581 3.47 1990 05/30/1990 374 3.18 2013 05/30/2013 323 2.72 2002 06/02/2002 324 2.98 2014 05/27/2014 568 3.40 2013 05/30/2013 323 2.72 2015 05/17/2015 515 3.34 2001 05/16/2001 224 2.63 2016 06/09/2016 426 3.08 2004 06/11/2004 213 2.56 2017 06/14/2017 883 4.17 1992 06/18/1992 150 2.45 2018 06/19/2018 1,250 4.59 2000 05/31/2000 145 2.33

Figure 1. Annual peak flows (probability plotting positions) and peak-flow frequency curve. EMA, Expected Moments Algorithm; Skew(G), analysis skew; PILF(LO), potentially Influencial low flow (low outlier).

Figure 2. Annual peak flows and perception thresholds.

1Definitions of types of PILF thresholds include: MGBT: PILF threshold calculated by using the multiple Grubbs-Beck Test as specifed in Bulletin 17C (England and others, 2016); Manual: PILF threshold based on a systematic peak flow selected by the peak-flow frequency analyst.

Figure 2. Annual peak flows and perception thresholds.

Flow, in cubicfeetsecond per 1947 //1947 1,080 -- Synthesized 1989 03/09/1989 1,430 4.51 1948 //1948 2,130 -- Synthesized 1996 02/08/1996 1,400 4.46 1949 //1949 772 -- Synthesized 1907 //1907 1,340 -- Synthesized 1,000 1950 //1950 1,250 -- Synthesized 1938 //1938 1,330 -- Synthesized 1951 //1951 993 -- Synthesized 1972 //1972 1,320 -- Synthesized 1952 //1952 490 -- Synthesized 1904 //1904 1,270 -- Synthesized 1953 //1953 2,140 -- Synthesized 1950 //1950 1,250 -- Synthesized 1954 //1954 607 -- Synthesized 1995 06/07/1995 1,240 4.17 1955 //1955 746 -- Synthesized 1970 //1970 1,200 -- Synthesized 1956 //1956 928 -- Synthesized 1998 07/04/1998 1,200 4.23 1957 //1957 1,050 -- Synthesized 1983 07/10/1983 1,190 4.09 1958 //1958 1,060 -- Synthesized 2014 03/06/2014 1,190 4.13 100

99 98 95 90 80 70 60 50 40 30 20 10 5 2 1 1959 //1959 1,070 -- Synthesized 2009 06/02/2009 1,180 4.23 99.5 0.5 0.2 1960 //1960 746 -- Synthesized 2007 06/07/2007 1,130 4.18 Exceedance probability, in percent 1961 //1961 814 -- Synthesized 1947 //1947 1,080 -- Synthesized 1962 //1962 783 -- Synthesized 1959 //1959 1,070 -- Synthesized Figure 1. Annual peak flows (plotting positions determined using the Cunnane formulation; Helsel and HIrsch, 2002) and peak-flow frequencycurve 1963 //1963 665 -- Synthesized 1958 //1958 1,060 -- Synthesized 1964 //1964 1,670 -- Synthesized 2003 06/01/2003 1,060 4.11 1965 //1965 1,460 -- Synthesized 1957 //1957 1,050 -- Synthesized 1966 //1966 324 -- Synthesized 1934 //1934 1,040 -- Synthesized 1967 //1967 1,020 -- Synthesized 1967 //1967 1,020 -- Synthesized 1Peak flows with a value of zero are not plotted in figure 1 . 1968 //1968 820 -- Synthesized 1991 06/08/1991 1,020 4.02 2In cases where the month, day, or both are not present in the date of a peak flow (as indicated by adjacent slash marks with no intervening values), 1969 //1969 556 -- Synthesized 2008 06/05/2008 1,020 4.11 the month, day, or both are unknown. 1970 //1970 1,200 -- Synthesized 1951 //1951 993 -- Synthesized 3Flood-frequency results not reported because of too many values less than the PILF threshold used in the at-site analysis. 1971 //1971 704 -- Synthesized 1956 //1956 928 -- Synthesized 4Definitions of peak-flow designations used in analysis include: 1972 //1972 1,320 -- Synthesized 2005 06/18/2005 848 3.90 PT definition: The peak flow is used to define perception thresholds in ungaged historical periods; 1973 //1973 723 -- Synthesized 2012 06/06/2012 840 3.80 Opportunistic: The peak flow was excluded from the analysis because it is outside of the systematic record and was of insufficient magnitude to 1974 //1974 2,260 -- Synthesized 1944 //1944 835 -- Synthesized determine nonexceedance during an ungaged period; 1975 //1975 2,990 -- Synthesized 1932 //1932 827 -- Synthesized PILF: The peak flow was identified as a potentially influential low flow; Synthesized: The peak flow was synthesized using Maintenance of Variance Extension Type III record extension. 1976 //1976 2,730 -- Synthesized 1968 //1968 820 -- Synthesized England, J.F., Jr., Cohn, T.A., Faber, B.A., Stedinger, J.R., Thomas, W.O., Jr., Veilleux, A.G., Kiang, J.E., and Mason, R.R., Jr., 2019, Guidelines 1977 //1977 503 -- Synthesized 1999 06/04/1999 819 3.83 for determining flood flow frequency—Bulletin 17C (ver. 1.1, May 2019): U.S. Geological Survey Techniques and Methods, book 4, chap. B5, 148 1978 //1978 1,430 -- Synthesized 1961 //1961 814 -- Synthesized p., https://doi.org/10.3133/tm4B5. 1979 05/25/1979 697 3.34 1936 //1936 810 -- Synthesized 1980 05/26/1980 1,710 4.58 1962 //1962 783 -- Synthesized 1981 05/23/1981 2,500 5.35 1949 //1949 772 -- Synthesized Helsel,D.R.,andHirsch,R.M.,2002,Statisticalmethodsinwaterresources:U.S.GeologicalSur-veyTechniquesofWater- 1982 06/25/1982 1,450 4.39 1955 //1955 746 -- Synthesized ResourcesInvestigations,book4,chap.A3,522p.,accessedOctober30,2017,athttps://pubs.usgs.gov/twri/twri4a3/. 1983 07/10/1983 1,190 4.09 1960 //1960 746 -- Synthesized 1984 06/22/1984 1,730 4.67 1973 //1973 723 -- Synthesized 1985 05/04/1985 492 3.16 1971 //1971 704 -- Synthesized 1986 02/25/1986 2,090 5.20 1979 05/25/1979 697 3.34 1987 05/28/1987 463 3.33 1963 //1963 665 -- Synthesized 1988 04/22/1988 409 3.20 2006 06/10/2006 654 3.66 1989 03/09/1989 1,430 4.51 2015 06/02/2015 647 3.54 1990 05/31/1990 507 3.34 1935 //1935 622 -- Synthesized 1991 06/08/1991 1,020 4.02 1993 06/17/1993 613 3.54 1992 11/05/1991 367 3.15 1954 //1954 607 -- Synthesized 1993 06/17/1993 613 3.54 1945 //1945 586 -- Synthesized 1994 05/13/1994 462 3.27 1969 //1969 556 -- Synthesized 1995 06/07/1995 1,240 4.17 1946 //1946 548 -- Synthesized 1996 02/08/1996 1,400 4.46 1930 //1930 537 -- Synthesized 1997 06/14/1997 2,020 5.07 1939 //1939 525 -- Synthesized 1998 07/04/1998 1,200 4.23 2016 05/21/2016 514 3.29 1999 06/04/1999 819 3.83 1990 05/31/1990 507 3.34 2000 11/26/1999 263 2.95 1977 //1977 503 -- Synthesized 2001 06/04/2001 310 3.04 1985 05/04/1985 492 3.16 2002 06/10/2002 461 3.34 1952 //1952 490 -- Synthesized 2003 06/01/2003 1,060 4.11 1905 //1905 484 -- Synthesized 2004 03/10/2004 286 2.98 1987 05/28/1987 463 3.33 2005 06/18/2005 848 3.90 1994 05/13/1994 462 3.27 2006 06/10/2006 654 3.66 2002 06/10/2002 461 3.34 2007 06/07/2007 1,130 4.18 1937 //1937 432 -- Synthesized 2008 06/05/2008 1,020 4.11 1906 //1906 428 -- Synthesized 2009 06/02/2009 1,180 4.23 1941 //1941 425 -- Synthesized 2010 06/17/2010 1,540 4.68 1940 //1940 420 -- Synthesized 2011 06/14/2011 1,970 5.02 1988 04/22/1988 409 3.20 2012 06/06/2012 840 3.80 2013 05/30/2013 381 3.09 2013 05/30/2013 381 3.09 1992 11/05/1991 367 3.15 2014 03/06/2014 1,190 4.13 1966 //1966 324 -- Synthesized 2015 06/02/2015 647 3.54 2001 06/04/2001 310 3.04 2016 05/21/2016 514 3.29 2004 03/10/2004 286 2.98 2017 06/14/2017 1,710 4.81 2000 11/26/1999 263 2.95 2018 06/22/2018 2,290 5.38 1931 //1931 189 -- Synthesized 12324400 Clark Fork above Little Blackfoot River near Garrison, Montana 12324400 Clark Fork above Little Blackfoot River near Garrison, Montana Analysis for unregulated period of record Analysis for unregulated period of record Analysis period of record, water years: 2009–18 Analysis period of record, water years: 2009–18 At-site peak-flow frequency analysis conducted on recorded data At-site peak-flow frequency analysis conducted on recorded data Table 1‐1 Table 1‐2 Table 1‐3 Table 1‐4 Table 1‐5 Table 1‐6 Table 1‐7 Table 1‐8 Table 1‐1 Table 1‐2 Table 1‐3 Table 1‐4 Table 1‐5 Table 1‐6 Table 1‐7 Table 1‐8 [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. PILF; potentially influential low flow; MGBT, multiple Grubbs-Beck test; -- not applicable or not available] [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends; --, not applicable or not available.] Peak-flow data Ranked (largest to smallest) peak-flow data Number of Number of Total number Contributing PILF Type of peak‐ recorded Skew type peak flows of years in Type of PILF flow Peak flow, in Gage Peak-flow Peak flow, in Gage Peak-flow drainage area, threshold, in Water 2 Water 2 peak flows used in 1 used for ungaged cubic feet per height, designation in cubic feet per height, designation in in square threshold cubic feet per frequency Date Date used in the analysis perception historical year 4 year 4 miles second second in feet analysis second in feet analysis analysis thresholds period(s) analysis 2009 06/01/2009 1,450 5.69 2011 06/10/2011 2,690 7.37 1,130 10 Weighted MGBT -- At-site 2010 06/18/2010 2,030 6.67 2018 06/20/2018 2,550 7.24 Peak flow, in cubic feet per second, for indicated annual exceedance probability (bold values), in percent 2011 06/10/2011 2,690 7.37 2010 06/18/2010 2,030 6.67 66.7 50 42.9 20 10 4 2 1.0 0.5 0.2 2012 06/06/2012 1,080 5.13 2017 06/14/2017 1,880 6.72 972 1,270 1,420 2,100 2,710 3,530 4,180 4,840 5,530 6,490 2013 05/31/2013 426 3.74 2009 06/01/2009 1,450 5.69 Upper and lower 90-percent confidence intervals, in cubic feet per second, for indicated annual exceedance probability, in percent 2014 06/28/2014 1,020 5.02 2012 06/06/2012 1,080 5.13 66.7 50 42.9 20 10 4 2 1.0 0.5 0.2 2015 06/03/2015 826 4.64 2014 06/28/2014 1,020 5.02 1,460 1,940 2,190 3,730 5,880 10,400 15,500 22,600 30,600 45,300 2016 05/27/2016 628 4.40 2015 06/03/2015 826 4.64 570 804 913 1,400 1,800 2,290 2,610 2,900 3,150 3,440 2017 06/14/2017 1,880 6.72 2016 05/27/2016 628 4.40 2018 06/20/2018 2,550 7.24 2013 05/31/2013 426 3.74

Figure 2. Annual peak flows and perception thresholds.

Exceedance probability, in percent 1979 //1979 1,230 -- Synthesized 2007 //2007 1,300 -- Synthesized 1980 //1980 2,720 -- Synthesized 1963 //1963 1,270 -- Synthesized Figure 1. Annual peak flows (plotting positions determined using the Cunnane formulation; Helsel and HIrsch, 2002) and peak-flow frequencycurve 1981 //1981 5,590 -- Historic; synthesized 1979 //1979 1,230 -- Synthesized 1982 //1982 1,660 -- Synthesized 1952 //1952 1,210 -- Synthesized 1983 //1983 1,300 -- Synthesized 1959 //1959 1,180 -- Synthesized 1984 //1984 1,860 -- Synthesized 1999 //1999 1,170 -- Synthesized 1985 //1985 825 -- Synthesized 1961 //1961 1,160 -- Synthesized 1Peak flows with a value of zero are not plotted in figure 1 . 1986 //1986 3,040 -- Synthesized 1978 //1978 1,120 -- Synthesized 2In cases where the month, day, or both are not present in the date of a peak flow (as indicated by adjacent slash marks with no intervening values), 1987 //1987 496 -- PILF; synthesized 1960 //1960 1,110 -- Synthesized the month, day, or both are unknown. 1988 //1988 540 -- PILF; synthesized 1955 //1955 1,090 -- Synthesized 3Flood-frequency results not reported because of too many values less than the PILF threshold used in the at-site analysis. 1989 //1989 1,440 -- Synthesized 1991 //1991 1,080 -- Synthesized 4Definitions of peak-flow designations used in analysis include: 1990 //1990 912 -- Synthesized 2012 06/06/2012 1,080 5.13 PT definition: The peak flow is used to define perception thresholds in ungaged historical periods; 1991 //1991 1,080 -- Synthesized 1969 //1969 1,070 -- Synthesized Opportunistic: The peak flow was excluded from the analysis because it is outside of the systematic record and was of insufficient magnitude to 1992 //1992 242 -- PILF; synthesized 2006 //2006 1,020 -- Synthesized determine nonexceedance during an ungaged period; 1993 //1993 796 -- Synthesized 2014 06/28/2014 1,020 5.02 PILF: The peak flow was identified as a potentially influential low flow; 1994 //1994 644 -- PILF; synthesized 1962 //1962 988 -- Synthesized England, J.F., Jr., Cohn, T.A., Faber, B.A., Stedinger, J.R., Thomas, W.O., Jr., Veilleux, A.G., Kiang, J.E., and Mason, R.R., Jr., 2019, Guidelines 1995 //1995 1,980 -- Synthesized 1968 //1968 955 -- Synthesized for determining flood flow frequency—Bulletin 17C (ver. 1.1, May 2019): U.S. Geological Survey Techniques and Methods, book 4, chap. B5, 148 1996 //1996 3,410 -- Synthesized 1990 //1990 912 -- Synthesized p., https://doi.org/10.3133/tm4B5. 1997 //1997 1,920 -- Synthesized 2015 06/03/2015 826 4.64 1998 //1998 1,650 -- Synthesized 1985 //1985 825 -- Synthesized 1999 //1999 1,170 -- Synthesized 1993 //1993 796 -- Synthesized Helsel,D.R.,andHirsch,R.M.,2002,Statisticalmethodsinwaterresources:U.S.GeologicalSur-veyTechniquesofWater- 2000 //2000 290 -- PILF; synthesized 2004 //2004 732 -- Synthesized ResourcesInvestigations,book4,chap.A3,522p.,accessedOctober30,2017,athttps://pubs.usgs.gov/twri/twri4a3/. 2001 //2001 629 -- PILF; synthesized 1939 //1939 708 -- Synthesized 2002 //2002 654 -- PILF; synthesized 2002 //2002 654 -- PILF; synthesi 2003 //2003 2,050 -- Synthesized 1994 //1994 644 -- PILF; synthesi 2004 //2004 732 -- Synthesized 1944 //1944 632 -- PILF; synthesi 2005 //2005 1,550 -- Synthesized 2001 //2001 629 -- PILF; synthesi 2006 //2006 1,020 -- Synthesized 2016 05/27/2016 628 4.40 PILF 2007 //2007 1,300 -- Synthesized 1945 //1945 608 -- PILF; synthesi 2008 //2008 1,350 -- Synthesized 1966 //1966 553 -- PILF; synthesi 2009 06/01/2009 1,450 5.69 1988 //1988 540 -- PILF; synthesi 2010 06/18/2010 2,030 6.67 1941 //1941 503 -- PILF; synthesi 2011 06/10/2011 2,690 7.37 1987 //1987 496 -- PILF; synthesi 2012 06/06/2012 1,080 5.13 1946 //1946 479 -- PILF; synthesi 2013 05/31/2013 426 3.74 PILF 2013 05/31/2013 426 3.74 PILF 2014 06/28/2014 1,020 5.02 1973 //1973 407 -- PILF; synthesi 2015 06/03/2015 826 4.64 1977 //1977 399 -- PILF; synthesi 2016 05/27/2016 628 4.40 PILF 1940 //1940 377 -- PILF; synthesi 2017 06/14/2017 1,880 6.72 2000 //2000 290 -- PILF; synthesi 2018 06/20/2018 2,550 7.24 1992 //1992 242 -- PILF; synthesi 12324680 Clark Fork at Goldcreek, Montana 12324680 Clark Fork at Goldcreek, Montana Analysis for unregulated period of record Analysis for unregulated period of record Analysis period of record, water years: 1978–2018 Analysis period of record, water years: 1978–2018 At-site peak-flow frequency analysis conducted on recorded data At-site peak-flow frequency analysis conducted on recorded data Table 1‐1 Table 1‐2 Table 1‐3 Table 1‐4 Table 1‐5 Table 1‐6 Table 1‐7 Table 1‐8 Table 1‐1 Table 1‐2 Table 1‐3 Table 1‐4 Table 1‐5 Table 1‐6 Table 1‐7 Table 1‐8 [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. PILF; potentially influential low flow; MGBT, multiple Grubbs-Beck test] [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends; --, not applicable or not available.] Peak-flow data Ranked (largest to smallest) peak-flow data Number of Number of Total number Contributing PILF Type of peak‐ recorded Skew type peak flows of years in Type of PILF flow Peak flow, in Gage Peak-flow Peak flow, in Gage Peak-flow drainage area, threshold, in Water 2 Water 2 peak flows used in 1 used for ungaged cubic feet per height, designation in cubic feet per height, designation in in square threshold cubic feet per frequency Date Date used in the analysis perception historical year 4 year 4 miles second second in feet analysis second in feet analysis analysis thresholds period(s) analysis 1978 05/24/1978 2,230 -- 1981 05/22/1981 12,000 11.17 1,774 41 Station Manual 941 At-site 1979 05/27/1979 2,470 6.41 1996 02/09/1996 7,160 8.95 Peak flow, in cubic feet per second, for indicated annual exceedance probability (bold values), in percent 1980 05/25/1980 5,640 8.25 2011 06/10/2011 6,410 8.97 66.7 50 42.9 20 10 4 2 1.0 0.5 0.2 1981 05/22/1981 12,000 11.17 1986 02/24/1986 6,350 8.50 1,800 2,380 2,670 4,170 5,640 7,840 9,740 11,900 14,300 17,900 1982 05/28/1982 3,370 7.09 1980 05/25/1980 5,640 8.25 Upper and lower 90-percent confidence intervals, in cubic feet per second, for indicated annual exceedance probability, in percent 1983 07/10/1983 2,600 6.85 2018 06/20/2018 5,320 8.45 66.7 50 42.9 20 10 4 2 1.0 0.5 0.2 1984 06/21/1984 3,800 7.42 2003 03/13/2003 4,200 7.78 2,240 2,980 3,370 5,550 8,370 15,100 24,600 38,900 57,100 94,800 1985 04/02/1985 1,620 6.04 2010 06/18/2010 4,160 7.76 1,430 1,910 2,140 3,300 4,380 5,830 6,940 8,040 9,140 10,600 1986 02/24/1986 6,350 8.50 1995 06/06/1995 4,060 7.76 1987 03/05/1987 951 5.38 1997 06/11/1997 3,920 7.63 1988 05/31/1988 1,040 5.55 1984 06/21/1984 3,800 7.42 1989 04/07/1989 2,900 6.99 2017 06/14/2017 3,530 7.43 1990 06/01/1990 1,800 6.31 1982 05/28/1982 3,370 7.09 1991 06/08/1991 2,140 6.52 1998 07/04/1998 3,350 7.36 1992 03/18/1992 448 4.83 PILF 2005 06/04/2005 3,140 7.25 1993 05/21/1993 1,560 6.19 1989 04/07/1989 2,900 6.99 1994 04/25/1994 1,250 5.91 2008 06/05/2008 2,720 7.01 1995 06/06/1995 4,060 7.76 2009 05/25/2009 2,700 7.13 1996 02/09/1996 7,160 8.95 2007 06/07/2007 2,610 7.04 1997 06/11/1997 3,920 7.63 1983 07/10/1983 2,600 6.85 1998 07/04/1998 3,350 7.36 1979 05/27/1979 2,470 6.41 1999 06/04/1999 2,330 6.64 1999 06/04/1999 2,330 6.64 2000 02/02/2000 543 4.90 PILF 1978 05/24/1978 2,230 -- 2001 06/05/2001 1,220 5.87 1991 06/08/1991 2,140 6.52 2002 06/11/2002 1,270 5.85 2014 05/25/2014 2,070 6.40 2003 03/13/2003 4,200 7.78 2006 06/10/2006 2,030 6.52 2004 03/08/2004 1,430 5.98 2012 04/27/2012 2,000 6.34 2005 06/04/2005 3,140 7.25 1990 06/01/1990 1,800 6.31 2006 06/10/2006 2,030 6.52 1985 04/02/1985 1,620 6.04 2007 06/07/2007 2,610 7.04 2015 06/03/2015 1,600 5.95 2008 06/05/2008 2,720 7.01 1993 05/21/1993 1,560 6.19 2009 05/25/2009 2,700 7.13 2004 03/08/2004 1,430 5.98 2010 06/18/2010 4,160 7.76 2002 06/11/2002 1,270 5.85 2011 06/10/2011 6,410 8.97 1994 04/25/1994 1,250 5.91 2012 04/27/2012 2,000 6.34 2001 06/05/2001 1,220 5.87 2013 06/04/2013 941 5.35 2016 05/22/2016 1,140 5.60 2014 05/25/2014 2,070 6.40 1988 05/31/1988 1,040 5.55 2015 06/03/2015 1,600 5.95 1987 03/05/1987 951 5.38 2016 05/22/2016 1,140 5.60 2013 06/04/2013 941 5.35 2017 06/14/2017 3,530 7.43 2000 02/02/2000 543 4.90 PILF 2018 06/20/2018 5,320 8.45 1992 03/18/1992 448 4.83 PILF

Figure 2. Annual peak flows and perception thresholds.

Peaks used in at-site analysis 1933 //1933 4,680 -- Synthesized 1965 //1965 5,470 -- Synthesized 1934 //1934 2,850 -- Synthesized 1948 //1948 5,340 -- Synthesized Potentially influential low flow (PILF) 1935 //1935 1,610 -- PILF; synthesized 2018 06/20/2018 5,320 8.45 1936 //1936 2,160 -- Synthesized 1950 //1950 5,200 -- Synthesized Weighted peak-flow frequency 1937 //1937 1,080 -- PILF; synthesized 1947 //1947 5,010 -- Synthesized curve 1938 //1938 3,110 -- Synthesized 1933 //1933 4,680 -- Synthesized Upper and lower 95-percent 1939 //1939 1,360 -- PILF; synthesized 1942 //1942 4,620 -- Synthesized confidence intervals 10,000 1940 //1940 647 -- PILF; synthesized 1943 //1943 4,430 -- Synthesized 1941 //1941 909 -- PILF; synthesized 1956 //1956 4,430 -- Synthesized 1942 //1942 4,620 -- Synthesized 1958 //1958 4,430 -- Synthesized 1943 //1943 4,430 -- Synthesized 1970 //1970 4,370 -- Synthesized 1944 //1944 1,190 -- PILF; synthesized 1901 //1901 4,250 -- Synthesized 1945 //1945 1,140 -- PILF; synthesized 2003 03/13/2003 4,200 7.78 1946 //1946 858 -- PILF; synthesized 1951 //1951 4,190 -- Synthesized 1947 //1947 5,010 -- Synthesized 1976 //1976 4,180 -- Synthesized Flow, in cubicfeetsecond per 1948 //1948 5,340 -- Synthesized 2010 06/18/2010 4,160 7.76 1949 //1949 3,160 -- Synthesized 1974 //1974 4,130 -- Synthesized 1,000 1950 //1950 5,200 -- Synthesized 1995 06/06/1995 4,060 7.76 1951 //1951 4,190 -- Synthesized 1964 //1964 4,010 -- Synthesized 1952 //1952 2,560 -- Synthesized 1997 06/11/1997 3,920 7.63 1953 //1953 5,810 -- Synthesized 1984 06/21/1984 3,800 7.42 1954 //1954 3,150 -- Synthesized 1907 //1907 3,740 -- Synthesized 1955 //1955 2,270 -- Synthesized 1904 //1904 3,540 -- Synthesized 1956 //1956 4,430 -- Synthesized 2017 06/14/2017 3,530 7.43 1957 //1957 3,150 -- Synthesized 1967 //1967 3,450 -- Synthesized 1958 //1958 4,430 -- Synthesized 1982 05/28/1982 3,370 7.09 100 1959 //1959 2,490 -- Synthesized 1998 07/04/1998 3,350 7.36 99.5 99 98 95 90 80 70 60 50 40 30 20 10 5 2 1 0.5 0.2 1960 //1960 2,330 -- Synthesized 1949 //1949 3,160 -- Synthesized 1961 //1961 2,440 -- Synthesized 1954 //1954 3,150 -- Synthesized Exceedance probability, in percent 1962 //1962 2,020 -- Synthesized 1957 //1957 3,150 -- Synthesized Figure 1. Annual peak flows (plotting positions determined using the Cunnane formulation; Helsel and HIrsch, 2002) and peak-flow frequencycurve 1963 //1963 2,710 -- Synthesized 2005 06/04/2005 3,140 7.25 1964 //1964 4,010 -- Synthesized 1938 //1938 3,110 -- Synthesized 1965 //1965 5,470 -- Synthesized 1989 04/07/1989 2,900 6.99 1966 //1966 1,020 -- PILF; synthesized 1971 //1971 2,880 -- Synthesized 1967 //1967 3,450 -- Synthesized 1934 //1934 2,850 -- Synthesized 1Peak flows with a value of zero are not plotted in figure 1 . 1968 //1968 1,940 -- Synthesized 2008 06/05/2008 2,720 7.01 2In cases where the month, day, or both are not present in the date of a peak flow (as indicated by adjacent slash marks with no intervening values), 1969 //1969 2,210 -- Synthesized 1963 //1963 2,710 -- Synthesized the month, day, or both are unknown. 1970 //1970 4,370 -- Synthesized 2009 05/25/2009 2,700 7.13 3Flood-frequency results not reported because of too many values less than the PILF threshold used in the at-site analysis. 1971 //1971 2,880 -- Synthesized 2007 06/07/2007 2,610 7.04 4Definitions of peak-flow designations used in analysis include: 1972 //1972 6,970 -- Synthesized 1983 07/10/1983 2,600 6.85 PT definition: The peak flow is used to define perception thresholds in ungaged historical periods; 1973 //1973 663 -- PILF; synthesized 1952 //1952 2,560 -- Synthesized Opportunistic: The peak flow was excluded from the analysis because it is outside of the systematic record and was of insufficient magnitude to 1974 //1974 4,130 -- Synthesized 1959 //1959 2,490 -- Synthesized determine nonexceedance during an ungaged period; 1975 //1975 6,770 -- Synthesized 1979 05/27/1979 2,470 6.41 PILF: The peak flow was identified as a potentially influential low flow; Synthesized: The peak flow was synthesized using Maintenance of Variance Extension Type III record extension. 1976 //1976 4,180 -- Synthesized 1961 //1961 2,440 -- Synthesized England, J.F., Jr., Cohn, T.A., Faber, B.A., Stedinger, J.R., Thomas, W.O., Jr., Veilleux, A.G., Kiang, J.E., and Mason, R.R., Jr., 2019, Guidelines 1977 //1977 649 -- PILF; synthesized 1960 //1960 2,330 -- Synthesized for determining flood flow frequency—Bulletin 17C (ver. 1.1, May 2019): U.S. Geological Survey Techniques and Methods, book 4, chap. B5, 148 1978 05/24/1978 2,230 -- 1999 06/04/1999 2,330 6.64 p., https://doi.org/10.3133/tm4B5. 1979 05/27/1979 2,470 6.41 1955 //1955 2,270 -- Synthesized 1980 05/25/1980 5,640 8.25 1978 05/24/1978 2,230 -- 1981 05/22/1981 12,000 11.17 1932 //1932 2,210 -- Synthesized Helsel,D.R.,andHirsch,R.M.,2002,Statisticalmethodsinwaterresources:U.S.GeologicalSur-veyTechniquesofWater- 1982 05/28/1982 3,370 7.09 1969 //1969 2,210 -- Synthesized ResourcesInvestigations,book4,chap.A3,522p.,accessedOctober30,2017,athttps://pubs.usgs.gov/twri/twri4a3/. 1983 07/10/1983 2,600 6.85 1936 //1936 2,160 -- Synthesized 1984 06/21/1984 3,800 7.42 1991 06/08/1991 2,140 6.52 1985 04/02/1985 1,620 6.04 PILF 2014 05/25/2014 2,070 6.40 1986 02/24/1986 6,350 8.50 2006 06/10/2006 2,030 6.52 1987 03/05/1987 951 5.38 PILF 1962 //1962 2,020 -- Synthesized 1988 05/31/1988 1,040 5.55 PILF 2012 04/27/2012 2,000 6.34 1989 04/07/1989 2,900 6.99 1968 //1968 1,940 -- Synthesized 1990 06/01/1990 1,800 6.31 PILF 1990 06/01/1990 1,800 6.31 PILF 1991 06/08/1991 2,140 6.52 1985 04/02/1985 1,620 6.04 PILF 1992 03/18/1992 448 4.83 PILF 1935 //1935 1,610 -- PILF; synthesi 1993 05/21/1993 1,560 6.19 PILF 2015 06/03/2015 1,600 5.95 PILF 1994 04/25/1994 1,250 5.91 PILF 1993 05/21/1993 1,560 6.19 PILF 1995 06/06/1995 4,060 7.76 2004 03/08/2004 1,430 5.98 PILF 1996 02/09/1996 7,160 8.95 1930 //1930 1,370 -- PILF; synthesi 1997 06/11/1997 3,920 7.63 1939 //1939 1,360 -- PILF; synthesi 1998 07/04/1998 3,350 7.36 2002 06/11/2002 1,270 5.85 PILF 1999 06/04/1999 2,330 6.64 1994 04/25/1994 1,250 5.91 PILF 2000 02/02/2000 543 4.90 PILF 1905 //1905 1,220 -- PILF; synthesi 2001 06/05/2001 1,220 5.87 PILF 2001 06/05/2001 1,220 5.87 PILF 2002 06/11/2002 1,270 5.85 PILF 1944 //1944 1,190 -- PILF; synthesi 2003 03/13/2003 4,200 7.78 1945 //1945 1,140 -- PILF; synthesi 2004 03/08/2004 1,430 5.98 PILF 2016 05/22/2016 1,140 5.60 PILF 2005 06/04/2005 3,140 7.25 1937 //1937 1,080 -- PILF; synthesi 2006 06/10/2006 2,030 6.52 1906 //1906 1,070 -- PILF; synthesi 2007 06/07/2007 2,610 7.04 1988 05/31/1988 1,040 5.55 PILF 2008 06/05/2008 2,720 7.01 1966 //1966 1,020 -- PILF; synthesi 2009 05/25/2009 2,700 7.13 1987 03/05/1987 951 5.38 PILF 2010 06/18/2010 4,160 7.76 2013 06/04/2013 941 5.35 PILF 2011 06/10/2011 6,410 8.97 1941 //1941 909 -- PILF; synthesi 2012 04/27/2012 2,000 6.34 1946 //1946 858 -- PILF; synthesi 2013 06/04/2013 941 5.35 PILF 1973 //1973 663 -- PILF; synthesi 2014 05/25/2014 2,070 6.40 1977 //1977 649 -- PILF; synthesi 2015 06/03/2015 1,600 5.95 PILF 1940 //1940 647 -- PILF; synthesi 2016 05/22/2016 1,140 5.60 PILF 2000 02/02/2000 543 4.90 PILF 2017 06/14/2017 3,530 7.43 1992 03/18/1992 448 4.83 PILF 2018 06/20/2018 5,320 8.45 1931 //1931 437 -- PILF; synthesi 12331800 Clark Fork near Drummond, Montana 12331800 Clark Fork near Drummond, Montana Analysis for unregulated period of record Analysis for unregulated period of record Analysis period of record, water years: 1967; 1973–90; 1992–2018 Analysis period of record, water years: 1967; 1973–90; 1992–2018 At-site peak-flow frequency analysis conducted on recorded data At-site peak-flow frequency analysis conducted on recorded data Table 1‐1 Table 1‐2 Table 1‐3 Table 1‐4 Table 1‐5 Table 1‐6 Table 1‐7 Table 1‐8 Table 1‐1 Table 1‐2 Table 1‐3 Table 1‐4 Table 1‐5 Table 1‐6 Table 1‐7 Table 1‐8 [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. PILF; potentially influential low flow; MGBT, multiple Grubbs-Beck test] [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends; --, not applicable or not available.] Peak-flow data Ranked (largest to smallest) peak-flow data Number of Number of Total number Contributing PILF Type of peak‐ recorded Skew type peak flows of years in Type of PILF flow Peak flow, in Gage Peak-flow Peak flow, in Gage Peak-flow drainage area, threshold, in Water 2 Water 2 peak flows used in 1 used for ungaged cubic feet per height, designation in cubic feet per height, designation in in square threshold cubic feet per frequency Date Date used in the analysis perception historical year 4 year 4 miles second second in feet analysis second in feet analysis analysis thresholds period(s) analysis 1967 06/14/1967 4,350 8.14 Opportunistic 1981 05/23/1981 15,800 12.44 2,516 46 Station Manual 1,130 At-site 1973 12/23/1972 850 7.86 PILF 1996 02/09/1996 9,800 10.03 Peak flow, in cubic feet per second, for indicated annual exceedance probability (bold values), in percent 1974 01/16/1974 5,200 9.70 1986 02/25/1986 8,760 9.61 66.7 50 42.9 20 10 4 2 1.0 0.5 0.2 1975 06/20/1975 8,490 10.60 1975 06/20/1975 8,490 10.60 2,200 3,040 3,460 5,580 7,580 10,400 12,800 15,200 17,900 21,600 1976 05/12/1976 5,270 8.70 2011 06/10/2011 8,220 9.79 Upper and lower 90-percent confidence intervals, in cubic feet per second, for indicated annual exceedance probability, in percent 1977 02/13/1977 832 -- PILF 2018 06/21/2018 6,260 8.76 66.7 50 42.9 20 10 4 2 1.0 0.5 0.2 1978 06/10/1978 2,800 6.88 1980 05/26/1980 6,030 9.58 2,790 3,860 4,410 7,330 10,800 18,100 27,000 39,900 58,600 92,500 1979 05/27/1979 2,710 6.67 2017 06/14/2017 5,900 8.39 1,660 2,390 2,730 4,390 5,900 7,880 9,310 10,700 11,900 13,400 1980 05/26/1980 6,030 9.58 1984 06/22/1984 5,540 8.09 1981 05/23/1981 15,800 12.44 1976 05/12/1976 5,270 8.70 1982 06/18/1982 3,710 7.55 1974 01/16/1974 5,200 9.70 1983 07/11/1983 2,990 6.89 1997 06/12/1997 5,000 7.71 1984 06/22/1984 5,540 8.09 1998 06/27/1998 4,830 7.34 1985 04/03/1985 1,590 6.04 2003 03/14/2003 4,580 7.71 1986 02/25/1986 8,760 9.61 1989 03/11/1989 4,540 7.70 1987 05/29/1987 1,220 5.60 1995 02/01/1995 4,490 7.55 1988 05/31/1988 1,420 5.88 1967 06/14/1967 4,350 8.14 Opportunistic 1989 03/11/1989 4,540 7.70 2010 06/18/2010 4,180 7.67 1990 06/01/1990 2,480 6.57 2009 05/26/2009 3,730 7.02 1992 03/19/1992 664 4.75 PILF 1982 06/18/1982 3,710 7.55 1993 06/17/1993 1,910 5.26 2005 06/04/2005 3,550 6.77 1994 04/25/1994 1,840 5.15 2007 06/07/2007 3,360 6.89 1995 02/01/1995 4,490 7.55 2008 06/05/2008 3,190 6.86 1996 02/09/1996 9,800 10.03 1983 07/11/1983 2,990 6.89 1997 06/12/1997 5,000 7.71 2014 03/11/2014 2,980 6.38 1998 06/27/1998 4,830 7.34 1999 06/04/1999 2,950 6.02 1999 06/04/1999 2,950 6.02 1978 06/10/1978 2,800 6.88 2000 02/02/2000 905 3.97 PILF 1979 05/27/1979 2,710 6.67 2001 06/05/2001 1,900 5.33 2006 06/11/2006 2,710 6.08 2002 06/11/2002 1,490 4.71 1990 06/01/1990 2,480 6.57 2003 03/14/2003 4,580 7.71 2012 04/27/2012 2,260 5.81 2004 03/09/2004 1,850 5.24 1993 06/17/1993 1,910 5.26 2005 06/04/2005 3,550 6.77 2001 06/05/2001 1,900 5.33 2006 06/11/2006 2,710 6.08 2004 03/09/2004 1,850 5.24 2007 06/07/2007 3,360 6.89 1994 04/25/1994 1,840 5.15 2008 06/05/2008 3,190 6.86 1985 04/03/1985 1,590 6.04 2009 05/26/2009 3,730 7.02 2015 06/03/2015 1,520 4.60 2010 06/18/2010 4,180 7.67 2002 06/11/2002 1,490 4.71 2011 06/10/2011 8,220 9.79 1988 05/31/1988 1,420 5.88 2012 04/27/2012 2,260 5.81 2016 05/28/2016 1,350 4.38 2013 06/04/2013 1,130 4.16 1987 05/29/1987 1,220 5.60 2014 03/11/2014 2,980 6.38 2013 06/04/2013 1,130 4.16 2015 06/03/2015 1,520 4.60 2000 02/02/2000 905 3.97 PILF 2016 05/28/2016 1,350 4.38 1973 12/23/1972 850 7.86 PILF 2017 06/14/2017 5,900 8.39 1977 02/13/1977 832 -- PILF 2018 06/21/2018 6,260 8.76 1992 03/19/1992 664 4.75 PILF

Figure 2. Annual peak flows and perception thresholds.

Weighted peak-flow frequency 1918 //1918 5,830 -- Synthesized 1964 //1964 6,100 -- Synthesized curve 1919 //1919 2,160 -- PILF; synthesized 1980 05/26/1980 6,030 9.58 Upper and lower 95-percent 1920 //1920 2,590 -- Synthesized 2017 06/14/2017 5,900 8.39 10,000 confidence intervals 1921 //1921 4,360 -- Synthesized 1918 //1918 5,830 -- Synthesized 1922 //1922 5,550 -- Synthesized 1933 //1933 5,700 -- Synthesized 1923 //1923 2,750 -- Synthesized 1922 //1922 5,550 -- Synthesized 1929 //1929 2,670 -- Synthesized 1984 06/22/1984 5,540 8.09 1930 //1930 1,750 -- PILF; synthesized 1965 //1965 5,270 -- Synthesized 1931 //1931 580 -- PILF; synthesized 1976 05/12/1976 5,270 8.70 1932 //1932 2,760 -- Synthesized 1943 //1943 5,230 -- Synthesized 1933 //1933 5,700 -- Synthesized 1901 //1901 5,200 -- Synthesized Flow, in cubicfeetsecond per 1934 //1934 3,540 -- Synthesized 1974 01/16/1974 5,200 9.70 1935 //1935 2,040 -- PILF; synthesized 1938 //1938 5,020 -- Synthesized 1,000 1936 //1936 2,700 -- Synthesized 1972 //1972 5,000 -- Synthesized 1937 //1937 1,390 -- PILF; synthesized 1997 06/12/1997 5,000 7.71 1938 //1938 5,020 -- Synthesized 1998 06/27/1998 4,830 7.34 1939 //1939 2,240 -- PILF; synthesized 1907 //1907 4,600 -- Synthesized 1940 //1940 1,300 -- PILF; synthesized 2003 03/14/2003 4,580 7.71 1941 //1941 1,320 -- PILF; synthesized 1989 03/11/1989 4,540 7.70 1942 //1942 6,400 -- Synthesized 1995 02/01/1995 4,490 7.55 1943 //1943 5,230 -- Synthesized 1950 //1950 4,430 -- Synthesized 1944 //1944 2,810 -- Synthesized 1904 //1904 4,360 -- Synthesized 100 1945 //1945 1,890 -- PILF; synthesized 1921 //1921 4,360 -- Synthesized 99.5 99 98 95 90 80 70 60 50 40 30 20 10 5 2 1 0.5 0.2 1946 //1946 1,750 -- PILF; synthesized 1967 06/14/1967 4,350 8.14 1947 //1947 3,770 -- Synthesized 1912 //1912 4,280 -- Synthesized Exceedance probability, in percent 1948 //1948 8,060 -- Synthesized 1970 //1970 4,210 -- Synthesized Figure 1. Annual peak flows (plotting positions determined using the Cunnane formulation; Helsel and HIrsch, 2002) and peak-flow frequencycurve 1949 //1949 2,570 -- Synthesized 2010 06/18/2010 4,180 7.67 1950 //1950 4,430 -- Synthesized 1947 //1947 3,770 -- Synthesized 1951 //1951 3,420 -- Synthesized 1959 //1959 3,730 -- Synthesized 1952 //1952 1,550 -- PILF; synthesized 2009 05/26/2009 3,730 7.02 1953 //1953 8,070 -- Synthesized 1982 06/18/1982 3,710 7.55 1Peak flows with a value of zero are not plotted in figure 1 . 1954 //1954 1,970 -- PILF; synthesized 1958 //1958 3,680 -- Synthesized 2In cases where the month, day, or both are not present in the date of a peak flow (as indicated by adjacent slash marks with no intervening values), 1955 //1955 2,480 -- PILF; synthesized 1957 //1957 3,640 -- Synthesized the month, day, or both are unknown. 1956 //1956 3,170 -- Synthesized 2005 06/04/2005 3,550 6.77 3Flood-frequency results not reported because of too many values less than the PILF threshold used in the at-site analysis. 1957 //1957 3,640 -- Synthesized 1934 //1934 3,540 -- Synthesized 4Definitions of peak-flow designations used in analysis include: 1958 //1958 3,680 -- Synthesized 1951 //1951 3,420 -- Synthesized PT definition: The peak flow is used to define perception thresholds in ungaged historical periods; 1959 //1959 3,730 -- Synthesized 2007 06/07/2007 3,360 6.89 Opportunistic: The peak flow was excluded from the analysis because it is outside of the systematic record and was of insufficient magnitude to 1960 //1960 2,480 -- PILF; synthesized 2008 06/05/2008 3,190 6.86 determine nonexceedance during an ungaged period; 1961 //1961 2,730 -- Synthesized 1956 //1956 3,170 -- Synthesized PILF: The peak flow was identified as a potentially influential low flow; Synthesized: The peak flow was synthesized using Maintenance of Variance Extension Type III record extension. 1962 //1962 2,620 -- Synthesized 1914 //1914 3,070 -- Synthesized England, J.F., Jr., Cohn, T.A., Faber, B.A., Stedinger, J.R., Thomas, W.O., Jr., Veilleux, A.G., Kiang, J.E., and Mason, R.R., Jr., 2019, Guidelines 1963 //1963 2,180 -- PILF; synthesized 1983 07/11/1983 2,990 6.89 for determining flood flow frequency—Bulletin 17C (ver. 1.1, May 2019): U.S. Geological Survey Techniques and Methods, book 4, chap. B5, 148 1964 //1964 6,100 -- Synthesized 2014 03/11/2014 2,980 6.38 p., https://doi.org/10.3133/tm4B5. 1965 //1965 5,270 -- Synthesized 1991 //1991 2,960 -- Synthesized 1966 //1966 972 -- PILF; synthesized 1999 06/04/1999 2,950 6.02 1967 06/14/1967 4,350 8.14 1911 //1911 2,870 -- Synthesized Helsel,D.R.,andHirsch,R.M.,2002,Statisticalmethodsinwaterresources:U.S.GeologicalSur-veyTechniquesofWater- 1968 //1968 2,760 -- Synthesized 1944 //1944 2,810 -- Synthesized ResourcesInvestigations,book4,chap.A3,522p.,accessedOctober30,2017,athttps://pubs.usgs.gov/twri/twri4a3/. 1969 //1969 1,780 -- PILF; synthesized 1978 06/10/1978 2,800 6.88 1970 //1970 4,210 -- Synthesized 1932 //1932 2,760 -- Synthesized 1971 //1971 2,320 -- PILF; synthesized 1968 //1968 2,760 -- Synthesized 1972 //1972 5,000 -- Synthesized 1923 //1923 2,750 -- Synthesized 1973 12/23/1972 850 7.86 PILF 1961 //1961 2,730 -- Synthesized 1974 01/16/1974 5,200 9.70 1979 05/27/1979 2,710 6.67 1975 06/20/1975 8,490 10.60 2006 06/11/2006 2,710 6.08 1976 05/12/1976 5,270 8.70 1936 //1936 2,700 -- Synthesized 1977 02/13/1977 832 -- PILF 1929 //1929 2,670 -- Synthesized 1978 06/10/1978 2,800 6.88 1962 //1962 2,620 -- Synthesized 1979 05/27/1979 2,710 6.67 1920 //1920 2,590 -- Synthesized 1980 05/26/1980 6,030 9.58 1949 //1949 2,570 -- Synthesized 1981 05/23/1981 15,800 12.44 1955 //1955 2,480 -- PILF; synthesi 1982 06/18/1982 3,710 7.55 1960 //1960 2,480 -- PILF; synthesi 1983 07/11/1983 2,990 6.89 1990 06/01/1990 2,480 6.57 PILF 1984 06/22/1984 5,540 8.09 1971 //1971 2,320 -- PILF; synthesi 1985 04/03/1985 1,590 6.04 PILF 2012 04/27/2012 2,260 5.81 PILF 1986 02/25/1986 8,760 9.61 1939 //1939 2,240 -- PILF; synthesi 1987 05/29/1987 1,220 5.60 PILF 1963 //1963 2,180 -- PILF; synthesi 1988 05/31/1988 1,420 5.88 PILF 1919 //1919 2,160 -- PILF; synthesi 1989 03/11/1989 4,540 7.70 1935 //1935 2,040 -- PILF; synthesi 1990 06/01/1990 2,480 6.57 PILF 1954 //1954 1,970 -- PILF; synthesi 1991 //1991 2,960 -- Synthesized 1993 06/17/1993 1,910 5.26 PILF 1992 03/19/1992 664 4.75 PILF 2001 06/05/2001 1,900 5.33 PILF 1993 06/17/1993 1,910 5.26 PILF 1945 //1945 1,890 -- PILF; synthesi 1994 04/25/1994 1,840 5.15 PILF 2004 03/09/2004 1,850 5.24 PILF 1995 02/01/1995 4,490 7.55 1994 04/25/1994 1,840 5.15 PILF 1996 02/09/1996 9,800 10.03 1969 //1969 1,780 -- PILF; synthesi 1997 06/12/1997 5,000 7.71 1930 //1930 1,750 -- PILF; synthesi 1998 06/27/1998 4,830 7.34 1946 //1946 1,750 -- PILF; synthesi 1999 06/04/1999 2,950 6.02 1985 04/03/1985 1,590 6.04 PILF 2000 02/02/2000 905 3.97 PILF 1905 //1905 1,570 -- PILF; synthesi 2001 06/05/2001 1,900 5.33 PILF 1952 //1952 1,550 -- PILF; synthesi 2002 06/11/2002 1,490 4.71 PILF 2015 06/03/2015 1,520 4.60 PILF 2003 03/14/2003 4,580 7.71 2002 06/11/2002 1,490 4.71 PILF 2004 03/09/2004 1,850 5.24 PILF 1988 05/31/1988 1,420 5.88 PILF 2005 06/04/2005 3,550 6.77 1937 //1937 1,390 -- PILF; synthesi 2006 06/11/2006 2,710 6.08 1906 //1906 1,380 -- PILF; synthesi 2007 06/07/2007 3,360 6.89 2016 05/28/2016 1,350 4.38 PILF 2008 06/05/2008 3,190 6.86 1941 //1941 1,320 -- PILF; synthesi 2009 05/26/2009 3,730 7.02 1940 //1940 1,300 -- PILF; synthesi 2010 06/18/2010 4,180 7.67 1987 05/29/1987 1,220 5.60 PILF 2011 06/10/2011 8,220 9.79 2013 06/04/2013 1,130 4.16 PILF 2012 04/27/2012 2,260 5.81 PILF 1966 //1966 972 -- PILF; synthesi 2013 06/04/2013 1,130 4.16 PILF 2000 02/02/2000 905 3.97 PILF 2014 03/11/2014 2,980 6.38 1973 12/23/1972 850 7.86 PILF 2015 06/03/2015 1,520 4.60 PILF 1977 02/13/1977 832 -- PILF 12332000 Middle Fork Rock Creek near Philipsburg, Montana 12332000 Middle Fork Rock Creek near Philipsburg, Montana Analysis for unregulated period of record Analysis for unregulated period of record Analysis period of record, water years: 1938–2018 Analysis period of record, water years: 1938–2018 At-site peak-flow frequency analysis conducted on recorded data At-site peak-flow frequency analysis conducted on recorded data Table 1‐1 Table 1‐2 Table 1‐3 Table 1‐4 Table 1‐5 Table 1‐6 Table 1‐7 Table 1‐8 Table 1‐1 Table 1‐2 Table 1‐3 Table 1‐4 Table 1‐5 Table 1‐6 Table 1‐7 Table 1‐8 [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. PILF; potentially influential low flow; MGBT, multiple Grubbs-Beck test] [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends; --, not applicable or not available.] Peak-flow data Ranked (largest to smallest) peak-flow data Number of Number of Total number Contributing PILF Type of peak‐ recorded Skew type peak flows of years in Type of PILF flow Peak flow, in Gage Peak-flow Peak flow, in Gage Peak-flow drainage area, threshold, in Water 2 Water 2 peak flows used in 1 used for ungaged cubic feet per height, designation in cubic feet per height, designation in in square threshold cubic feet per frequency Date Date used in the analysis perception historical year 4 year 4 miles second second in feet analysis second in feet analysis analysis thresholds period(s) analysis 1938 05/29/1938 992 -- 1974 06/16/1974 1,680 5.58 121 81 Weighted MGBT 670 At-site 1939 05/04/1939 611 -- PILF 2003 05/31/2003 1,670 4.37 Peak flow, in cubic feet per second, for indicated annual exceedance probability (bold values), in percent 1940 05/26/1940 395 -- PILF 1972 06/02/1972 1,590 5.23 66.7 50 42.9 20 10 4 2 1.0 0.5 0.2 1941 06/05/1941 482 -- PILF 1996 06/09/1996 1,540 5.41 749 901 968 1,240 1,440 1,650 1,800 1,930 2,050 2,190 1942 05/26/1942 1,250 -- 2017 06/13/2017 1,490 4.15 Upper and lower 90-percent confidence intervals, in cubic feet per second, for indicated annual exceedance probability, in percent 1943 06/19/1943 1,220 -- 2011 06/09/2011 1,470 4.11 66.7 50 42.9 20 10 4 2 1.0 0.5 0.2 1944 06/13/1944 565 -- PILF 1997 06/01/1997 1,460 4.46 825 989 1,060 1,370 1,610 1,910 2,140 2,370 2,600 2,930 1945 06/23/1945 550 -- PILF 1953 06/13/1953 1,430 -- 620 803 874 1,130 1,310 1,490 1,600 1,690 1,760 1,840 1946 06/08/1946 466 -- PILF 2018 05/11/2018 1,390 4.42 1947 05/09/1947 1,310 -- 1965 06/12/1965 1,380 4.96 1948 05/29/1948 1,360 -- 1986 05/30/1986 1,380 5.18 1949 06/01/1949 1,000 -- 1948 05/29/1948 1,360 -- 1950 06/18/1950 1,340 -- 1950 06/18/1950 1,340 -- 1951 05/28/1951 1,180 -- 1947 05/09/1947 1,310 -- 1952 06/06/1952 884 -- 1978 06/10/1978 1,260 5.01 1953 06/13/1953 1,430 -- 1942 05/26/1942 1,250 -- 1954 05/21/1954 998 -- 1995 06/06/1995 1,250 4.52 1955 06/12/1955 824 -- 1943 06/19/1943 1,220 -- 1956 05/25/1956 1,220 4.73 1956 05/25/1956 1,220 4.73 1957 06/03/1957 999 4.56 1958 05/25/1958 1,220 4.89 1958 05/25/1958 1,220 4.89 1970 06/06/1970 1,210 4.71 1959 06/15/1959 870 4.23 1951 05/28/1951 1,180 -- 1960 06/04/1960 836 4.16 1982 06/24/1982 1,180 4.89 1961 05/30/1961 860 4.15 1975 07/05/1975 1,170 4.88 1962 06/14/1962 770 3.97 2014 05/29/2014 1,160 3.83 1963 06/01/1963 914 4.29 1964 06/08/1964 1,150 4.72 1964 06/08/1964 1,150 4.72 1981 05/26/1981 1,150 4.84 1965 06/12/1965 1,380 4.96 2009 06/01/2009 1,140 3.81 1966 05/30/1966 515 3.59 PILF 1984 06/21/1984 1,110 4.77 1967 06/09/1967 969 4.37 2006 05/21/2006 1,100 3.77 1968 06/04/1968 752 4.07 1999 05/29/1999 1,040 3.86 1969 05/20/1969 812 4.16 1949 06/01/1949 1,000 -- 1970 06/06/1970 1,210 4.71 1957 06/03/1957 999 4.56 1971 05/30/1971 948 4.34 1954 05/21/1954 998 -- 1972 06/02/1972 1,590 5.23 2008 05/21/2008 993 3.65 1973 05/20/1973 550 3.99 PILF 1938 05/29/1938 992 -- 1974 06/16/1974 1,680 5.58 1967 06/09/1967 969 4.37 1975 07/05/1975 1,170 4.88 1991 06/07/1991 959 4.31 1976 05/28/1976 930 4.48 1971 05/30/1971 948 4.34 1977 06/07/1977 352 3.37 PILF 1976 05/28/1976 930 4.48 1978 06/10/1978 1,260 5.01 1963 06/01/1963 914 4.29 1979 05/27/1979 881 4.42 2010 06/05/2010 892 3.53 1980 05/10/1980 717 4.13 1952 06/06/1952 884 -- 1981 05/26/1981 1,150 4.84 1979 05/27/1979 881 4.42 1982 06/24/1982 1,180 4.89 1983 05/30/1983 875 4.41 1983 05/30/1983 875 4.41 1959 06/15/1959 870 4.23 1984 06/21/1984 1,110 4.77 1961 05/30/1961 860 4.15 Figure 1. Annual peak flows (probability plotting positions) and peak-flow frequency curve. EMA, Expected Moments Algorithm; Skew(G), analysis 1985 06/02/1985 559 3.78 PILF 1960 06/04/1960 836 4.16 skew; PILF(LO), potentially Influencial low flow (low outlier). 1986 05/30/1986 1,380 5.18 1955 06/12/1955 824 -- 1987 05/01/1987 430 3.55 PILF 1969 05/20/1969 812 4.16 1988 05/25/1988 581 3.73 PILF 2012 05/22/2012 790 3.40 1989 06/16/1989 702 4.07 2002 06/02/2002 775 3.38 1990 06/11/1990 680 4.00 1962 06/14/1962 770 3.97 1991 06/07/1991 959 4.31 1968 06/04/1968 752 4.07 1992 05/09/1992 304 2.54 PILF 2007 06/11/2007 731 3.32 1993 05/21/1993 687 3.59 1980 05/10/1980 717 4.13 1994 05/13/1994 520 3.18 PILF 1989 06/16/1989 702 4.07 1995 06/06/1995 1,250 4.52 1993 05/21/1993 687 3.59 1996 06/09/1996 1,540 5.41 1990 06/11/1990 680 4.00 1997 06/01/1997 1,460 4.46 1998 06/26/1998 670 3.18 1998 06/26/1998 670 3.18 1939 05/04/1939 611 -- PILF 1999 05/29/1999 1,040 3.86 1988 05/25/1988 581 3.73 PILF 2000 05/23/2000 343 2.63 PILF 1944 06/13/1944 565 -- PILF 2001 05/15/2001 459 2.87 PILF 1985 06/02/1985 559 3.78 PILF 2002 06/02/2002 775 3.38 1945 06/23/1945 550 -- PILF 2003 05/31/2003 1,670 4.37 1973 05/20/1973 550 3.99 PILF 2004 06/10/2004 547 3.01 PILF 2004 06/10/2004 547 3.01 PILF 2005 05/19/2005 535 3.01 PILF 2005 05/19/2005 535 3.01 PILF 2006 05/21/2006 1,100 3.77 1994 05/13/1994 520 3.18 PILF 2007 06/11/2007 731 3.32 2015 05/17/2015 518 2.99 PILF 2008 05/21/2008 993 3.65 2016 06/08/2016 518 3.24 PILF 2009 06/01/2009 1,140 3.81 1966 05/30/1966 515 3.59 PILF 2010 06/05/2010 892 3.53 1941 06/05/1941 482 -- PILF 2011 06/09/2011 1,470 4.11 2013 05/14/2013 479 2.88 PILF 2012 05/22/2012 790 3.40 1946 06/08/1946 466 -- PILF 2013 05/14/2013 479 2.88 PILF 2001 05/15/2001 459 2.87 PILF 2014 05/29/2014 1,160 3.83 1987 05/01/1987 430 3.55 PILF 2015 05/17/2015 518 2.99 PILF 1940 05/26/1940 395 -- PILF 2016 06/08/2016 518 3.24 PILF 1977 06/07/1977 352 3.37 PILF 2017 06/13/2017 1,490 4.15 2000 05/23/2000 343 2.63 PILF Figure 2. Annual peak flows and perception thresholds. 2018 05/11/2018 1,390 4.42 1992 05/09/1992 304 2.54 PILF

1Definitions of types of PILF thresholds include: MGBT: PILF threshold calculated by using the multiple Grubbs-Beck Test as specifed in Bulletin 17C (England and others, 2016);

Weighted peak-flow frequency 1937 //1937 366 -- PILF; synthesized 1950 06/18/1950 1,340 -- curve 1938 05/29/1938 992 -- 1947 05/09/1947 1,310 -- Upper and lower 95-percent 1939 05/04/1939 611 -- PILF 1978 06/10/1978 1,260 5.01 confidence intervals 1940 05/26/1940 395 -- PILF 1942 05/26/1942 1,250 -- 1941 06/05/1941 482 -- PILF 1995 06/06/1995 1,250 4.52 1942 05/26/1942 1,250 -- 1943 06/19/1943 1,220 -- 1943 06/19/1943 1,220 -- 1956 05/25/1956 1,220 4.73 1944 06/13/1944 565 -- PILF 1958 05/25/1958 1,220 4.89 1,000 1945 06/23/1945 550 -- PILF 1970 06/06/1970 1,210 4.71 1946 06/08/1946 466 -- PILF 1933 //1933 1,200 -- Synthesized 1947 05/09/1947 1,310 -- 1951 05/28/1951 1,180 -- Flow, in cubicfeetsecond per 1948 05/29/1948 1,360 -- 1982 06/24/1982 1,180 4.89 1949 06/01/1949 1,000 -- 1975 07/05/1975 1,170 4.88 1950 06/18/1950 1,340 -- 2014 05/29/2014 1,160 3.83 1951 05/28/1951 1,180 -- 1964 06/08/1964 1,150 4.72 1952 06/06/1952 884 -- 1981 05/26/1981 1,150 4.84 1953 06/13/1953 1,430 -- 2009 06/01/2009 1,140 3.81 1954 05/21/1954 998 -- 1901 //1901 1,110 -- Synthesized 1955 06/12/1955 824 -- 1984 06/21/1984 1,110 4.77 1956 05/25/1956 1,220 4.73 2006 05/21/2006 1,100 3.77 1957 06/03/1957 999 4.56 1999 05/29/1999 1,040 3.86 1958 05/25/1958 1,220 4.89 1907 //1907 1,030 -- Synthesized 100 1959 06/15/1959 870 4.23 1904 //1904 1,000 -- Synthesized 99.5 99 98 95 90 80 70 60 50 40 30 20 10 5 2 1 0.5 0.2 1960 06/04/1960 836 4.16 1949 06/01/1949 1,000 -- 1961 05/30/1961 860 4.15 1957 06/03/1957 999 4.56 Exceedance probability, in percent 1962 06/14/1962 770 3.97 1954 05/21/1954 998 -- Figure 1. Annual peak flows (plotting positions determined using the Cunnane formulation; Helsel and HIrsch, 2002) and peak-flow frequencycurve 1963 06/01/1963 914 4.29 2008 05/21/2008 993 3.65 1964 06/08/1964 1,150 4.72 1938 05/29/1938 992 -- 1965 06/12/1965 1,380 4.96 1967 06/09/1967 969 4.37 1966 05/30/1966 515 3.59 PILF 1991 06/07/1991 959 4.31 1967 06/09/1967 969 4.37 1971 05/30/1971 948 4.34 1Peak flows with a value of zero are not plotted in figure 1 . 1968 06/04/1968 752 4.07 1976 05/28/1976 930 4.48 2In cases where the month, day, or both are not present in the date of a peak flow (as indicated by adjacent slash marks with no intervening values), 1969 05/20/1969 812 4.16 1963 06/01/1963 914 4.29 the month, day, or both are unknown. 1970 06/06/1970 1,210 4.71 2010 06/05/2010 892 3.53 3Flood-frequency results not reported because of too many values less than the PILF threshold used in the at-site analysis. 1971 05/30/1971 948 4.34 1952 06/06/1952 884 -- 4Definitions of peak-flow designations used in analysis include: 1972 06/02/1972 1,590 5.23 1979 05/27/1979 881 4.42 PT definition: The peak flow is used to define perception thresholds in ungaged historical periods; 1973 05/20/1973 550 3.99 PILF 1983 05/30/1983 875 4.41 Opportunistic: The peak flow was excluded from the analysis because it is outside of the systematic record and was of insufficient magnitude to 1974 06/16/1974 1,680 5.58 1959 06/15/1959 870 4.23 determine nonexceedance during an ungaged period; 1975 07/05/1975 1,170 4.88 1936 //1936 869 -- Synthesized PILF: The peak flow was identified as a potentially influential low flow; Synthesized: The peak flow was synthesized using Maintenance of Variance Extension Type III record extension. 1976 05/28/1976 930 4.48 1961 05/30/1961 860 4.15 England, J.F., Jr., Cohn, T.A., Faber, B.A., Stedinger, J.R., Thomas, W.O., Jr., Veilleux, A.G., Kiang, J.E., and Mason, R.R., Jr., 2019, Guidelines 1977 06/07/1977 352 3.37 PILF 1960 06/04/1960 836 4.16 for determining flood flow frequency—Bulletin 17C (ver. 1.1, May 2019): U.S. Geological Survey Techniques and Methods, book 4, chap. B5, 148 1978 06/10/1978 1,260 5.01 1932 //1932 832 -- Synthesized p., https://doi.org/10.3133/tm4B5. 1979 05/27/1979 881 4.42 1934 //1934 829 -- Synthesized 1980 05/10/1980 717 4.13 1955 06/12/1955 824 -- 1981 05/26/1981 1,150 4.84 1969 05/20/1969 812 4.16 Helsel,D.R.,andHirsch,R.M.,2002,Statisticalmethodsinwaterresources:U.S.GeologicalSur-veyTechniquesofWater- 1982 06/24/1982 1,180 4.89 2012 05/22/2012 790 3.40 ResourcesInvestigations,book4,chap.A3,522p.,accessedOctober30,2017,athttps://pubs.usgs.gov/twri/twri4a3/. 1983 05/30/1983 875 4.41 2002 06/02/2002 775 3.38 1984 06/21/1984 1,110 4.77 1962 06/14/1962 770 3.97 1985 06/02/1985 559 3.78 PILF 1968 06/04/1968 752 4.07 1986 05/30/1986 1,380 5.18 2007 06/11/2007 731 3.32 1987 05/01/1987 430 3.55 PILF 1980 05/10/1980 717 4.13 1988 05/25/1988 581 3.73 PILF 1989 06/16/1989 702 4.07 1989 06/16/1989 702 4.07 1993 05/21/1993 687 3.59 1990 06/11/1990 680 4.00 1990 06/11/1990 680 4.00 1991 06/07/1991 959 4.31 1998 06/26/1998 670 3.18 1992 05/09/1992 304 2.54 PILF 1935 //1935 645 -- PILF; synthesi 1993 05/21/1993 687 3.59 1939 05/04/1939 611 -- PILF 1994 05/13/1994 520 3.18 PILF 1988 05/25/1988 581 3.73 PILF 1995 06/06/1995 1,250 4.52 1944 06/13/1944 565 -- PILF 1996 06/09/1996 1,540 5.41 1985 06/02/1985 559 3.78 PILF 1997 06/01/1997 1,460 4.46 1930 //1930 556 -- PILF; synthesi 1998 06/26/1998 670 3.18 1945 06/23/1945 550 -- PILF 1999 05/29/1999 1,040 3.86 1973 05/20/1973 550 3.99 PILF 2000 05/23/2000 343 2.63 PILF 2004 06/10/2004 547 3.01 PILF 2001 05/15/2001 459 2.87 PILF 2005 05/19/2005 535 3.01 PILF 2002 06/02/2002 775 3.38 1905 //1905 531 -- PILF; synthesi 2003 05/31/2003 1,670 4.37 1994 05/13/1994 520 3.18 PILF 2004 06/10/2004 547 3.01 PILF 2015 05/17/2015 518 2.99 PILF 2005 05/19/2005 535 3.01 PILF 2016 06/08/2016 518 3.24 PILF 2006 05/21/2006 1,100 3.77 1966 05/30/1966 515 3.59 PILF 2007 06/11/2007 731 3.32 1906 //1906 490 -- PILF; synthesi 2008 05/21/2008 993 3.65 1941 06/05/1941 482 -- PILF 2009 06/01/2009 1,140 3.81 2013 05/14/2013 479 2.88 PILF 2010 06/05/2010 892 3.53 1946 06/08/1946 466 -- PILF 2011 06/09/2011 1,470 4.11 2001 05/15/2001 459 2.87 PILF 2012 05/22/2012 790 3.40 1987 05/01/1987 430 3.55 PILF 2013 05/14/2013 479 2.88 PILF 1940 05/26/1940 395 -- PILF 2014 05/29/2014 1,160 3.83 1931 //1931 382 -- PILF; synthesi 2015 05/17/2015 518 2.99 PILF 1937 //1937 366 -- PILF; synthesi 2016 06/08/2016 518 3.24 PILF 1977 06/07/1977 352 3.37 PILF 2017 06/13/2017 1,490 4.15 2000 05/23/2000 343 2.63 PILF 2018 05/11/2018 1,390 4.42 1992 05/09/1992 304 2.54 PILF 12334510 Rock Creek near Clinton, Montana 12334510 Rock Creek near Clinton, Montana Analysis for unregulated period of record Analysis for unregulated period of record Analysis period of record, water years: 1972–2018 Analysis period of record, water years: 1972–2018 At-site peak-flow frequency analysis conducted on recorded data At-site peak-flow frequency analysis conducted on recorded data Table 1‐1 Table 1‐2 Table 1‐3 Table 1‐4 Table 1‐5 Table 1‐6 Table 1‐7 Table 1‐8 Table 1‐1 Table 1‐2 Table 1‐3 Table 1‐4 Table 1‐5 Table 1‐6 Table 1‐7 Table 1‐8 [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. PILF; potentially influential low flow; MGBT, multiple Grubbs-Beck test; -- not applicable or not available] [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends; --, not applicable or not available.] Peak-flow data Ranked (largest to smallest) peak-flow data Number of Number of Total number Contributing PILF Type of peak‐ recorded Skew type peak flows of years in Type of PILF flow Peak flow, in Gage Peak-flow Peak flow, in Gage Peak-flow drainage area, threshold, in Water 2 Water 2 peak flows used in 1 used for ungaged cubic feet per height, designation in cubic feet per height, designation in in square threshold cubic feet per frequency Date Date used in the analysis perception historical year 4 year 4 miles second second in feet analysis second in feet analysis analysis thresholds period(s) analysis 1972 06/01/1972 6,500 8.50 1972 06/01/1972 6,500 8.50 889 47 Weighted MGBT -- At-site 1973 05/25/1973 1,640 4.68 2018 05/10/2018 5,830 8.63 Peak flow, in cubic feet per second, for indicated annual exceedance probability (bold values), in percent 1974 06/16/1974 5,050 7.23 2017 06/14/2017 5,540 8.53 66.7 50 42.9 20 10 4 2 1.0 0.5 0.2 1975 06/20/1975 5,520 7.49 1997 05/18/1997 5,530 7.81 2,520 3,130 3,420 4,610 5,520 6,590 7,320 8,010 8,650 9,460 1976 05/15/1976 5,190 7.46 1975 06/20/1975 5,520 7.49 Upper and lower 90-percent confidence intervals, in cubic feet per second, for indicated annual exceedance probability, in percent 1977 05/25/1977 922 4.49 2011 05/26/2011 5,210 8.17 66.7 50 42.9 20 10 4 2 1.0 0.5 0.2 1978 06/08/1978 3,640 6.87 1976 05/15/1976 5,190 7.46 2,950 3,650 3,970 5,380 6,670 8,470 9,890 11,400 13,000 15,400 1979 05/27/1979 3,390 6.82 1981 05/22/1981 5,140 7.53 2,100 2,670 2,930 3,990 4,780 5,600 6,040 6,380 6,640 6,890 1980 05/10/1980 3,340 6.86 2009 05/25/2009 5,090 7.65 1981 05/22/1981 5,140 7.53 1974 06/16/1974 5,050 7.23 1982 06/17/1982 4,870 7.53 1982 06/17/1982 4,870 7.53 1983 05/30/1983 2,870 6.70 2008 05/20/2008 4,370 7.37 1984 05/16/1984 3,420 7.21 2014 05/27/2014 4,310 8.18 1985 06/02/1985 1,950 5.95 1996 06/10/1996 4,270 7.23 1986 05/29/1986 4,060 7.11 1986 05/29/1986 4,060 7.11 1987 05/01/1987 1,680 5.66 2003 05/31/2003 3,950 7.60 1988 05/25/1988 1,870 5.84 1999 06/04/1999 3,770 7.47 1989 05/11/1989 3,090 6.46 2012 04/27/2012 3,690 8.16 1990 05/31/1990 2,450 5.97 1978 06/08/1978 3,640 6.87 1991 06/07/1991 3,370 6.75 1998 06/26/1998 3,630 7.16 1992 05/09/1992 1,030 4.96 2006 05/20/2006 3,620 6.94 1993 05/16/1993 2,690 6.29 1984 05/16/1984 3,420 7.21 1994 05/13/1994 1,820 5.37 1979 05/27/1979 3,390 6.82 1995 06/06/1995 3,140 6.63 1991 06/07/1991 3,370 6.75 1996 06/10/1996 4,270 7.23 1980 05/10/1980 3,340 6.86 1997 05/18/1997 5,530 7.81 1995 06/06/1995 3,140 6.63 1998 06/26/1998 3,630 7.16 1989 05/11/1989 3,090 6.46 1999 06/04/1999 3,770 7.47 1983 05/30/1983 2,870 6.70 2000 05/29/2000 1,050 4.99 1993 05/16/1993 2,690 6.29 2001 05/15/2001 1,560 5.63 2010 06/08/2010 2,680 -- 2002 05/31/2002 2,540 6.56 2002 05/31/2002 2,540 6.56 2003 05/31/2003 3,950 7.60 2013 05/14/2013 2,500 7.63 2004 06/11/2004 1,310 5.25 1990 05/31/1990 2,450 5.97 2005 05/20/2005 2,230 5.91 2007 05/13/2007 2,390 6.23 2006 05/20/2006 3,620 6.94 2005 05/20/2005 2,230 5.91 2007 05/13/2007 2,390 6.23 2016 05/09/2016 2,180 7.26 2008 05/20/2008 4,370 7.37 1985 06/02/1985 1,950 5.95 2009 05/25/2009 5,090 7.65 2015 05/17/2015 1,890 7.02 2010 06/08/2010 2,680 -- 1988 05/25/1988 1,870 5.84 2011 05/26/2011 5,210 8.17 1994 05/13/1994 1,820 5.37 2012 04/27/2012 3,690 8.16 1987 05/01/1987 1,680 5.66 2013 05/14/2013 2,500 7.63 1973 05/25/1973 1,640 4.68 2014 05/27/2014 4,310 8.18 2001 05/15/2001 1,560 5.63 2015 05/17/2015 1,890 7.02 2004 06/11/2004 1,310 5.25 2016 05/09/2016 2,180 7.26 2000 05/29/2000 1,050 4.99 2017 06/14/2017 5,540 8.53 1992 05/09/1992 1,030 4.96 2018 05/10/2018 5,830 8.63 1977 05/25/1977 922 4.49 Figure 1. Annual peak flows (probability plotting positions) and peak-flow frequency curve. EMA, Expected Moments Algorithm; Skew(G), analysis skew; PILF(LO), potentially Influencial low flow (low outlier).

Figure 2. Annual peak flows and perception thresholds.

England, J.F., Jr., Cohn, T.A., Faber, B.A., Stedinger, J.R., Thomas, W.O., Jr., Veilleux, A.G., Kiang, J.E., and Mason, R.R., Jr., 2019, Guidelines for determining flood flow frequency—Bulletin 17C (ver. 1.1, May 2019): U.S. Geological Survey Techniques and Methods, book 4, chap. B5, 148 p., https://doi.org/10.3133/tm4B5. 12334510 Rock Creek near Clinton, Montana 12334510 Rock Creek near Clinton, Montana Analysis for unregulated period of record Analysis for unregulated period of record Analysis period of record, water years: 1899–1908; 1930–2018 Analysis period of record, water years: 1899–1908; 1930–2018 Peak-flow frequency analysis conducted on recorded and synthesized data Peak-flow frequency analysis conducted on recorded and synthesized data Table 1‐1 Table 1‐2 Table 1‐3 Table 1‐4 Table 1‐5 Table 1‐6 Table 1‐7 Table 1‐8 Table 1‐1 Table 1‐2 Table 1‐3 Table 1‐4 Table 1‐5 Table 1‐6 Table 1‐7 Table 1‐8 [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. PILF; potentially influential low flow; MGBT, multiple Grubbs-Beck test] [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends; --, not applicable or not available.] Peak-flow data Ranked (largest to smallest) peak-flow data Contributing Type of peak‐ Peak flow, in Gage Peak-flow Peak flow, in Gage Peak-flow drainage area, flow Water Water 2 cubic feet per height, designation in 2 cubic feet per height, designation in in square frequency year Date year Date second in feet 4 second in feet 4 miles analysis analysis analysis 1899 //1899 7,910 -- Synthesized 1908 //1908 10,400 -- Historic; synthe 889 MOVE.3 1900 //1900 5,580 -- Synthesized 1899 //1899 7,910 -- Synthesized Peak flow, in cubic feet per second, for indicated annual exceedance probability (bold values), in percent 1901 //1901 4,410 -- Synthesized 1902 //1902 6,830 -- Synthesized 66.7 50 42.9 20 10 4 2 1.0 0.5 0.2 1902 //1902 6,830 -- Synthesized 1948 //1948 6,620 -- Synthesized 2,720 3,340 3,630 4,840 5,780 6,900 7,690 8,440 9,160 10,100 1903 //1903 5,450 -- Synthesized 1972 06/01/1972 6,500 8.50 Upper and lower 90-percent confidence intervals, in cubic feet per second, for indicated annual exceedance probability, in percent 1904 //1904 3,890 -- Synthesized 1964 //1964 6,230 -- Synthesized 66.7 50 42.9 20 10 4 2 1.0 0.5 0.2 1905 //1905 1,870 -- PILF; synthesized 2018 05/10/2018 5,830 8.63 3,050 3,740 4,070 5,480 6,650 8,220 9,480 10,900 12,400 14,600 1906 //1906 1,700 -- PILF; synthesized 1947 //1947 5,630 -- Synthesized 2,250 2,940 3,220 4,310 5,120 6,020 6,580 7,070 7,480 7,950 1907 //1907 4,040 -- Synthesized 1900 //1900 5,580 -- Synthesized 1908 //1908 10,400 -- Historic; synthesized 2017 06/14/2017 5,540 8.53 1930 //1930 1,850 -- PILF; synthesized 1997 05/18/1997 5,530 7.81 100,000 EXPLANATION 1931 //1931 1,220 -- PILF; synthesized 1975 06/20/1975 5,520 7.49 1932 //1932 2,900 -- Synthesized 1903 //1903 5,450 -- Synthesized Peaks used in at-site analysis 1933 //1933 4,370 -- Synthesized 1956 //1956 5,450 -- Synthesized 1934 //1934 2,890 -- Synthesized 2011 05/26/2011 5,210 8.17 Potentially influential low flow (PILF) 1935 //1935 2,190 -- PILF; synthesized 1976 05/15/1976 5,190 7.46 1936 //1936 3,040 -- Synthesized 1981 05/22/1981 5,140 7.53 Weighted peak-flow frequency 1937 //1937 1,160 -- PILF; synthesized 2009 05/25/2009 5,090 7.65 curve 1938 //1938 4,220 -- Synthesized 1974 06/16/1974 5,050 7.23 Upper and lower 95-percent 1939 //1939 2,410 -- Synthesized 1982 06/17/1982 4,870 7.53 10,000 confidence intervals 1940 //1940 1,460 -- PILF; synthesized 1953 //1953 4,820 -- Synthesized 1941 //1941 1,020 -- PILF; synthesized 1967 //1967 4,790 -- Synthesized 1942 //1942 3,520 -- Synthesized 1950 //1950 4,680 -- Synthesized 1943 //1943 3,880 -- Synthesized 1901 //1901 4,410 -- Synthesized 1944 //1944 1,480 -- PILF; synthesized 1933 //1933 4,370 -- Synthesized 1945 //1945 2,050 -- PILF; synthesized 2008 05/20/2008 4,370 7.37 1946 //1946 2,120 -- PILF; synthesized 2014 05/27/2014 4,310 8.18 1947 //1947 5,630 -- Synthesized 1971 //1971 4,280 -- Synthesized Flow, in cubicfeetsecond per 1948 //1948 6,620 -- Synthesized 1970 //1970 4,270 -- Synthesized 1949 //1949 4,030 -- Synthesized 1996 06/10/1996 4,270 7.23 1,000 1950 //1950 4,680 -- Synthesized 1938 //1938 4,220 -- Synthesized 1951 //1951 3,780 -- Synthesized 1959 //1959 4,190 -- Synthesized 1952 //1952 3,070 -- Synthesized 1986 05/29/1986 4,060 7.11 1953 //1953 4,820 -- Synthesized 1907 //1907 4,040 -- Synthesized 1954 //1954 4,010 -- Synthesized 1949 //1949 4,030 -- Synthesized 1955 //1955 3,120 -- Synthesized 1954 //1954 4,010 -- Synthesized 1956 //1956 5,450 -- Synthesized 2003 05/31/2003 3,950 7.60 1957 //1957 3,650 -- Synthesized 1965 //1965 3,920 -- Synthesized 1958 //1958 3,840 -- Synthesized 1904 //1904 3,890 -- Synthesized 100 1959 //1959 4,190 -- Synthesized 1943 //1943 3,880 -- Synthesized 99.5 99 98 95 90 80 70 60 50 40 30 20 10 5 2 1 0.5 0.2 1960 //1960 3,070 -- Synthesized 1958 //1958 3,840 -- Synthesized

Exceedance probability, in percent 1961 //1961 3,360 -- Synthesized 1951 //1951 3,780 -- Synthesized 1962 //1962 2,900 -- Synthesized 1999 06/04/1999 3,770 7.47 Figure 1. Annual peak flows (plotting positions determined using the Cunnane formulation; Helsel and HIrsch, 2002) and peak-flow frequencycurve 1963 //1963 2,740 -- Synthesized 2012 04/27/2012 3,690 8.16 1964 //1964 6,230 -- Synthesized 1957 //1957 3,650 -- Synthesized 1965 //1965 3,920 -- Synthesized 1978 06/08/1978 3,640 6.87 1966 //1966 1,770 -- PILF; synthesized 1998 06/26/1998 3,630 7.16 1967 //1967 4,790 -- Synthesized 2006 05/20/2006 3,620 6.94 1Peak flows with a value of zero are not plotted in figure 1 . 1968 //1968 2,760 -- Synthesized 1942 //1942 3,520 -- Synthesized 2In cases where the month, day, or both are not present in the date of a peak flow (as indicated by adjacent slash marks with no intervening values), 1969 //1969 3,280 -- Synthesized 1984 05/16/1984 3,420 7.21 the month, day, or both are unknown. 1970 //1970 4,270 -- Synthesized 1979 05/27/1979 3,390 6.82 3Flood-frequency results not reported because of too many values less than the PILF threshold used in the at-site analysis. 1971 //1971 4,280 -- Synthesized 1991 06/07/1991 3,370 6.75 4Definitions of peak-flow designations used in analysis include: 1972 06/01/1972 6,500 8.50 1961 //1961 3,360 -- Synthesized PT definition: The peak flow is used to define perception thresholds in ungaged historical periods; 1973 05/25/1973 1,640 4.68 PILF 1980 05/10/1980 3,340 6.86 Opportunistic: The peak flow was excluded from the analysis because it is outside of the systematic record and was of insufficient magnitude to 1974 06/16/1974 5,050 7.23 1969 //1969 3,280 -- Synthesized determine nonexceedance during an ungaged period; 1975 06/20/1975 5,520 7.49 1995 06/06/1995 3,140 6.63 PILF: The peak flow was identified as a potentially influential low flow; Synthesized: The peak flow was synthesized using Maintenance of Variance Extension Type III record extension. 1976 05/15/1976 5,190 7.46 1955 //1955 3,120 -- Synthesized England, J.F., Jr., Cohn, T.A., Faber, B.A., Stedinger, J.R., Thomas, W.O., Jr., Veilleux, A.G., Kiang, J.E., and Mason, R.R., Jr., 2019, Guidelines 1977 05/25/1977 922 4.49 PILF 1989 05/11/1989 3,090 6.46 for determining flood flow frequency—Bulletin 17C (ver. 1.1, May 2019): U.S. Geological Survey Techniques and Methods, book 4, chap. B5, 148 1978 06/08/1978 3,640 6.87 1952 //1952 3,070 -- Synthesized p., https://doi.org/10.3133/tm4B5. 1979 05/27/1979 3,390 6.82 1960 //1960 3,070 -- Synthesized 1980 05/10/1980 3,340 6.86 1936 //1936 3,040 -- Synthesized 1981 05/22/1981 5,140 7.53 1932 //1932 2,900 -- Synthesized Helsel,D.R.,andHirsch,R.M.,2002,Statisticalmethodsinwaterresources:U.S.GeologicalSur-veyTechniquesofWater- 1982 06/17/1982 4,870 7.53 1962 //1962 2,900 -- Synthesized ResourcesInvestigations,book4,chap.A3,522p.,accessedOctober30,2017,athttps://pubs.usgs.gov/twri/twri4a3/. 1983 05/30/1983 2,870 6.70 1934 //1934 2,890 -- Synthesized 1984 05/16/1984 3,420 7.21 1983 05/30/1983 2,870 6.70 1985 06/02/1985 1,950 5.95 PILF 1968 //1968 2,760 -- Synthesized 1986 05/29/1986 4,060 7.11 1963 //1963 2,740 -- Synthesized 1987 05/01/1987 1,680 5.66 PILF 1993 05/16/1993 2,690 6.29 1988 05/25/1988 1,870 5.84 PILF 2010 06/08/2010 2,680 -- 1989 05/11/1989 3,090 6.46 2002 05/31/2002 2,540 6.56 1990 05/31/1990 2,450 5.97 2013 05/14/2013 2,500 7.63 1991 06/07/1991 3,370 6.75 1990 05/31/1990 2,450 5.97 1992 05/09/1992 1,030 4.96 PILF 1939 //1939 2,410 -- Synthesized 1993 05/16/1993 2,690 6.29 2007 05/13/2007 2,390 6.23 1994 05/13/1994 1,820 5.37 PILF 2005 05/20/2005 2,230 5.91 PILF 1995 06/06/1995 3,140 6.63 1935 //1935 2,190 -- PILF; synthesi 1996 06/10/1996 4,270 7.23 2016 05/09/2016 2,180 7.26 PILF 1997 05/18/1997 5,530 7.81 1946 //1946 2,120 -- PILF; synthesi 1998 06/26/1998 3,630 7.16 1945 //1945 2,050 -- PILF; synthesi 1999 06/04/1999 3,770 7.47 1985 06/02/1985 1,950 5.95 PILF 2000 05/29/2000 1,050 4.99 PILF 2015 05/17/2015 1,890 7.02 PILF 2001 05/15/2001 1,560 5.63 PILF 1905 //1905 1,870 -- PILF; synthesi 2002 05/31/2002 2,540 6.56 1988 05/25/1988 1,870 5.84 PILF 2003 05/31/2003 3,950 7.60 1930 //1930 1,850 -- PILF; synthesi 2004 06/11/2004 1,310 5.25 PILF 1994 05/13/1994 1,820 5.37 PILF 2005 05/20/2005 2,230 5.91 PILF 1966 //1966 1,770 -- PILF; synthesi 2006 05/20/2006 3,620 6.94 1906 //1906 1,700 -- PILF; synthesi 2007 05/13/2007 2,390 6.23 1987 05/01/1987 1,680 5.66 PILF 2008 05/20/2008 4,370 7.37 1973 05/25/1973 1,640 4.68 PILF 2009 05/25/2009 5,090 7.65 2001 05/15/2001 1,560 5.63 PILF 2010 06/08/2010 2,680 -- 1944 //1944 1,480 -- PILF; synthesi 2011 05/26/2011 5,210 8.17 1940 //1940 1,460 -- PILF; synthesi 2012 04/27/2012 3,690 8.16 2004 06/11/2004 1,310 5.25 PILF 2013 05/14/2013 2,500 7.63 1931 //1931 1,220 -- PILF; synthesi 2014 05/27/2014 4,310 8.18 1937 //1937 1,160 -- PILF; synthesi 2015 05/17/2015 1,890 7.02 PILF 2000 05/29/2000 1,050 4.99 PILF 2016 05/09/2016 2,180 7.26 PILF 1992 05/09/1992 1,030 4.96 PILF 2017 06/14/2017 5,540 8.53 1941 //1941 1,020 -- PILF; synthesi 2018 05/10/2018 5,830 8.63 1977 05/25/1977 922 4.49 PILF 12334550 Clark Fork at Turah Bridge, near Bonner, Montana 12334550 Clark Fork at Turah Bridge, near Bonner, Montana Analysis for unregulated period of record Analysis for regulated period of record Analysis period of record, water years: 1986–2018 Analysis for unregulated period of record At-site peak-flow frequency analysis conducted on recorded data At-site peak-flow frequency analysis conducted on recorded data Table 1‐1 Table 1‐2 Table 1‐3 Table 1‐4 Table 1‐5 Table 1‐6 Table 1‐7 Table 1‐8 Table 1‐1 Table 1‐2 Table 1‐3 Table 1‐4 Table 1‐5 Table 1‐6 Table 1‐7 Table 1‐8 [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. PILF; potentially influential low flow; MGBT, multiple Grubbs-Beck test; -- not applicable or not available] [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends; --, not applicable or not available.] Peak-flow data Ranked (largest to smallest) peak-flow data Number of Number of Total number Contributing PILF Type of peak‐ recorded Skew type peak flows of years in Type of PILF flow Peak flow, in Gage Peak-flow Peak flow, in Gage Peak-flow drainage area, threshold, in Water 2 Water 2 peak flows used in 1 used for ungaged cubic feet per height, designation in cubic feet per height, designation in in square threshold cubic feet per frequency Date Date used in the analysis perception historical year 4 year 4 miles second second in feet analysis second in feet analysis analysis thresholds period(s) analysis 1986 02/25/1986 9,700 6.52 2011 06/11/2011 13,300 8.86 3,657 33 Weighted MGBT -- At-site 1987 05/01/1987 2,370 4.36 1996 02/09/1996 12,400 9.05 Peak flow, in cubic feet per second, for indicated annual exceedance probability (bold values), in percent 1988 06/01/1988 2,740 4.64 2018 05/11/2018 12,300 9.40 66.7 50 42.9 20 10 4 2 1.0 0.5 0.2 1989 05/11/1989 4,690 5.80 2017 06/14/2017 9,950 9.15 4,110 5,300 5,870 8,420 10,500 13,200 15,200 17,200 19,200 21,700 1990 06/01/1990 4,730 5.96 1997 06/02/1997 9,870 8.20 Upper and lower 90-percent confidence intervals, in cubic feet per second, for indicated annual exceedance probability, in percent 1991 06/08/1991 5,520 6.26 1986 02/25/1986 9,700 6.52 66.7 50 42.9 20 10 4 2 1.0 0.5 0.2 1992 05/01/1992 1,380 3.58 2009 05/25/2009 8,510 7.47 5,100 6,540 7,240 10,700 14,200 19,800 24,800 30,500 37,100 47,600 1993 05/17/1993 4,570 5.90 2012 04/27/2012 8,470 7.54 3,190 4,240 4,730 6,850 8,540 10,500 11,700 12,700 13,500 14,400 1994 04/25/1994 3,480 5.11 1998 06/27/1998 7,390 7.26 1995 06/07/1995 6,950 6.80 2003 05/30/2003 7,150 7.53 1996 02/09/1996 12,400 9.05 1995 06/07/1995 6,950 6.80 1997 06/02/1997 9,870 8.20 1999 06/04/1999 6,700 7.38 1998 06/27/1998 7,390 7.26 2014 05/27/2014 6,530 7.72 1999 06/04/1999 6,700 7.38 2010 06/18/2010 6,110 7.12 2000 06/01/2000 1,510 5.19 2008 05/21/2008 6,080 7.01 2001 05/16/2001 3,390 6.12 1991 06/08/1991 5,520 6.26 2002 06/03/2002 3,910 6.33 2007 06/13/2007 5,510 6.88 2003 05/30/2003 7,150 7.53 2005 06/05/2005 5,180 6.72 2004 06/12/2004 2,620 5.63 2006 05/20/2006 4,810 6.67 2005 06/05/2005 5,180 6.72 1990 06/01/1990 4,730 5.96 2006 05/20/2006 4,810 6.67 1989 05/11/1989 4,690 5.80 2007 06/13/2007 5,510 6.88 1993 05/17/1993 4,570 5.90 2008 05/21/2008 6,080 7.01 2002 06/03/2002 3,910 6.33 2009 05/25/2009 8,510 7.47 1994 04/25/1994 3,480 5.11 2010 06/18/2010 6,110 7.12 2001 05/16/2001 3,390 6.12 2011 06/11/2011 13,300 8.86 2016 05/09/2016 3,250 5.95 2012 04/27/2012 8,470 7.54 2013 05/14/2013 2,920 5.77 2013 05/14/2013 2,920 5.77 2015 06/03/2015 2,880 5.70 2014 05/27/2014 6,530 7.72 1988 06/01/1988 2,740 4.64 2015 06/03/2015 2,880 5.70 2004 06/12/2004 2,620 5.63 2016 05/09/2016 3,250 5.95 1987 05/01/1987 2,370 4.36 2017 06/14/2017 9,950 9.15 2000 06/01/2000 1,510 5.19 2018 05/11/2018 12,300 9.40 1992 05/01/1992 1,380 3.58

Figure 2. Annual peak flows and perception thresholds.

2Peak-flow data with a value of zero are not plotted in figures 1 or 2. 3In cases where the month, day, or both are not present in the date of a peak flow (as indicated by adjacent slash marks with no intervening values), the month, day, or both are unknown. 4Definitions of peak-flow designations used in analysis include: PT definition: The peak flow is used to define perception thresholds in ungaged historical periods; Opportunistic: The peak flow was excluded from the analysis because it is outside of the systematic record and was of insufficient magnitude to determine nonexceedance during an ungaged period; PILF: The peak flow was identified as a potentially influential low flow; Synthesized: The peak flow was synthesized using Maintenance of Variance Extension Type III record extension. England, J.F., Jr., Cohn, T.A., Faber, B.A., Stedinger, J.R., Thomas, W.O., Jr., Veilleux, A.G., Kiang, J.E., and Mason, R.R., Jr., 2019, Guidelines for determining flood flow frequency—Bulletin 17C (ver. 1.1, May 2019): U.S. Geological Survey Techniques and Methods, book 4, chap. B5, 148 p., https://doi.org/10.3133/tm4B5. Note:X23A0T Not all footnotes are applicable for each frequency analysis. Even if a footnote is not applicable for a given frequency analysis, it is retained in the worksheet for convenience and to maintain consistency among the various similarly structured frequency-analysis worksheets. Also, not all table columns are applicable for each frequency analysis. Even if a table column is not applicable for a given frequency analysis, it is retained in the worksheet for convenience and to maintain consistency among the various similarly structured frequency-analysis worksheets. 12334550 Clark Fork at Turah Bridge near Bonner ,Montana 12334550 Clark Fork at Turah Bridge near Bonner ,Montana Analysis for unregulated period of record Analysis for unregulated period of record Analysis period of record, water years: 1899 - 2018 Analysis period of record, water years: 1899 - 2018 Peak-flow frequency analysis conducted on recorded and synthesized data Peak-flow frequency analysis conducted on recorded and synthesized data Table 1-1 Table 1-2 Table 1-3 Table 1-4 Table 1-5 Table 1-6 Table 1-7 Table 1-8 [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. PILF; potentially influential low flow; MGBT, multiple Grubbs-Beck test; -- not applicable or not available] [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends; --, not applicable or not available.] Peak-flow data3 Ranked (largest to smallest) peak-flow data3 Number of Number of Total number Contributing PILF Type of peak- recorded Skew type peak flows of years in Type of PILF Peak flow, in Gage Peak-flow Peak flow, in Gage drainage area, threshold, in flow Water 4 Water 4 Peak-flow designation in peak flows used in 1 used for ungaged cubic feet per height, designation in cubic feet per height, in square threshold cubic feet per frequency Date Date 5 used in the analysis perception historical year 5 year analysis miles second analysis second in feet analysis second in feet analysis thresholds period(s) 1899 18,600 Synthesized 1908 26,300 Synthesized 3,657 113 Weighted FIXED 4,570 7 MOVE3 1900 11,900 Synthesized 1981 22,100 Synthesized Peak flow, in cubic feet per second, for indicated annual exceedance probability (bold values), in percent 1901 8,840 Synthesized 1899 18,600 Synthesized 66.7 50 42.9 20 10 4 2 1.0 0.5 0.2 1902 15,400 Synthesized 1902 15,400 Synthesized 5,040 6,260 6,840 9,570 12,000 15,200 17,700 20,400 23,200 27,000 1903 11,600 Synthesized 1913 14,800 Synthesized Upper and lower 90-percent confidence intervals, in cubic feet per second, for indicated annual exceedance probability, in percent 1904 7,540 Synthesized 2011 6/11/2011 13,300 8.86 66.7 50 42.9 20 10 4 2 1.0 0.5 0.2 1905 2,970 PILF , Synthesized 1917 13,200 Synthesized 5,849 7,295 8,015 11,533 15,064 20,971 26,789 34,079 43,147 58,472 1906 2,640 PILF , Synthesized 1975 13,000 Synthesized 3,889 5,233 5,784 8,032 9,822 11,943 13,359 14,647 15,748 16,961 1907 7,910 Synthesized 1996 2/9/1996 12,400 9.05 1908 26,300 Synthesized 1972 12,300 Synthesized 1911 5,420 Synthesized 2018 5/11/2018 12,300 9.40 1912 7,480 Synthesized 1900 11,900 Synthesized 1913 14,800 Synthesized 1903 11,600 Synthesized 1914 5,730 Synthesized 1916 11,100 Synthesized 1915 1,860 PILF , Synthesized 1953 10,700 Synthesized 1916 11,100 Synthesized 1965 10,200 Synthesized 1917 13,200 Synthesized 1948 10,000 Synthesized 1918 9,600 Synthesized 2017 6/14/2017 9,950 9.15 1919 4,310 PILF , Synthesized 1997 6/2/1997 9,870 8.20 1920 4,990 Synthesized 1950 9,850 Synthesized 1921 7,600 Synthesized 1980 9,730 Synthesized 1922 9,230 Synthesized 1986 2/25/1986 9,700 6.52 1923 5,240 Synthesized 1918 9,600 Synthesized 1929 5,110 Synthesized 1947 9,570 Synthesized 1930 3,150 PILF , Synthesized 1933 9,350 Synthesized 1931 1,860 PILF , Synthesized 1922 9,230 Synthesized 1932 5,560 Synthesized 1984 9,050 Synthesized 1933 9,350 Synthesized 1942 9,000 Synthesized 1934 5,530 Synthesized 1901 8,840 Synthesized 1935 3,890 PILF , Synthesized 1943 8,720 Synthesized 1936 5,910 Synthesized 1956 8,720 Synthesized 1937 1,750 PILF , Synthesized 1958 8,720 Synthesized 1938 6,650 Synthesized 1976 8,670 Synthesized 1939 3,530 PILF , Synthesized 1970 8,620 Synthesized 1940 2,000 PILF , Synthesized 1974 8,580 Synthesized 1941 2,590 PILF , Synthesized 2009 5/25/2009 8,510 7.47 1942 9,000 Synthesized 2012 4/27/2012 8,470 7.54 1943 8,720 Synthesized 1951 8,340 Synthesized 1944 3,190 PILF , Synthesized 1964 8,070 Synthesized 1945 3,080 PILF , Synthesized 1907 7,910 Synthesized 1946 2,480 PILF , Synthesized 1921 7,600 Synthesized 1947 9,570 Synthesized 1904 7,540 Synthesized 1948 10,000 Synthesized 1912 7,480 Synthesized Figure 1. Annual peak flows (probability plotting positions) and peak-flow frequency curve. EMA, Expected Moments Algorithm; Skew(G), analysis 1949 6,720 Synthesized 1998 6/27/1998 7,390 7.26 skew; PILF(LO), Potentially Influential Low Flow (low outlier). 1950 9,850 Synthesized 1967 7,370 Synthesized 1951 8,340 Synthesized 2003 5/30/2003 7,150 7.53 1952 5,720 Synthesized 1995 6/7/1995 6,950 6.80 1953 10,700 Synthesized 1949 6,720 Synthesized 1954 6,710 Synthesized 1954 6,710 Synthesized 1955 5,220 Synthesized 1957 6,710 Synthesized 1956 8,720 Synthesized 1999 6/4/1999 6,700 7.38 1957 6,710 Synthesized 1938 6,650 Synthesized 1958 8,720 Synthesized 2014 5/27/2014 6,530 7.72 1959 5,600 Synthesized 1982 6,440 Synthesized 1960 5,320 Synthesized 1971 6,270 Synthesized 1961 5,520 Synthesized 2010 6/18/2010 6,110 7.12 1962 4,780 Synthesized 2008 5/21/2008 6,080 7.01 1963 5,980 Synthesized 1963 5,980 Synthesized 1964 8,070 Synthesized 1936 5,910 Synthesized 1965 10,200 Synthesized 1914 5,730 Synthesized 1966 2,830 PILF , Synthesized 1952 5,720 Synthesized 1967 7,370 Synthesized 1959 5,600 Synthesized 1968 4,630 Synthesized 1932 5,560 Synthesized 1969 5,120 Synthesized 1934 5,530 Synthesized 1970 8,620 Synthesized 1961 5,520 Synthesized 1971 6,270 Synthesized 1991 6/8/1991 5,520 6.26 1972 12,300 Synthesized 2007 6/13/2007 5,510 6.88 1973 1,840 PILF , Synthesized 1911 5,420 Synthesized 1974 8,580 Synthesized 1983 5,360 Synthesized 1975 13,000 Synthesized 1960 5,320 Synthesized 1976 8,670 Synthesized 1923 5,240 Synthesized 1977 1,810 PILF , Synthesized 1955 5,220 Synthesized 1978 5,070 Synthesized 2005 6/5/2005 5,180 6.72 1979 4,930 Synthesized 1969 5,120 Synthesized 1980 9,730 Synthesized 1929 5,110 Synthesized 1981 22,100 Synthesized 1978 5,070 Synthesized 1982 6,440 Synthesized 1920 4,990 Synthesized 1Definitions of types of PILF thresholds include: 1983 5,360 Synthesized 1979 4,930 Synthesized MGBT: PILF threshold calculated by using the multiple Grubbs-Beck Test as specified in Bulletin 17C (England and others, 2016); 1984 9,050 Synthesized 2006 5/20/2006 4,810 6.67 Manual: PILF threshold based on a systematic peak flow selected by the peak-flow frequency analyst. 1985 3,130 PILF , Synthesized 1962 4,780 Synthesized 2Flood-frequency results not reported because of too many values less than the PILF threshold used in the at-site analysis. 1986 2/25/1986 9,700 6.52 1990 6/1/1990 4,730 5.96 3Peak-flow data with a value of zero are not plotted in figures. 1987 5/1/1987 2,370 4.36 PILF 1989 5/11/1989 4,690 5.80 4In cases where the month, day, or both are not present in the date of a peak flow (as indicated by adjacent slash marks with no intervening 1988 6/1/1988 2,740 4.64 PILF 1968 4,630 Synthesized values), the month, day, or both are unknown. 1989 5/11/1989 4,690 5.80 1993 5/17/1993 4,570 5.90 5Definitions of peak-flow designations used in analysis include: 1990 6/1/1990 4,730 5.96 1919 4,310 PILF , Synthesized PT definition: The peak flow is used to define perception thresholds in ungaged historical periods; 1991 6/8/1991 5,520 6.26 2002 6/3/2002 3,910 6.33 PILF Opportunistic: The peak flow was excluded from the analysis because it is outside of the systematic record and was of insufficient magnitude to 1992 5/1/1992 1,380 3.58 PILF 1935 3,890 PILF , Synthesized determine nonexceedance during an ungaged period; 1993 5/17/1993 4,570 5.90 1939 3,530 PILF , Synthesized PILF: The peak flow was identified as a potentially influential low flow; 1994 4/25/1994 3,480 5.11 PILF 1994 4/25/1994 3,480 5.11 PILF Synthesized: The peak flow was synthesized using Maintenance of Variance Extension Type III record extension. 1995 6/7/1995 6,950 6.80 2001 5/16/2001 3,390 6.12 PILF England, J.F., Jr., Cohn, T.A., Faber, B.A., Stedinger, J.R., Thomas, W.O., Jr., Veilleux, A.G., Kiang, J.E., and Mason, R.R., Jr., 2019, Guidelines 1996 2/9/1996 12,400 9.05 2016 5/9/2016 3,250 5.95 PILF for determining flood flow frequency—Bulletin 17C (ver. 1.1, May 2019): U.S. Geological Survey Techniques and Methods, book 4, chap. B5, 148 1997 6/2/1997 9,870 8.20 1944 3,190 PILF , Synthesized p., https://doi.org/10.3133/tm4B5. 1998 6/27/1998 7,390 7.26 1930 3,150 PILF , Synthesized 1999 6/4/1999 6,700 7.38 1985 3,130 PILF , Synthesized 2000 6/1/2000 1,510 5.19 PILF 1945 3,080 PILF , Synthesized 2001 5/16/2001 3,390 6.12 PILF 1905 2,970 PILF , Synthesized 2002 6/3/2002 3,910 6.33 PILF 2013 5/14/2013 2,920 5.77 PILF 2003 5/30/2003 7,150 7.53 2015 6/3/2015 2,880 5.70 PILF 2004 6/12/2004 2,620 5.63 PILF 1966 2,830 PILF , Synthesized 2005 6/5/2005 5,180 6.72 1988 6/1/1988 2,740 4.64 PILF 2006 5/20/2006 4,810 6.67 1906 2,640 PILF , Synthesized 2007 6/13/2007 5,510 6.88 2004 6/12/2004 2,620 5.63 PILF 2008 5/21/2008 6,080 7.01 1941 2,590 PILF , Synthesized 2009 5/25/2009 8,510 7.47 1946 2,480 PILF , Synthesized 2010 6/18/2010 6,110 7.12 1987 5/1/1987 2,370 4.36 PILF 2011 6/11/2011 13,300 8.86 1940 2,000 PILF , Synthesized 2012 4/27/2012 8,470 7.54 1915 1,860 PILF , Synthesized 2013 5/14/2013 2,920 5.77 PILF 1931 1,860 PILF , Synthesized 2014 5/27/2014 6,530 7.72 1973 1,840 PILF , Synthesized 2015 6/3/2015 2,880 5.70 PILF 1977 1,810 PILF , Synthesized 2016 5/9/2016 3,250 5.95 PILF 1937 1,750 PILF , Synthesized 2017 6/14/2017 9,950 9.15 2000 6/1/2000 1,510 5.19 PILF 2018 5/11/2018 12,300 9.40 1992 5/1/1992 1,380 3.58 PILF 12340500 Clark Fork above Missoula, Montana 12340500 Clark Fork above Missoula, Montana Analysis for unregulated period of record Analysis for unregulated period of record Analysis period of record, water years: 1899–1908; 1930–2018 Analysis period of record, water years: 1899–1908; 1930–2018 At-site peak-flow frequency analysis conducted on recorded data At-site peak-flow frequency analysis conducted on recorded data Table 1‐1 Table 1‐2 Table 1‐3 Table 1‐4 Table 1‐5 Table 1‐6 Table 1‐7 Table 1‐8 Table 1‐1 Table 1‐2 Table 1‐3 Table 1‐4 Table 1‐5 Table 1‐6 Table 1‐7 Table 1‐8 [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. PILF; potentially influential low flow; MGBT, multiple Grubbs-Beck test; -- not applicable or not available] [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends; --, not applicable or not available.] Peak-flow data Ranked (largest to smallest) peak-flow data Number of Number of Total number Contributing PILF Type of peak‐ recorded Skew type peak flows of years in Type of PILF flow Peak flow, in Gage Peak-flow Peak flow, in Gage Peak-flow drainage area, threshold, in Water 2 Water 2 peak flows used in 1 used for ungaged cubic feet per height, designation in cubic feet per height, designation in in square threshold cubic feet per frequency Date Date used in the analysis perception historical year 4 year 4 miles second second in feet analysis second in feet analysis analysis thresholds period(s) analysis 1899 06/20/1899 36,400 -- 1908 06//1908 48,000 -- 6,021 99 Weighted MGBT -- At-site 1900 05/12/1900 25,600 -- 1899 06/20/1899 36,400 -- Peak flow, in cubic feet per second, for indicated annual exceedance probability (bold values), in percent 1901 05/18/1901 20,200 -- 2018 05/11/2018 32,500 13.83 66.7 50 42.9 20 10 4 2 1.0 0.5 0.2 1902 05/30/1902 31,400 -- 1975 06/21/1975 32,300 13.75 11,800 14,700 16,000 21,700 26,300 32,000 36,100 40,100 44,000 49,100 1903 06/04/1903 25,000 -- 1964 06/10/1964 31,700 13.35 Upper and lower 90-percent confidence intervals, in cubic feet per second, for indicated annual exceedance probability, in percent 1904 05/23/1904 17,800 -- 1948 05/23/1948 31,500 13.07 Historic 66.7 50 42.9 20 10 4 2 1.0 0.5 0.2 1905 06/08/1905 8,500 -- 1902 05/30/1902 31,400 -- 13,200 16,300 17,700 24,100 29,700 37,600 43,900 50,600 57,700 67,900 1906 06/07/1906 7,740 -- 1981 05/24/1981 29,500 13.38 10,600 13,200 14,400 19,700 23,800 28,500 31,500 34,100 36,300 38,900 1907 05/21/1907 18,500 -- 1953 06/04/1953 28,800 12.34 1908 06//1908 48,000 -- 2011 06/09/2011 28,500 12.72 1930 04/28/1930 9,200 6.54 1972 06/01/1972 27,400 12.27 1931 06/21/1931 4,150 4.36 1997 05/18/1997 27,000 12.59 1932 05/14/1932 12,800 7.65 1900 05/12/1900 25,600 -- 1933 06/02/1933 21,600 9.90 1976 05/15/1976 25,500 12.00 1934 04/13/1934 15,300 8.42 1903 06/04/1903 25,000 -- 1935 05/25/1935 10,300 6.74 1947 05/09/1947 24,200 11.10 1936 05/16/1936 12,600 7.51 1933 06/02/1933 21,600 9.90 1937 05/05/1937 7,790 5.80 1956 05/23/1956 21,200 10.27 1938 05/30/1938 19,700 10.01 1967 06/03/1967 21,100 10.67 1939 05/05/1939 11,000 7.10 1974 06/17/1974 20,900 10.68 1940 05/13/1940 6,140 5.17 1951 05/15/1951 20,400 10.04 1941 06/06/1941 5,400 4.89 1959 06/07/1959 20,300 10.34 1942 05/27/1942 17,400 9.11 1901 05/18/1901 20,200 -- 1943 04/20/1943 18,700 9.50 1950 06/19/1950 19,800 9.87 1944 05/20/1944 8,230 5.99 1938 05/30/1938 19,700 10.01 1945 06/07/1945 9,860 6.75 1971 05/14/1971 19,400 10.14 1946 05/29/1946 10,100 6.84 1957 05/21/1957 18,900 9.61 1947 05/09/1947 24,200 11.10 1943 04/20/1943 18,700 9.50 1948 05/23/1948 31,500 13.07 Historic 1907 05/21/1907 18,500 -- 1949 05/17/1949 15,200 8.53 1982 06/17/1982 18,500 10.31 1950 06/19/1950 19,800 9.87 1996 06/10/1996 18,500 9.94 1951 05/15/1951 20,400 10.04 2012 04/27/2012 18,500 9.70 1952 05/16/1952 15,500 8.62 1980 05/27/1980 18,300 10.03 1953 06/04/1953 28,800 12.34 1954 05/21/1954 17,900 9.42 1954 05/21/1954 17,900 9.42 1904 05/23/1904 17,800 -- 1955 05/23/1955 13,900 8.22 1965 06/18/1965 17,700 9.76 1956 05/23/1956 21,200 10.27 1970 05/27/1970 17,700 9.76 1957 05/21/1957 18,900 9.61 1979 05/27/1979 17,500 9.77 1958 05/22/1958 16,800 9.00 2008 05/21/2008 17,500 9.48 1959 06/07/1959 20,300 10.34 2009 05/31/2009 17,500 9.49 1960 05/14/1960 12,100 7.86 1942 05/27/1942 17,400 9.11 1961 05/27/1961 15,800 9.06 2014 05/27/2014 17,300 9.30 1962 05/26/1962 14,200 8.65 1958 05/22/1958 16,800 9.00 1963 06/05/1963 10,400 7.23 1999 06/04/1999 16,300 9.59 1964 06/10/1964 31,700 13.35 2003 06/01/2003 15,900 9.48 1965 06/18/1965 17,700 9.76 2017 06/14/2017 15,900 9.07 1966 05/11/1966 10,300 7.29 1961 05/27/1961 15,800 9.06 Figure 1. Annual peak flows (probability plotting positions) and peak-flow frequency curve. EMA, Expected Moments Algorithm; Skew(G), analysis 1967 06/03/1967 21,100 10.67 1952 05/16/1952 15,500 8.62 skew; PILF(LO), potentially Influencial low flow (low outlier). 1968 06/13/1968 11,900 7.89 1934 04/13/1934 15,300 8.42 1969 05/15/1969 13,400 8.42 1989 05/11/1989 15,300 9.27 1970 05/27/1970 17,700 9.76 1949 05/17/1949 15,200 8.53 Figure 1. Annual peak flows (probability plotting positions) and peak-flow frequency curve. 1971 05/14/1971 19,400 10.14 1991 05/20/1991 14,500 9.01 1972 06/01/1972 27,400 12.27 1962 05/26/1962 14,200 8.65 1973 05/21/1973 7,790 6.19 1978 06/09/1978 14,200 8.73 1974 06/17/1974 20,900 10.68 1955 05/23/1955 13,900 8.22 1975 06/21/1975 32,300 13.75 1983 05/20/1983 13,900 8.78 1976 05/15/1976 25,500 12.00 1984 06/22/1984 13,800 8.76 1977 05/04/1977 3,520 4.19 1986 05/30/1986 13,600 8.67 1978 06/09/1978 14,200 8.73 1969 05/15/1969 13,400 8.42 1979 05/27/1979 17,500 9.77 1995 06/07/1995 13,400 8.62 1980 05/27/1980 18,300 10.03 1990 05/31/1990 13,000 8.45 1981 05/24/1981 29,500 13.38 1993 05/16/1993 13,000 8.48 1982 06/17/1982 18,500 10.31 2013 05/14/2013 13,000 8.00 1983 05/20/1983 13,900 8.78 1998 06/21/1998 12,900 8.26 1984 06/22/1984 13,800 8.76 2006 05/20/2006 12,900 8.43 1985 05/25/1985 9,250 7.03 1932 05/14/1932 12,800 7.65 1986 05/30/1986 13,600 8.67 1936 05/16/1936 12,600 7.51 1987 05/01/1987 7,270 6.17 2002 05/30/2002 12,300 8.21 1988 05/18/1988 6,460 5.79 1960 05/14/1960 12,100 7.86 1989 05/11/1989 15,300 9.27 1968 06/13/1968 11,900 7.89 1990 05/31/1990 13,000 8.45 1939 05/05/1939 11,000 7.10 1991 05/20/1991 14,500 9.01 1994 04/25/1994 10,600 7.57 1992 05/01/1992 6,150 5.64 1963 06/05/1963 10,400 7.23 1993 05/16/1993 13,000 8.48 1935 05/25/1935 10,300 6.74 1994 04/25/1994 10,600 7.57 1966 05/11/1966 10,300 7.29 1995 06/07/1995 13,400 8.62 2010 06/18/2010 10,200 7.11 1996 06/10/1996 18,500 9.94 1946 05/29/1946 10,100 6.84 1997 05/18/1997 27,000 12.59 1945 06/07/1945 9,860 6.75 1998 06/21/1998 12,900 8.26 2005 05/20/2005 9,400 7.09 1999 06/04/1999 16,300 9.59 2007 05/04/2007 9,320 7.06 2000 04/23/2000 6,770 5.94 1985 05/25/1985 9,250 7.03 2001 05/15/2001 6,990 6.04 1930 04/28/1930 9,200 6.54 2002 05/30/2002 12,300 8.21 1905 06/08/1905 8,500 -- 2003 06/01/2003 15,900 9.48 1944 05/20/1944 8,230 5.99 Figure 2. Annual peak flows and perception thresholds. 2004 06/13/2004 7,290 6.18 2016 05/09/2016 8,010 6.24 2005 05/20/2005 9,400 7.09 1937 05/05/1937 7,790 5.80 2006 05/20/2006 12,900 8.43 1973 05/21/1973 7,790 6.19 2007 05/04/2007 9,320 7.06 1906 06/07/1906 7,740 -- 2008 05/21/2008 17,500 9.48 2004 06/13/2004 7,290 6.18 1Definitions of types of PILF thresholds include: 2009 05/31/2009 17,500 9.49 1987 05/01/1987 7,270 6.17 MGBT: PILF threshold calculated by using the multiple Grubbs-Beck Test as specifed in Bulletin 17C (England and others, 2016); 2010 06/18/2010 10,200 7.11 2015 06/03/2015 7,050 5.91 Manual: PILF threshold based on a systematic peak flow selected by the peak-flow frequency analyst. 2011 06/09/2011 28,500 12.72 2001 05/15/2001 6,990 6.04 2012 04/27/2012 18,500 9.70 2000 04/23/2000 6,770 5.94 2Peak-flow data with a value of zero are not plotted in figures 1 or 2. 2013 05/14/2013 13,000 8.00 1988 05/18/1988 6,460 5.79 3In cases where the month, day, or both are not present in the date of a peak flow (as indicated by adjacent slash marks with no intervening values), 2014 05/27/2014 17,300 9.30 1992 05/01/1992 6,150 5.64 the month, day, or both are unknown. 2015 06/03/2015 7,050 5.91 1940 05/13/1940 6,140 5.17 4Definitions of peak-flow designations used in analysis include: PT definition: The peak flow is used to define perception thresholds in ungaged historical periods; 2016 05/09/2016 8,010 6.24 1941 06/06/1941 5,400 4.89 Opportunistic: The peak flow was excluded from the analysis because it is outside of the systematic record and was of insufficient magnitude to 2017 06/14/2017 15,900 9.07 1931 06/21/1931 4,150 4.36 determine nonexceedance during an ungaged period; 2018 05/11/2018 32,500 13.83 1977 05/04/1977 3,520 4.19 PILF: The peak flow was identified as a potentially influential low flow; Synthesized: The peak flow was synthesized using Maintenance of Variance Extension Type III record extension. England, J.F., Jr., Cohn, T.A., Faber, B.A., Stedinger, J.R., Thomas, W.O., Jr., Veilleux, A.G., Kiang, J.E., and Mason, R.R., Jr., 2019, Guidelines for determining flood flow frequency—Bulletin 17C (ver. 1.1, May 2019): U.S. Geological Survey Techniques and Methods, book 4, chap. B5, 148 p., https://doi.org/10.3133/tm4B5. 12340500 Clark Fork above Missoula, Montan 12340500 Clark Fork above Missoula, Montan Analysis for unregulated period of record Analysis for unregulated period of record Analysis period of record, water years: 1899–1908; 1911–23; 1929–2018 Analysis period of record, water years: 1899–1908; 1911–23; 1929–2018 Peak-flow frequency analysis conducted on recorded and synthesized data Peak-flow frequency analysis conducted on recorded and synthesized data Table 1‐1 Table 1‐2 Table 1‐3 Table 1‐4 Table 1‐5 Table 1‐6 Table 1‐7 Table 1‐8 Table 1‐1 Table 1‐2 Table 1‐3 Table 1‐4 Table 1‐5 Table 1‐6 Table 1‐7 Table 1‐8 [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. PILF; potentially influential low flow; MGBT, multiple Grubbs-Beck test; -- not applicable or not available] [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends; --, not applicable or not available.] Peak-flow data Ranked (largest to smallest) peak-flow data Contributing Type of peak‐ Peak flow, in Gage Peak-flow Peak flow, in Gage Peak-flow drainage area, flow Water Water 2 cubic feet per height, designation in 2 cubic feet per height, designation in in square frequency year Date year Date second in feet 4 second in feet 4 miles analysis analysis analysis 1899 06/20/1899 36,400 -- 1908 06//1908 48,000 -- 6,021 MOVE.3 1900 05/12/1900 25,600 -- 1899 06/20/1899 36,400 -- Peak flow, in cubic feet per second, for indicated annual exceedance probability (bold values), in percent 1901 05/18/1901 20,200 -- 2018 05/11/2018 32,500 13.83 66.7 50 42.9 20 10 4 2 1.0 0.5 0.2 1902 05/30/1902 31,400 -- 1975 06/21/1975 32,300 13.75 12,100 15,000 16,300 22,100 26,600 32,100 36,100 39,900 43,600 48,400 1903 06/04/1903 25,000 -- 1964 06/10/1964 31,700 13.35 Upper and lower 90-percent confidence intervals, in cubic feet per second, for indicated annual exceedance probability, in percent 1904 05/23/1904 17,800 -- 1948 05/23/1948 31,500 13.07 Historic 66.7 50 42.9 20 10 4 2 1.0 0.5 0.2 1905 06/08/1905 8,500 -- 1902 05/30/1902 31,400 -- 15,400 18,900 20,400 27,800 34,800 45,700 55,100 65,400 76,800 93,400 1906 06/07/1906 7,740 -- 1981 05/24/1981 29,500 13.38 9,010 11,400 12,400 17,000 20,200 23,400 24,500 24,700 --5 --5 1907 05/21/1907 18,500 -- 1913 //1913 28,900 -- Synthesized 1908 06//1908 48,000 -- 1953 06/04/1953 28,800 12.34 1911 //1911 13,700 -- Synthesized 2011 06/09/2011 28,500 12.72 100,000 EXPLANATION 1912 //1912 17,400 -- Synthesized 1972 06/01/1972 27,400 12.27 1913 //1913 28,900 -- Synthesized 1997 05/18/1997 27,000 12.59 Peaks used in at-site analysis 1914 //1914 14,300 -- Synthesized 1917 //1917 26,500 -- Synthesized 1915 //1915 6,230 -- Synthesized 1900 05/12/1900 25,600 -- Potentially influential low flow (PILF) 1916 //1916 23,300 -- Synthesized 1976 05/15/1976 25,500 12.00 1917 //1917 26,500 -- Synthesized 1903 06/04/1903 25,000 -- Weighted peak-flow frequency 1918 //1918 21,000 -- Synthesized 1947 05/09/1947 24,200 11.10 curve 1919 //1919 11,600 -- Synthesized 1916 //1916 23,300 -- Synthesized Upper and lower 95-percent 1920 //1920 12,900 -- Synthesized 1933 06/02/1933 21,600 9.90 confidence intervals 1921 //1921 17,600 -- Synthesized 1956 05/23/1956 21,200 10.27 1922 //1922 20,300 -- Synthesized 1967 06/03/1967 21,100 10.67 1923 //1923 13,400 -- Synthesized 1918 //1918 21,000 -- Synthesized 1929 //1929 13,100 -- Synthesized 1974 06/17/1974 20,900 10.68 1930 04/28/1930 9,200 6.54 1951 05/15/1951 20,400 10.04 10,000 1931 06/21/1931 4,150 4.36 1922 //1922 20,300 -- Synthesized 1932 05/14/1932 12,800 7.65 1959 06/07/1959 20,300 10.34

Flow, in cubicfeetsecond per 1933 06/02/1933 21,600 9.90 1901 05/18/1901 20,200 -- 1934 04/13/1934 15,300 8.42 1950 06/19/1950 19,800 9.87 1935 05/25/1935 10,300 6.74 1938 05/30/1938 19,700 10.01 1936 05/16/1936 12,600 7.51 1971 05/14/1971 19,400 10.14 1937 05/05/1937 7,790 5.80 1957 05/21/1957 18,900 9.61 1938 05/30/1938 19,700 10.01 1943 04/20/1943 18,700 9.50 1939 05/05/1939 11,000 7.10 1907 05/21/1907 18,500 -- 1940 05/13/1940 6,140 5.17 1982 06/17/1982 18,500 10.31 1941 06/06/1941 5,400 4.89 1996 06/10/1996 18,500 9.94 1942 05/27/1942 17,400 9.11 2012 04/27/2012 18,500 9.70 1943 04/20/1943 18,700 9.50 1980 05/27/1980 18,300 10.03 1944 05/20/1944 8,230 5.99 1954 05/21/1954 17,900 9.42 1,000

99 98 95 90 80 70 60 50 40 30 20 10 5 2 1 1945 06/07/1945 9,860 6.75 1904 05/23/1904 17,800 -- 99.5 0.5 0.2 1946 05/29/1946 10,100 6.84 1965 06/18/1965 17,700 9.76 Exceedance probability, in percent 1947 05/09/1947 24,200 11.10 1970 05/27/1970 17,700 9.76 1948 05/23/1948 31,500 13.07 Historic 1921 //1921 17,600 -- Synthesized Figure 1. Annual peak flows (plotting positions determined using the Cunnane formulation; Helsel and HIrsch, 2002) and peak-flow frequencycurve 1949 05/17/1949 15,200 8.53 1979 05/27/1979 17,500 9.77 1950 06/19/1950 19,800 9.87 2008 05/21/2008 17,500 9.48 1951 05/15/1951 20,400 10.04 2009 05/31/2009 17,500 9.49 1952 05/16/1952 15,500 8.62 1912 //1912 17,400 -- Synthesized 1953 06/04/1953 28,800 12.34 1942 05/27/1942 17,400 9.11 1Peak flows with a value of zero are not plotted in figure 1 . 1954 05/21/1954 17,900 9.42 2014 05/27/2014 17,300 9.30 2In cases where the month, day, or both are not present in the date of a peak flow (as indicated by adjacent slash marks with no intervening values), 1955 05/23/1955 13,900 8.22 1958 05/22/1958 16,800 9.00 the month, day, or both are unknown. 1956 05/23/1956 21,200 10.27 1999 06/04/1999 16,300 9.59 3Flood-frequency results not reported because of too many values less than the PILF threshold used in the at-site analysis. 1957 05/21/1957 18,900 9.61 2003 06/01/2003 15,900 9.48 4Definitions of peak-flow designations used in analysis include: 1958 05/22/1958 16,800 9.00 2017 06/14/2017 15,900 9.07 PT definition: The peak flow is used to define perception thresholds in ungaged historical periods; 1959 06/07/1959 20,300 10.34 1961 05/27/1961 15,800 9.06 Opportunistic: The peak flow was excluded from the analysis because it is outside of the systematic record and was of insufficient magnitude to 1960 05/14/1960 12,100 7.86 1952 05/16/1952 15,500 8.62 determine nonexceedance during an ungaged period; 1961 05/27/1961 15,800 9.06 1934 04/13/1934 15,300 8.42 PILF: The peak flow was identified as a potentially influential low flow; Synthesized: The peak flow was synthesized using Maintenance of Variance Extension Type III record extension. 1962 05/26/1962 14,200 8.65 1989 05/11/1989 15,300 9.27 1963 06/05/1963 10,400 7.23 1949 05/17/1949 15,200 8.53 5Lower confidence levels calculated for the 0.5 and 0.2 percent AEP using eq. 28 from Sando and McCarthy (2018) were considered unreliable, 1964 06/10/1964 31,700 13.35 1991 05/20/1991 14,500 9.01 because they are lower than the value for the 1-percent AEP, and are not presented. 1965 06/18/1965 17,700 9.76 1914 //1914 14,300 -- Synthesized 1966 05/11/1966 10,300 7.29 1962 05/26/1962 14,200 8.65 1967 06/03/1967 21,100 10.67 1978 06/09/1978 14,200 8.73 England, J.F., Jr., Cohn, T.A., Faber, B.A., Stedinger, J.R., Thomas, W.O., Jr., Veilleux, A.G., Kiang, J.E., and Mason, R.R., Jr., 2019, Guidelines 1968 06/13/1968 11,900 7.89 1955 05/23/1955 13,900 8.22 for determining flood flow frequency—Bulletin 17C (ver. 1.1, May 2019): U.S. Geological Survey Techniques and Methods, book 4, chap. B5, 148 1969 05/15/1969 13,400 8.42 1983 05/20/1983 13,900 8.78 p., https://doi.org/10.3133/tm4B5. 1970 05/27/1970 17,700 9.76 1984 06/22/1984 13,800 8.76 1971 05/14/1971 19,400 10.14 1911 //1911 13,700 -- Synthesized 1972 06/01/1972 27,400 12.27 1986 05/30/1986 13,600 8.67 Helsel,D.R.,andHirsch,R.M.,2002,Statisticalmethodsinwaterresources:U.S.GeologicalSur-veyTechniquesofWater- 1973 05/21/1973 7,790 6.19 1923 //1923 13,400 -- Synthesized ResourcesInvestigations,book4,chap.A3,522p.,accessedOctober30,2017,athttps://pubs.usgs.gov/twri/twri4a3/. 1974 06/17/1974 20,900 10.68 1969 05/15/1969 13,400 8.42 1975 06/21/1975 32,300 13.75 1995 06/07/1995 13,400 8.62 1976 05/15/1976 25,500 12.00 1929 //1929 13,100 -- Synthesized 1977 05/04/1977 3,520 4.19 1990 05/31/1990 13,000 8.45 1978 06/09/1978 14,200 8.73 1993 05/16/1993 13,000 8.48 1979 05/27/1979 17,500 9.77 2013 05/14/2013 13,000 8.00 1980 05/27/1980 18,300 10.03 1920 //1920 12,900 -- Synthesized 1981 05/24/1981 29,500 13.38 1998 06/21/1998 12,900 8.26 1982 06/17/1982 18,500 10.31 2006 05/20/2006 12,900 8.43 1983 05/20/1983 13,900 8.78 1932 05/14/1932 12,800 7.65 1984 06/22/1984 13,800 8.76 1936 05/16/1936 12,600 7.51 1985 05/25/1985 9,250 7.03 2002 05/30/2002 12,300 8.21 1986 05/30/1986 13,600 8.67 1960 05/14/1960 12,100 7.86 1987 05/01/1987 7,270 6.17 1968 06/13/1968 11,900 7.89 1988 05/18/1988 6,460 5.79 1919 //1919 11,600 -- Synthesized 1989 05/11/1989 15,300 9.27 1939 05/05/1939 11,000 7.10 1990 05/31/1990 13,000 8.45 1994 04/25/1994 10,600 7.57 1991 05/20/1991 14,500 9.01 1963 06/05/1963 10,400 7.23 1992 05/01/1992 6,150 5.64 1935 05/25/1935 10,300 6.74 1993 05/16/1993 13,000 8.48 1966 05/11/1966 10,300 7.29 1994 04/25/1994 10,600 7.57 2010 06/18/2010 10,200 7.11 1995 06/07/1995 13,400 8.62 1946 05/29/1946 10,100 6.84 1996 06/10/1996 18,500 9.94 1945 06/07/1945 9,860 6.75 1997 05/18/1997 27,000 12.59 2005 05/20/2005 9,400 7.09 1998 06/21/1998 12,900 8.26 2007 05/04/2007 9,320 7.06 1999 06/04/1999 16,300 9.59 1985 05/25/1985 9,250 7.03 2000 04/23/2000 6,770 5.94 1930 04/28/1930 9,200 6.54 2001 05/15/2001 6,990 6.04 1905 06/08/1905 8,500 -- 2002 05/30/2002 12,300 8.21 1944 05/20/1944 8,230 5.99 2003 06/01/2003 15,900 9.48 2016 05/09/2016 8,010 6.24 2004 06/13/2004 7,290 6.18 1937 05/05/1937 7,790 5.80 2005 05/20/2005 9,400 7.09 1973 05/21/1973 7,790 6.19 2006 05/20/2006 12,900 8.43 1906 06/07/1906 7,740 -- 2007 05/04/2007 9,320 7.06 2004 06/13/2004 7,290 6.18 2008 05/21/2008 17,500 9.48 1987 05/01/1987 7,270 6.17 2009 05/31/2009 17,500 9.49 2015 06/03/2015 7,050 5.91 2010 06/18/2010 10,200 7.11 2001 05/15/2001 6,990 6.04 2011 06/09/2011 28,500 12.72 2000 04/23/2000 6,770 5.94 2012 04/27/2012 18,500 9.70 1988 05/18/1988 6,460 5.79 2013 05/14/2013 13,000 8.00 1915 //1915 6,230 -- Synthesized 2014 05/27/2014 17,300 9.30 1992 05/01/1992 6,150 5.64 2015 06/03/2015 7,050 5.91 1940 05/13/1940 6,140 5.17 12342500 West Fork Bitterroot River near Conner, Montana 12342500 West Fork Bitterroot River near Conner, Montana Analysis for total period of record Analysis for total period of record Analysis period of record, water years: 1941–2018 Analysis period of record, water years: 1941–2018 Peak-flow frequency analsys determined from regional regression equations Peak-flow frequency analsys determined from regional regression equations Table 1‐1 Table 1‐2 Table 1‐3 Table 1‐4 Table 1‐5 Table 1‐6 Table 1‐7 Table 1‐8 Table 1‐1 Table 1‐2 Table 1‐3 Table 1‐4 Table 1‐5 Table 1‐6 Table 1‐7 Table 1‐8 [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. PILF; potentially influential low flow; MGBT, multiple Grubbs-Beck test] [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends; --, not applicable or not available.] Peak-flow data Ranked (largest to smallest) peak-flow data Number of Number of Total number Contributing PILF Type of peak‐ recorded Skew type peak flows of years in Type of PILF flow Peak flow, in Gage Peak-flow Peak flow, in Gage Peak-flow drainage area, threshold, in Water 2 Water 2 peak flows used in 1 used for ungaged cubic feet per height, designation in cubic feet per height, designation in in square threshold cubic feet per frequency Date Date used in the analysis perception historical year 4 year 4 miles second second in feet analysis second in feet analysis analysis thresholds period(s) analysis 1941 05/28/1941 820 3.35 PILF 1947 05/09/1947 4,060 6.18 317 78 Weighted MGBT 1,530 At-site 1942 05/26/1942 2,490 5.10 1948 05/29/1948 3,880 6.08 Peak flow, in cubic feet per second, for indicated annual exceedance probability (bold values), in percent 1943 05/31/1943 2,290 4.92 1974 06/16/1974 3,380 5.54 66.7 50 42.9 20 10 4 2 1.0 0.5 0.2 1944 05/19/1944 832 3.31 PILF 1956 05/23/1956 3,260 5.52 1,550 1,890 2,050 2,670 3,130 3,650 3,990 4,310 4,600 4,940 1945 06/06/1945 1,300 3.97 PILF 1997 05/17/1997 3,260 5.49 Upper and lower 90-percent confidence intervals, in cubic feet per second, for indicated annual exceedance probability, in percent 1946 05/28/1946 1,110 3.86 PILF 1972 06/03/1972 3,230 5.44 66.7 50 42.9 20 10 4 2 1.0 0.5 0.2 1947 05/09/1947 4,060 6.18 1964 06/09/1964 3,120 5.47 1,730 2,090 2,260 2,990 3,590 4,350 4,900 5,450 6,000 6,770 1948 05/29/1948 3,880 6.08 2018 05/10/2018 3,100 5.20 1,140 1,620 1,800 2,420 2,820 3,250 3,500 3,720 3,900 4,090 1949 05/16/1949 2,280 4.78 1982 05/27/1982 3,080 5.21 1950 06/07/1950 1,860 4.38 2003 05/31/2003 3,070 5.21 1951 05/25/1951 2,020 4.45 1976 05/11/1976 2,980 5.23 1952 05/27/1952 1,670 4.18 1996 06/10/1996 2,920 5.09 1953 06/14/1953 2,390 4.88 1957 05/10/1957 2,860 5.24 1954 05/21/1954 1,850 4.37 2009 05/20/2009 2,790 5.17 1955 06/15/1955 2,090 4.54 1971 05/13/1971 2,780 5.16 1956 05/23/1956 3,260 5.52 2014 05/24/2014 2,760 5.05 1957 05/10/1957 2,860 5.24 1984 05/31/1984 2,600 4.84 1958 05/22/1958 2,440 4.93 2008 05/20/2008 2,560 4.96 1959 06/06/1959 2,330 4.78 1970 05/27/1970 2,550 4.96 1960 05/13/1960 2,280 4.80 1942 05/26/1942 2,490 5.10 1961 05/27/1961 1,820 4.33 1967 05/24/1967 2,490 4.94 1962 05/10/1962 1,300 3.70 PILF 1975 06/20/1975 2,470 4.85 1963 05/25/1963 1,600 4.04 1958 05/22/1958 2,440 4.93 1964 06/09/1964 3,120 5.47 1953 06/14/1953 2,390 4.88 1965 05/31/1965 2,200 4.70 2011 06/08/2011 2,390 4.80 1966 05/29/1966 652 2.85 PILF 1978 06/08/1978 2,370 4.77 1967 05/24/1967 2,490 4.94 1959 06/06/1959 2,330 4.78 1968 06/04/1968 1,650 4.10 1943 05/31/1943 2,290 4.92 1969 05/20/1969 1,990 4.44 1949 05/16/1949 2,280 4.78 1970 05/27/1970 2,550 4.96 1960 05/13/1960 2,280 4.80 1971 05/13/1971 2,780 5.16 2012 04/27/2012 2,250 4.67 1972 06/03/1972 3,230 5.44 1965 05/31/1965 2,200 4.70 1973 04/23/1973 1,620 4.06 2017 05/13/2017 2,200 4.51 1974 06/16/1974 3,380 5.54 1986 05/30/1986 2,160 4.54 1975 06/20/1975 2,470 4.85 1955 06/15/1955 2,090 4.54 1976 05/11/1976 2,980 5.23 1999 05/27/1999 2,050 4.46 1977 06/08/1977 859 3.06 PILF 1951 05/25/1951 2,020 4.45 1978 06/08/1978 2,370 4.77 1969 05/20/1969 1,990 4.44 1979 05/25/1979 1,980 4.36 1979 05/25/1979 1,980 4.36 1980 06/03/1980 975 3.23 PILF 1983 05/30/1983 1,970 4.30 1981 05/26/1981 1,740 4.08 2006 05/20/2006 1,870 4.28 1982 05/27/1982 3,080 5.21 1950 06/07/1950 1,860 4.38 1983 05/30/1983 1,970 4.30 1954 05/21/1954 1,850 4.37 1984 05/31/1984 2,600 4.84 1961 05/27/1961 1,820 4.33 1985 05/25/1985 830 3.00 PILF 1981 05/26/1981 1,740 4.08 1986 05/30/1986 2,160 4.54 2002 05/31/2002 1,710 4.11 1987 07/17/1987 311 1.99 PILF 2013 05/14/2013 1,680 3.98 Figure 1. Annual peak flows (probability plotting positions) and peak-flow frequency curve. EMA, Expected Moments Algorithm; Skew(G), analysis 1988 05/26/1988 1,010 3.23 PILF 1952 05/27/1952 1,670 4.18 skew; PILF(LO), potentially Influencial low flow (low outlier). 1989 05/11/1989 1,450 3.77 PILF 1968 06/04/1968 1,650 4.10 1990 05/30/1990 931 3.12 PILF 1973 04/23/1973 1,620 4.06 1991 06/07/1991 1,240 3.58 PILF 1963 05/25/1963 1,600 4.04 1992 05/21/1992 493 2.37 PILF 2010 06/07/2010 1,530 3.90 1993 05/21/1993 1,280 3.62 PILF 1995 06/06/1995 1,460 3.78 PILF 1994 05/15/1994 786 2.90 PILF 1989 05/11/1989 1,450 3.77 PILF 1995 06/06/1995 1,460 3.78 PILF 1945 06/06/1945 1,300 3.97 PILF 1996 06/10/1996 2,920 5.09 1962 05/10/1962 1,300 3.70 PILF 1997 05/17/1997 3,260 5.49 2015 05/17/2015 1,290 3.50 PILF 1998 05/11/1998 1,110 3.42 PILF 1993 05/21/1993 1,280 3.62 PILF 1999 05/27/1999 2,050 4.46 1991 06/07/1991 1,240 3.58 PILF 2000 05/24/2000 913 3.12 PILF 2016 05/09/2016 1,170 3.54 PILF 2001 05/16/2001 1,020 3.26 PILF 1946 05/28/1946 1,110 3.86 PILF 2002 05/31/2002 1,710 4.11 1998 05/11/1998 1,110 3.42 PILF 2003 05/31/2003 3,070 5.21 2007 05/13/2007 1,090 3.36 PILF 2004 05/22/2004 712 2.80 PILF 2001 05/16/2001 1,020 3.26 PILF 2005 05/21/2005 991 3.22 PILF 1988 05/26/1988 1,010 3.23 PILF 2006 05/20/2006 1,870 4.28 2005 05/21/2005 991 3.22 PILF 2007 05/13/2007 1,090 3.36 PILF 1980 06/03/1980 975 3.23 PILF 2008 05/20/2008 2,560 4.96 1990 05/30/1990 931 3.12 PILF 2009 05/20/2009 2,790 5.17 2000 05/24/2000 913 3.12 PILF 2010 06/07/2010 1,530 3.90 1977 06/08/1977 859 3.06 PILF 2011 06/08/2011 2,390 4.80 1944 05/19/1944 832 3.31 PILF 2012 04/27/2012 2,250 4.67 1985 05/25/1985 830 3.00 PILF 2013 05/14/2013 1,680 3.98 1941 05/28/1941 820 3.35 PILF 2014 05/24/2014 2,760 5.05 1994 05/15/1994 786 2.90 PILF 2015 05/17/2015 1,290 3.50 PILF 2004 05/22/2004 712 2.80 PILF 2016 05/09/2016 1,170 3.54 PILF 1966 05/29/1966 652 2.85 PILF 2017 05/13/2017 2,200 4.51 1992 05/21/1992 493 2.37 PILF 2018 05/10/2018 3,100 5.20 1987 07/17/1987 311 1.99 PILF

Figure 2. Annual peak flows and perception thresholds.

Peaks used in at-site analysis 1938 //1938 1,700 -- Synthesized 1957 05/10/1957 2,860 5.24 1939 //1939 1,460 -- Synthesized 2009 05/20/2009 2,790 5.17 Potentially influential low flow (PILF) 1940 //1940 859 -- PILF; synthesized 1971 05/13/1971 2,780 5.16 1941 05/28/1941 820 3.35 PILF 2014 05/24/2014 2,760 5.05 Weighted peak-flow frequency 1942 05/26/1942 2,490 5.10 1933 //1933 2,630 -- Synthesized curve 1943 05/31/1943 2,290 4.92 1984 05/31/1984 2,600 4.84 Upper and lower 95-percent 1944 05/19/1944 832 3.31 PILF 1903 //1903 2,560 -- Synthesized confidence intervals 1945 06/06/1945 1,300 3.97 PILF 2008 05/20/2008 2,560 4.96 1946 05/28/1946 1,110 3.86 PILF 1970 05/27/1970 2,550 4.96 1947 05/09/1947 4,060 6.18 1942 05/26/1942 2,490 5.10 1948 05/29/1948 3,880 6.08 1967 05/24/1967 2,490 4.94 1949 05/16/1949 2,280 4.78 1975 06/20/1975 2,470 4.85 1,000 1950 06/07/1950 1,860 4.38 1958 05/22/1958 2,440 4.93 1951 05/25/1951 2,020 4.45 1953 06/14/1953 2,390 4.88 1952 05/27/1952 1,670 4.18 2011 06/08/2011 2,390 4.80 Flow, in cubicfeetsecond per 1953 06/14/1953 2,390 4.88 1978 06/08/1978 2,370 4.77 1954 05/21/1954 1,850 4.37 1959 06/06/1959 2,330 4.78 1955 06/15/1955 2,090 4.54 1901 //1901 2,320 -- Synthesized 1956 05/23/1956 3,260 5.52 1904 //1904 2,300 -- Synthesized 1957 05/10/1957 2,860 5.24 1943 05/31/1943 2,290 4.92 1958 05/22/1958 2,440 4.93 1900 //1900 2,280 -- Synthesized 1959 06/06/1959 2,330 4.78 1949 05/16/1949 2,280 4.78 1960 05/13/1960 2,280 4.80 1960 05/13/1960 2,280 4.80 1961 05/27/1961 1,820 4.33 2012 04/27/2012 2,250 4.67 1962 05/10/1962 1,300 3.70 PILF 1965 05/31/1965 2,200 4.70 1963 05/25/1963 1,600 4.04 2017 05/13/2017 2,200 4.51 100 1964 06/09/1964 3,120 5.47 1986 05/30/1986 2,160 4.54 99.5 99 98 95 90 80 70 60 50 40 30 20 10 5 2 1 0.5 0.2 1965 05/31/1965 2,200 4.70 1955 06/15/1955 2,090 4.54 1966 05/29/1966 652 2.85 PILF 1999 05/27/1999 2,050 4.46 Exceedance probability, in percent 1967 05/24/1967 2,490 4.94 1951 05/25/1951 2,020 4.45 Figure 1. Annual peak flows (plotting positions determined using the Cunnane formulation; Helsel and HIrsch, 2002) and peak-flow frequencycurve 1968 06/04/1968 1,650 4.10 1969 05/20/1969 1,990 4.44 1969 05/20/1969 1,990 4.44 1979 05/25/1979 1,980 4.36 1970 05/27/1970 2,550 4.96 1983 05/30/1983 1,970 4.30 1971 05/13/1971 2,780 5.16 2006 05/20/2006 1,870 4.28 1972 06/03/1972 3,230 5.44 1950 06/07/1950 1,860 4.38 1Peak flows with a value of zero are not plotted in figure 1 . 1973 04/23/1973 1,620 4.06 1954 05/21/1954 1,850 4.37 2In cases where the month, day, or both are not present in the date of a peak flow (as indicated by adjacent slash marks with no intervening values), 1974 06/16/1974 3,380 5.54 1961 05/27/1961 1,820 4.33 the month, day, or both are unknown. 1975 06/20/1975 2,470 4.85 1936 //1936 1,760 -- Synthesized 3Flood-frequency results not reported because of too many values less than the PILF threshold used in the at-site analysis. 1976 05/11/1976 2,980 5.23 1981 05/26/1981 1,740 4.08 4Definitions of peak-flow designations used in analysis include: 1977 06/08/1977 859 3.06 PILF 2002 05/31/2002 1,710 4.11 PT definition: The peak flow is used to define perception thresholds in ungaged historical periods; 1978 06/08/1978 2,370 4.77 1938 //1938 1,700 -- Synthesized Opportunistic: The peak flow was excluded from the analysis because it is outside of the systematic record and was of insufficient magnitude to 1979 05/25/1979 1,980 4.36 2013 05/14/2013 1,680 3.98 determine nonexceedance during an ungaged period; 1980 06/03/1980 975 3.23 PILF 1932 //1932 1,670 -- Synthesized PILF: The peak flow was identified as a potentially influential low flow; Synthesized: The peak flow was synthesized using Maintenance of Variance Extension Type III record extension. 1981 05/26/1981 1,740 4.08 1952 05/27/1952 1,670 4.18 England, J.F., Jr., Cohn, T.A., Faber, B.A., Stedinger, J.R., Thomas, W.O., Jr., Veilleux, A.G., Kiang, J.E., and Mason, R.R., Jr., 2019, Guidelines 1982 05/27/1982 3,080 5.21 1934 //1934 1,660 -- Synthesized for determining flood flow frequency—Bulletin 17C (ver. 1.1, May 2019): U.S. Geological Survey Techniques and Methods, book 4, chap. B5, 148 1983 05/30/1983 1,970 4.30 1968 06/04/1968 1,650 4.10 p., https://doi.org/10.3133/tm4B5. 1984 05/31/1984 2,600 4.84 1973 04/23/1973 1,620 4.06 1985 05/25/1985 830 3.00 PILF 1963 05/25/1963 1,600 4.04 1986 05/30/1986 2,160 4.54 2010 06/07/2010 1,530 3.90 Helsel,D.R.,andHirsch,R.M.,2002,Statisticalmethodsinwaterresources:U.S.GeologicalSur-veyTechniquesofWater- 1987 07/17/1987 311 1.99 PILF 1939 //1939 1,460 -- Synthesized ResourcesInvestigations,book4,chap.A3,522p.,accessedOctober30,2017,athttps://pubs.usgs.gov/twri/twri4a3/. 1988 05/26/1988 1,010 3.23 PILF 1995 06/06/1995 1,460 3.78 1989 05/11/1989 1,450 3.77 1989 05/11/1989 1,450 3.77 1990 05/30/1990 931 3.12 PILF 1945 06/06/1945 1,300 3.97 PILF 1991 06/07/1991 1,240 3.58 PILF 1962 05/10/1962 1,300 3.70 PILF 1992 05/21/1992 493 2.37 PILF 2015 05/17/2015 1,290 3.50 PILF 1993 05/21/1993 1,280 3.62 PILF 1993 05/21/1993 1,280 3.62 PILF 1994 05/15/1994 786 2.90 PILF 1991 06/07/1991 1,240 3.58 PILF 1995 06/06/1995 1,460 3.78 1935 //1935 1,220 -- PILF; synthesi 1996 06/10/1996 2,920 5.09 2016 05/09/2016 1,170 3.54 PILF 1997 05/17/1997 3,260 5.49 1946 05/28/1946 1,110 3.86 PILF 1998 05/11/1998 1,110 3.42 PILF 1998 05/11/1998 1,110 3.42 PILF 1999 05/27/1999 2,050 4.46 2007 05/13/2007 1,090 3.36 PILF 2000 05/24/2000 913 3.12 PILF 2001 05/16/2001 1,020 3.26 PILF 2001 05/16/2001 1,020 3.26 PILF 1930 //1930 1,010 -- PILF; synthesi 2002 05/31/2002 1,710 4.11 1988 05/26/1988 1,010 3.23 PILF 2003 05/31/2003 3,070 5.21 2005 05/21/2005 991 3.22 PILF 2004 05/22/2004 712 2.80 PILF 1980 06/03/1980 975 3.23 PILF 2005 05/21/2005 991 3.22 PILF 1990 05/30/1990 931 3.12 PILF 2006 05/20/2006 1,870 4.28 2000 05/24/2000 913 3.12 PILF 2007 05/13/2007 1,090 3.36 PILF 1940 //1940 859 -- PILF; synthesi 2008 05/20/2008 2,560 4.96 1977 06/08/1977 859 3.06 PILF 2009 05/20/2009 2,790 5.17 1944 05/19/1944 832 3.31 PILF 2010 06/07/2010 1,530 3.90 1985 05/25/1985 830 3.00 PILF 2011 06/08/2011 2,390 4.80 1941 05/28/1941 820 3.35 PILF 2012 04/27/2012 2,250 4.67 1994 05/15/1994 786 2.90 PILF 2013 05/14/2013 1,680 3.98 2004 05/22/2004 712 2.80 PILF 2014 05/24/2014 2,760 5.05 1966 05/29/1966 652 2.85 PILF 2015 05/17/2015 1,290 3.50 PILF 1931 //1931 639 -- PILF; synthesi 2016 05/09/2016 1,170 3.54 PILF 1937 //1937 606 -- PILF; synthesi 2017 05/13/2017 2,200 4.51 1992 05/21/1992 493 2.37 PILF 2018 05/10/2018 3,100 5.20 1987 07/17/1987 311 1.99 PILF 12343400 East Fork Bitterroot River near Conner, Montana 12343400 East Fork Bitterroot River near Conner, Montana Analysis for unregulated period of record Analysis for unregulated period of record Analysis period of record, water years: 1937–72; 2001–4 Analysis period of record, water years: 1937–72; 2001–4 At-site peak-flow frequency analysis conducted on recorded data At-site peak-flow frequency analysis conducted on recorded data Table 1‐1 Table 1‐2 Table 1‐3 Table 1‐4 Table 1‐5 Table 1‐6 Table 1‐7 Table 1‐8 Table 1‐1 Table 1‐2 Table 1‐3 Table 1‐4 Table 1‐5 Table 1‐6 Table 1‐7 Table 1‐8 [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. PILF; potentially influential low flow; MGBT, multiple Grubbs-Beck test; -- not applicable or not available] [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends; --, not applicable or not available.] Peak-flow data Ranked (largest to smallest) peak-flow data Number of Number of Total number Contributing PILF Type of peak‐ recorded Skew type peak flows of years in Type of PILF flow Peak flow, in Gage Peak-flow Peak flow, in Gage Peak-flow drainage area, threshold, in Water 2 Water 2 peak flows used in 1 used for ungaged cubic feet per height, designation in cubic feet per height, designation in in square threshold cubic feet per frequency Date Date used in the analysis perception historical year 4 year 4 miles second second in feet analysis second in feet analysis analysis thresholds period(s) analysis 1937 05/26/1937 700 -- 1972 06/02/1972 4,000 7.17 380 40 Weighted MGBT -- At-site 1938 05/29/1938 1,810 5.78 1948 05/29/1948 3,760 -- Peak flow, in cubic feet per second, for indicated annual exceedance probability (bold values), in percent 1939 05/05/1939 1,320 4.90 1947 05/09/1947 3,660 5.78 66.7 50 42.9 20 10 4 2 1.0 0.5 0.2 1940 05/13/1940 738 -- 2003 05/31/2003 3,620 6.88 1,540 1,910 2,070 2,790 3,340 4,010 4,480 4,930 5,360 5,900 1941 06/05/1941 715 -- 1964 06/08/1964 3,270 6.70 Upper and lower 90-percent confidence intervals, in cubic feet per second, for indicated annual exceedance probability, in percent 1942 05/26/1942 1,990 5.28 1965 06/12/1965 3,270 6.87 66.7 50 42.9 20 10 4 2 1.0 0.5 0.2 1943 05/31/1943 2,110 5.30 1956 05/25/1956 3,000 6.44 1,820 2,230 2,430 3,300 4,120 5,320 6,300 7,360 8,520 10,200 1944 06/02/1944 1,010 3.88 1949 05/17/1949 2,910 5.07 1,280 1,610 1,760 2,390 2,850 3,360 3,660 3,890 4,080 4,280 1945 06/05/1945 1,030 4.02 1971 05/29/1971 2,650 6.37 1946 05/28/1946 1,310 4.35 1970 06/08/1970 2,640 6.36 1947 05/09/1947 3,660 5.78 1958 05/25/1958 2,560 6.16 1948 05/29/1948 3,760 -- 1951 05/24/1951 2,530 4.65 1949 05/17/1949 2,910 5.07 1967 06/07/1967 2,430 6.23 1950 06/16/1950 2,380 4.70 1950 06/16/1950 2,380 4.70 1951 05/24/1951 2,530 4.65 1969 05/20/1969 2,320 6.16 1952 05/29/1952 1,590 3.64 1953 06/13/1953 2,240 4.75 1953 06/13/1953 2,240 4.75 1943 05/31/1943 2,110 5.30 1954 05/21/1954 1,990 5.05 1959 06/07/1959 2,090 5.83 1955 06/15/1955 1,540 4.76 1957 06/03/1957 2,040 5.77 1956 05/25/1956 3,000 6.44 1942 05/26/1942 1,990 5.28 1957 06/03/1957 2,040 5.77 1954 05/21/1954 1,990 5.05 1958 05/25/1958 2,560 6.16 1938 05/29/1938 1,810 5.78 1959 06/07/1959 2,090 5.83 1963 06/04/1963 1,770 5.50 1960 06/04/1960 1,730 5.64 1960 06/04/1960 1,730 5.64 1961 05/31/1961 1,720 5.55 1961 05/31/1961 1,720 5.55 1962 06/14/1962 1,420 5.31 1968 06/03/1968 1,680 5.65 1963 06/04/1963 1,770 5.50 2002 05/31/2002 1,610 5.46 1964 06/08/1964 3,270 6.70 1952 05/29/1952 1,590 3.64 1965 06/12/1965 3,270 6.87 1955 06/15/1955 1,540 4.76 1966 05/10/1966 848 4.74 1962 06/14/1962 1,420 5.31 1967 06/07/1967 2,430 6.23 1939 05/05/1939 1,320 4.90 1968 06/03/1968 1,680 5.65 1946 05/28/1946 1,310 4.35 1969 05/20/1969 2,320 6.16 2001 05/16/2001 1,190 5.04 1970 06/08/1970 2,640 6.36 1945 06/05/1945 1,030 4.02 1971 05/29/1971 2,650 6.37 1944 06/02/1944 1,010 3.88 1972 06/02/1972 4,000 7.17 2004 05/06/2004 887 4.68 2001 05/16/2001 1,190 5.04 1966 05/10/1966 848 4.74 2002 05/31/2002 1,610 5.46 1940 05/13/1940 738 -- 2003 05/31/2003 3,620 6.88 1941 06/05/1941 715 -- 2004 05/06/2004 887 4.68 1937 05/26/1937 700 --

Figure 2. Annual peak flows and perception thresholds.

Exceedance probability, in percent 1971 05/29/1971 2,650 6.37 2013 //2013 1,880 -- Synthesized 1972 06/02/1972 4,000 7.17 1979 //1979 1,860 -- Synthesized Figure 1. Annual peak flows (plotting positions determined using the Cunnane formulation; Helsel and HIrsch, 2002) and peak-flow frequencycurve 1973 //1973 826 -- Synthesized 1938 05/29/1938 1,810 5.78 1974 //1974 6,040 -- Synthesized 1936 //1936 1,790 -- Synthesized 1975 //1975 2,510 -- Synthesized 1963 06/04/1963 1,770 5.50 1976 //1976 2,990 -- Synthesized 1960 06/04/1960 1,730 5.64 1977 //1977 785 -- Synthesized 1961 05/31/1961 1,720 5.55 1Peak flows with a value of zero are not plotted in figure 1 . 1978 //1978 2,280 -- Synthesized 1932 //1932 1,700 -- Synthesized 2In cases where the month, day, or both are not present in the date of a peak flow (as indicated by adjacent slash marks with no intervening values), 1979 //1979 1,860 -- Synthesized 1934 //1934 1,690 -- Synthesized the month, day, or both are unknown. 1980 //1980 981 -- Synthesized 1968 06/03/1968 1,680 5.65 3Flood-frequency results not reported because of too many values less than the PILF threshold used in the at-site analysis. 1981 //1981 1,580 -- Synthesized 2007 //2007 1,650 -- Synthesized 4Definitions of peak-flow designations used in analysis include: 1982 //1982 4,920 -- Synthesized 2002 05/31/2002 1,610 5.46 PT definition: The peak flow is used to define perception thresholds in ungaged historical periods; 1983 //1983 1,930 -- Synthesized 2010 //2010 1,610 -- Synthesized Opportunistic: The peak flow was excluded from the analysis because it is outside of the systematic record and was of insufficient magnitude to 1984 //1984 2,420 -- Synthesized 1952 05/29/1952 1,590 3.64 determine nonexceedance during an ungaged period; 1985 //1985 995 -- Synthesized 1981 //1981 1,580 -- Synthesized PILF: The peak flow was identified as a potentially influential low flow; Synthesized: The peak flow was synthesized using Maintenance of Variance Extension Type III record extension. 1986 //1986 2,420 -- Synthesized 1995 //1995 1,570 -- Synthesized England, J.F., Jr., Cohn, T.A., Faber, B.A., Stedinger, J.R., Thomas, W.O., Jr., Veilleux, A.G., Kiang, J.E., and Mason, R.R., Jr., 2019, Guidelines 1987 //1987 766 -- Synthesized 1955 06/15/1955 1,540 4.76 for determining flood flow frequency—Bulletin 17C (ver. 1.1, May 2019): U.S. Geological Survey Techniques and Methods, book 4, chap. B5, 148 1988 //1988 1,090 -- Synthesized 1991 //1991 1,460 -- Synthesized p., https://doi.org/10.3133/tm4B5. 1989 //1989 1,290 -- Synthesized 1962 06/14/1962 1,420 5.31 1990 //1990 1,020 -- Synthesized 1993 //1993 1,370 -- Synthesized 1991 //1991 1,460 -- Synthesized 1939 05/05/1939 1,320 4.90 Helsel,D.R.,andHirsch,R.M.,2002,Statisticalmethodsinwaterresources:U.S.GeologicalSur-veyTechniquesofWater- 1992 //1992 585 -- Synthesized 1946 05/28/1946 1,310 4.35 ResourcesInvestigations,book4,chap.A3,522p.,accessedOctober30,2017,athttps://pubs.usgs.gov/twri/twri4a3/. 1993 //1993 1,370 -- Synthesized 1989 //1989 1,290 -- Synthesized 1994 //1994 725 -- Synthesized 1935 //1935 1,260 -- Synthesized 1995 //1995 1,570 -- Synthesized 2015 //2015 1,230 -- Synthesized 1996 //1996 3,080 -- Synthesized 2016 //2016 1,230 -- Synthesized 1997 //1997 3,370 -- Synthesized 2001 05/16/2001 1,190 5.04 1998 //1998 1,050 -- Synthesized 1988 //1988 1,090 -- Synthesized 1999 //1999 2,090 -- Synthesized 1930 //1930 1,060 -- Synthesized 2000 //2000 942 -- Synthesized 1998 //1998 1,050 -- Synthesized 2001 05/16/2001 1,190 5.04 1945 06/05/1945 1,030 4.02 2002 05/31/2002 1,610 5.46 1990 //1990 1,020 -- Synthesized 2003 05/31/2003 3,620 6.88 1944 06/02/1944 1,010 3.88 2004 05/06/2004 887 4.68 1985 //1985 995 -- Synthesized 2005 //2005 874 -- Synthesized 1980 //1980 981 -- Synthesized 2006 //2006 2,420 -- Synthesized 2000 //2000 942 -- Synthesized 2007 //2007 1,650 -- Synthesized 2004 05/06/2004 887 4.68 2008 //2008 2,720 -- Synthesized 2005 //2005 874 -- Synthesized 2009 //2009 2,300 -- Synthesized 1966 05/10/1966 848 4.74 2010 //2010 1,610 -- Synthesized 1973 //1973 826 -- Synthesized 2011 //2011 2,520 -- Synthesized 1977 //1977 785 -- Synthesized 2012 //2012 2,500 -- Synthesized 1987 //1987 766 -- Synthesized 2013 //2013 1,880 -- Synthesized 1940 05/13/1940 738 -- 2014 //2014 2,670 -- Synthesized 1994 //1994 725 -- Synthesized 2015 //2015 1,230 -- Synthesized 1941 06/05/1941 715 -- 2016 //2016 1,230 -- Synthesized 1937 05/26/1937 700 -- 2017 //2017 2,120 -- Synthesized 1931 //1931 687 -- Synthesized 2018 //2018 2,910 -- Synthesized 1992 //1992 585 -- Synthesized 12344000 Bitterroot River near Darby, Montana 12344000 Bitterroot River near Darby, Montana Analysis for total period of record Analysis for total period of record Analysis period of record, water years: 1938–2018 Analysis period of record, water years: 1938–2018 Peak-flow frequency analsys determined from regional regression equations Peak-flow frequency analsys determined from regional regression equations Table 1‐1 Table 1‐2 Table 1‐3 Table 1‐4 Table 1‐5 Table 1‐6 Table 1‐7 Table 1‐8 Table 1‐1 Table 1‐2 Table 1‐3 Table 1‐4 Table 1‐5 Table 1‐6 Table 1‐7 Table 1‐8 [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. PILF; potentially influential low flow; MGBT, multiple Grubbs-Beck test] [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends; --, not applicable or not available.] Peak-flow data Ranked (largest to smallest) peak-flow data Number of Number of Total number Contributing PILF Type of peak‐ recorded Skew type peak flows of years in Type of PILF flow Peak flow, in Gage Peak-flow Peak flow, in Gage Peak-flow drainage area, threshold, in Water 2 Water 2 peak flows used in 1 used for ungaged cubic feet per height, designation in cubic feet per height, designation in in square threshold cubic feet per frequency Date Date used in the analysis perception historical year 4 year 4 miles second second in feet analysis second in feet analysis analysis thresholds period(s) analysis 1938 05/29/1938 5,480 6.51 1947 05/09/1947 11,500 8.18 1,050 81 Weighted MGBT 5,480 At-site 1939 04/30/1939 4,920 6.10 PILF 1948 05/29/1948 11,300 8.12 Peak flow, in cubic feet per second, for indicated annual exceedance probability (bold values), in percent 1940 05/12/1940 3,360 4.83 PILF 1974 06/17/1974 11,100 8.42 66.7 50 42.9 20 10 4 2 1.0 0.5 0.2 1941 05/13/1941 2,420 4.18 PILF 1956 05/24/1956 10,500 7.85 5,300 6,260 6,690 8,390 9,620 11,000 11,900 12,700 13,500 14,400 1942 05/26/1942 5,930 6.23 1972 06/03/1972 10,500 7.83 Upper and lower 90-percent confidence intervals, in cubic feet per second, for indicated annual exceedance probability, in percent 1943 05/31/1943 6,820 6.64 2003 05/31/2003 10,300 8.45 66.7 50 42.9 20 10 4 2 1.0 0.5 0.2 1944 05/31/1944 3,060 4.75 PILF 1997 05/17/1997 10,100 8.43 5,790 6,790 7,260 9,230 10,800 12,800 14,300 15,600 17,000 18,900 1945 06/01/1945 3,960 5.10 PILF 2009 05/20/2009 9,510 8.18 4,020 5,360 5,950 7,730 8,810 9,930 10,600 11,200 11,700 12,300 1946 05/28/1946 4,060 5.17 PILF 1964 06/08/1964 9,450 7.37 1947 05/09/1947 11,500 8.18 1996 06/09/1996 9,320 8.26 1948 05/29/1948 11,300 8.12 2008 05/20/2008 9,220 8.05 1949 05/16/1949 7,440 6.55 1976 05/11/1976 9,080 7.29 1950 06/17/1950 6,930 6.33 1982 05/27/1982 9,000 7.10 1951 05/24/1951 7,080 6.47 2018 05/10/2018 8,870 7.88 1952 06/07/1952 5,520 5.78 1967 05/24/1967 8,750 7.18 1953 06/13/1953 8,200 7.04 1971 05/29/1971 8,670 7.12 1954 05/21/1954 7,420 6.70 2011 06/08/2011 8,620 7.65 1955 06/15/1955 6,770 6.46 1970 06/05/1970 8,470 6.96 1956 05/24/1956 10,500 7.85 2014 05/24/2014 8,230 7.84 1957 05/11/1957 6,030 5.87 1953 06/13/1953 8,200 7.04 1958 05/23/1958 7,870 6.92 2006 05/20/2006 7,970 7.48 1959 06/07/1959 6,850 6.39 1958 05/23/1958 7,870 6.92 1960 06/04/1960 6,780 6.22 1975 06/16/1975 7,810 6.75 1961 05/27/1961 6,240 6.33 2012 04/27/2012 7,780 7.27 1962 06/14/1962 4,370 5.10 PILF 1984 05/31/1984 7,560 6.26 1963 05/25/1963 5,590 5.66 1986 05/30/1986 7,560 6.44 1964 06/08/1964 9,450 7.37 1949 05/16/1949 7,440 6.55 1965 06/12/1965 7,070 6.53 1954 05/21/1954 7,420 6.70 1966 05/29/1966 2,960 4.31 PILF 1978 06/08/1978 7,160 6.49 1967 05/24/1967 8,750 7.18 1951 05/24/1951 7,080 6.47 1968 06/03/1968 5,820 5.77 1965 06/12/1965 7,070 6.53 1969 05/20/1969 6,780 6.22 1950 06/17/1950 6,930 6.33 1970 06/05/1970 8,470 6.96 1959 06/07/1959 6,850 6.39 1971 05/29/1971 8,670 7.12 1943 05/31/1943 6,820 6.64 1972 06/03/1972 10,500 7.83 1960 06/04/1960 6,780 6.22 1973 05/20/1973 2,950 4.16 PILF 1969 05/20/1969 6,780 6.22 1974 06/17/1974 11,100 8.42 1955 06/15/1955 6,770 6.46 1975 06/16/1975 7,810 6.75 2017 06/13/2017 6,720 6.80 1976 05/11/1976 9,080 7.29 1999 05/26/1999 6,660 6.89 1977 06/07/1977 2,820 3.76 PILF 2010 06/05/2010 6,570 6.78 1978 06/08/1978 7,160 6.49 1961 05/27/1961 6,240 6.33 1979 05/27/1979 5,990 5.89 1983 05/29/1983 6,190 5.75 1980 06/13/1980 3,430 4.24 PILF 2002 05/31/2002 6,140 6.57 1981 05/26/1981 5,200 5.43 PILF 2013 05/14/2013 6,050 6.70 1982 05/27/1982 9,000 7.10 1957 05/11/1957 6,030 5.87 1983 05/29/1983 6,190 5.75 1979 05/27/1979 5,990 5.89 1984 05/31/1984 7,560 6.26 1942 05/26/1942 5,930 6.23 Figure 1. Annual peak flows (probability plotting positions) and peak-flow frequency curve. EMA, Expected Moments Algorithm; Skew(G), analysis 1985 05/24/1985 3,470 4.09 PILF 1968 06/03/1968 5,820 5.77 skew; PILF(LO), potentially Influencial low flow (low outlier). 1986 05/30/1986 7,560 6.44 1963 05/25/1963 5,590 5.66 1987 05/01/1987 2,760 3.76 PILF 1952 06/07/1952 5,520 5.78 1988 05/25/1988 3,750 5.51 PILF 1938 05/29/1938 5,480 6.51 1989 05/11/1989 4,370 5.92 PILF 1981 05/26/1981 5,200 5.43 PILF 1990 06/11/1990 3,540 5.33 PILF 1995 06/04/1995 5,160 6.40 PILF 1991 06/07/1991 4,870 6.23 PILF 1939 04/30/1939 4,920 6.10 PILF 1992 05/07/1992 2,180 4.28 PILF 1991 06/07/1991 4,870 6.23 PILF 1993 05/21/1993 4,590 6.06 PILF 1993 05/21/1993 4,590 6.06 PILF 1994 05/13/1994 2,630 4.67 PILF 2007 11/08/2006 4,410 5.60 PILF 1995 06/04/1995 5,160 6.40 PILF 1962 06/14/1962 4,370 5.10 PILF 1996 06/09/1996 9,320 8.26 1989 05/11/1989 4,370 5.92 PILF 1997 05/17/1997 10,100 8.43 2016 05/09/2016 4,190 5.50 PILF 1998 05/27/1998 3,650 5.14 PILF 2015 05/17/2015 4,180 5.32 PILF 1999 05/26/1999 6,660 6.89 1946 05/28/1946 4,060 5.17 PILF 2000 05/23/2000 3,310 4.86 PILF 1945 06/01/1945 3,960 5.10 PILF 2001 05/16/2001 3,670 5.03 PILF 1988 05/25/1988 3,750 5.51 PILF 2002 05/31/2002 6,140 6.57 2001 05/16/2001 3,670 5.03 PILF 2003 05/31/2003 10,300 8.45 1998 05/27/1998 3,650 5.14 PILF 2004 06/06/2004 2,950 4.51 PILF 2005 05/19/2005 3,600 5.00 PILF 2005 05/19/2005 3,600 5.00 PILF 1990 06/11/1990 3,540 5.33 PILF 2006 05/20/2006 7,970 7.48 1985 05/24/1985 3,470 4.09 PILF 2007 11/08/2006 4,410 5.60 PILF 1980 06/13/1980 3,430 4.24 PILF 2008 05/20/2008 9,220 8.05 1940 05/12/1940 3,360 4.83 PILF 2009 05/20/2009 9,510 8.18 2000 05/23/2000 3,310 4.86 PILF 2010 06/05/2010 6,570 6.78 1944 05/31/1944 3,060 4.75 PILF 2011 06/08/2011 8,620 7.65 1966 05/29/1966 2,960 4.31 PILF 2012 04/27/2012 7,780 7.27 1973 05/20/1973 2,950 4.16 PILF 2013 05/14/2013 6,050 6.70 2004 06/06/2004 2,950 4.51 PILF 2014 05/24/2014 8,230 7.84 1977 06/07/1977 2,820 3.76 PILF 2015 05/17/2015 4,180 5.32 PILF 1987 05/01/1987 2,760 3.76 PILF 2016 05/09/2016 4,190 5.50 PILF 1994 05/13/1994 2,630 4.67 PILF 2017 06/13/2017 6,720 6.80 1941 05/13/1941 2,420 4.18 PILF Figure 2. Annual peak flows and perception thresholds. 2018 05/10/2018 8,870 7.88 1992 05/07/1992 2,180 4.28 PILF

Peaks used in at-site analysis 1938 05/29/1938 5,480 6.51 1982 05/27/1982 9,000 7.10 1939 04/30/1939 4,920 6.10 2018 05/10/2018 8,870 7.88 Potentially influential low flow (PILF) 1940 05/12/1940 3,360 4.83 PILF 1967 05/24/1967 8,750 7.18 1941 05/13/1941 2,420 4.18 PILF 1903 //1903 8,680 -- Synthesized Weighted peak-flow frequency 1942 05/26/1942 5,930 6.23 1971 05/29/1971 8,670 7.12 curve 1943 05/31/1943 6,820 6.64 2011 06/08/2011 8,620 7.65 Upper and lower 95-percent 1944 05/31/1944 3,060 4.75 PILF 1970 06/05/1970 8,470 6.96 confidence intervals 1945 06/01/1945 3,960 5.10 PILF 2014 05/24/2014 8,230 7.84 1946 05/28/1946 4,060 5.17 PILF 1953 06/13/1953 8,200 7.04 1947 05/09/1947 11,500 8.18 1933 //1933 8,070 -- Synthesized 1948 05/29/1948 11,300 8.12 2006 05/20/2006 7,970 7.48 1949 05/16/1949 7,440 6.55 1901 //1901 7,960 -- Synthesized 10,000 1950 06/17/1950 6,930 6.33 1904 //1904 7,910 -- Synthesized 1951 05/24/1951 7,080 6.47 1958 05/23/1958 7,870 6.92 1952 06/07/1952 5,520 5.78 1900 //1900 7,850 -- Synthesized Flow, in cubicfeetsecond per 1953 06/13/1953 8,200 7.04 1975 06/16/1975 7,810 6.75 1954 05/21/1954 7,420 6.70 2012 04/27/2012 7,780 7.27 1955 06/15/1955 6,770 6.46 1984 05/31/1984 7,560 6.26 1956 05/24/1956 10,500 7.85 1986 05/30/1986 7,560 6.44 1957 05/11/1957 6,030 5.87 1949 05/16/1949 7,440 6.55 1958 05/23/1958 7,870 6.92 1954 05/21/1954 7,420 6.70 1959 06/07/1959 6,850 6.39 1978 06/08/1978 7,160 6.49 1960 06/04/1960 6,780 6.22 1951 05/24/1951 7,080 6.47 1961 05/27/1961 6,240 6.33 1965 06/12/1965 7,070 6.53 1962 06/14/1962 4,370 5.10 PILF 1950 06/17/1950 6,930 6.33 1963 05/25/1963 5,590 5.66 1959 06/07/1959 6,850 6.39 1,000 1964 06/08/1964 9,450 7.37 1943 05/31/1943 6,820 6.64 99.5 99 98 95 90 80 70 60 50 40 30 20 10 5 2 1 0.5 0.2 1965 06/12/1965 7,070 6.53 1960 06/04/1960 6,780 6.22 1966 05/29/1966 2,960 4.31 PILF 1969 05/20/1969 6,780 6.22 Exceedance probability, in percent 1967 05/24/1967 8,750 7.18 1955 06/15/1955 6,770 6.46 Figure 1. Annual peak flows (plotting positions determined using the Cunnane formulation; Helsel and HIrsch, 2002) and peak-flow frequencycurve 1968 06/03/1968 5,820 5.77 2017 06/13/2017 6,720 6.80 1969 05/20/1969 6,780 6.22 1999 05/26/1999 6,660 6.89 1970 06/05/1970 8,470 6.96 2010 06/05/2010 6,570 6.78 1971 05/29/1971 8,670 7.12 1961 05/27/1961 6,240 6.33 1972 06/03/1972 10,500 7.83 1983 05/29/1983 6,190 5.75 1Peak flows with a value of zero are not plotted in figure 1 . 1973 05/20/1973 2,950 4.16 PILF 2002 05/31/2002 6,140 6.57 2In cases where the month, day, or both are not present in the date of a peak flow (as indicated by adjacent slash marks with no intervening values), 1974 06/17/1974 11,100 8.42 2013 05/14/2013 6,050 6.70 the month, day, or both are unknown. 1975 06/16/1975 7,810 6.75 1957 05/11/1957 6,030 5.87 3Flood-frequency results not reported because of too many values less than the PILF threshold used in the at-site analysis. 1976 05/11/1976 9,080 7.29 1979 05/27/1979 5,990 5.89 4Definitions of peak-flow designations used in analysis include: 1977 06/07/1977 2,820 3.76 PILF 1942 05/26/1942 5,930 6.23 PT definition: The peak flow is used to define perception thresholds in ungaged historical periods; 1978 06/08/1978 7,160 6.49 1968 06/03/1968 5,820 5.77 Opportunistic: The peak flow was excluded from the analysis because it is outside of the systematic record and was of insufficient magnitude to 1979 05/27/1979 5,990 5.89 1936 //1936 5,810 -- Synthesized determine nonexceedance during an ungaged period; 1980 06/13/1980 3,430 4.24 PILF 1963 05/25/1963 5,590 5.66 PILF: The peak flow was identified as a potentially influential low flow; Synthesized: The peak flow was synthesized using Maintenance of Variance Extension Type III record extension. 1981 05/26/1981 5,200 5.43 1932 //1932 5,560 -- Synthesized England, J.F., Jr., Cohn, T.A., Faber, B.A., Stedinger, J.R., Thomas, W.O., Jr., Veilleux, A.G., Kiang, J.E., and Mason, R.R., Jr., 2019, Guidelines 1982 05/27/1982 9,000 7.10 1934 //1934 5,540 -- Synthesized for determining flood flow frequency—Bulletin 17C (ver. 1.1, May 2019): U.S. Geological Survey Techniques and Methods, book 4, chap. B5, 148 1983 05/29/1983 6,190 5.75 1952 06/07/1952 5,520 5.78 p., https://doi.org/10.3133/tm4B5. 1984 05/31/1984 7,560 6.26 1938 05/29/1938 5,480 6.51 1985 05/24/1985 3,470 4.09 PILF 1981 05/26/1981 5,200 5.43 1986 05/30/1986 7,560 6.44 1995 06/04/1995 5,160 6.40 Helsel,D.R.,andHirsch,R.M.,2002,Statisticalmethodsinwaterresources:U.S.GeologicalSur-veyTechniquesofWater- 1987 05/01/1987 2,760 3.76 PILF 1939 04/30/1939 4,920 6.10 ResourcesInvestigations,book4,chap.A3,522p.,accessedOctober30,2017,athttps://pubs.usgs.gov/twri/twri4a3/. 1988 05/25/1988 3,750 5.51 PILF 1991 06/07/1991 4,870 6.23 1989 05/11/1989 4,370 5.92 PILF 1993 05/21/1993 4,590 6.06 PILF 1990 06/11/1990 3,540 5.33 PILF 2007 11/08/2006 4,410 5.60 PILF 1991 06/07/1991 4,870 6.23 1962 06/14/1962 4,370 5.10 PILF 1992 05/07/1992 2,180 4.28 PILF 1989 05/11/1989 4,370 5.92 PILF 1993 05/21/1993 4,590 6.06 PILF 1935 //1935 4,300 -- PILF; synthesi 1994 05/13/1994 2,630 4.67 PILF 2016 05/09/2016 4,190 5.50 PILF 1995 06/04/1995 5,160 6.40 2015 05/17/2015 4,180 5.32 PILF 1996 06/09/1996 9,320 8.26 1946 05/28/1946 4,060 5.17 PILF 1997 05/17/1997 10,100 8.43 1945 06/01/1945 3,960 5.10 PILF 1998 05/27/1998 3,650 5.14 PILF 1988 05/25/1988 3,750 5.51 PILF 1999 05/26/1999 6,660 6.89 1930 //1930 3,700 -- PILF; synthesi 2000 05/23/2000 3,310 4.86 PILF 2001 05/16/2001 3,670 5.03 PILF 2001 05/16/2001 3,670 5.03 PILF 1998 05/27/1998 3,650 5.14 PILF 2002 05/31/2002 6,140 6.57 2005 05/19/2005 3,600 5.00 PILF 2003 05/31/2003 10,300 8.45 1990 06/11/1990 3,540 5.33 PILF 2004 06/06/2004 2,950 4.51 PILF 1985 05/24/1985 3,470 4.09 PILF 2005 05/19/2005 3,600 5.00 PILF 1980 06/13/1980 3,430 4.24 PILF 2006 05/20/2006 7,970 7.48 1940 05/12/1940 3,360 4.83 PILF 2007 11/08/2006 4,410 5.60 PILF 2000 05/23/2000 3,310 4.86 PILF 2008 05/20/2008 9,220 8.05 1944 05/31/1944 3,060 4.75 PILF 2009 05/20/2009 9,510 8.18 1966 05/29/1966 2,960 4.31 PILF 2010 06/05/2010 6,570 6.78 1973 05/20/1973 2,950 4.16 PILF 2011 06/08/2011 8,620 7.65 2004 06/06/2004 2,950 4.51 PILF 2012 04/27/2012 7,780 7.27 1977 06/07/1977 2,820 3.76 PILF 2013 05/14/2013 6,050 6.70 1987 05/01/1987 2,760 3.76 PILF 2014 05/24/2014 8,230 7.84 1994 05/13/1994 2,630 4.67 PILF 2015 05/17/2015 4,180 5.32 PILF 1931 //1931 2,530 -- PILF; synthesi 2016 05/09/2016 4,190 5.50 PILF 1937 //1937 2,420 -- PILF; synthesi 2017 06/13/2017 6,720 6.80 1941 05/13/1941 2,420 4.18 PILF 2018 05/10/2018 8,870 7.88 1992 05/07/1992 2,180 4.28 PILF Note: Not all footnotes are applicable for each frequency analysis. Even if a footnote is not applicable for a given frequency analysis, it is retained in the worksheet for convenience and to maintain consistency among the various similarly structured frequency-analysis worksheets. Also, not all table columns are applicable for each frequency analysis. Even if a table column is not applicable for a given frequency analysis, it is retained in the worksheet for convenience and to maintain consistency among the various similarly structured frequency-analysis worksheets. 12350250 Bitterroot River at Bell Crossing near Victor ,Montana 12350250 Bitterroot River at Bell Crossing near Victor ,Montana Analysis for total period of record Analysis for total period of record Analysis period of record, water years: 1987 - 2018 Analysis period of record, water years: 1987 - 2018 At-site peak-flow frequency analysis conducted on recorded data At-site peak-flow frequency analysis conducted on recorded data Table 1-1 Table 1-2 Table 1-3 Table 1-4 Table 1-5 Table 1-6 Table 1-7 Table 1-8 [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. PILF; potentially influential low flow; MGBT, multiple Grubbs-Beck test; -- not applicable or not available] [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends; --, not applicable or not available.] Peak-flow data3 Ranked (largest to smallest) peak-flow data3 Number of Number of Total number Contributing PILF Type of peak- recorded Skew type peak flows of years in Type of PILF Peak flow, in Gage Peak-flow Peak flow, in Gage Peak-flow drainage area, threshold, in flow Water 4 Water 4 peak flows used in 1 used for ungaged cubic feet per height, designation in cubic feet per height, designation in in square threshold cubic feet per frequency Date Date used in the analysis perception historical year 5 year 5 miles second analysis second in feet analysis second in feet analysis analysis thresholds period(s) 1987 5/1/1987 6,980 7.52 1996 6/9/1996 18,700 10.07 1,944.0 32 Weighted MGBT 0 At-site 1988 6/5/1988 8,500 8.04 2003 5/31/2003 17,600 10.82 Peak flow, in cubic feet per second, for indicated annual exceedance probability (bold values), in percent 1989 6/16/1989 10,600 9.18 2018 5/10/2018 15,100 10.44 266.7 50 42.9 20 10 4 2 1.0 0.5 0.2 1990 5/30/1990 7,820 8.22 1995 6/4/1995 14,100 9.29 6 9,610 11,100 11,800 14,600 16,600 19,000 20,700 22,300 23,800 25,700 1991 6/6/1991 11,300 9.23 1997 5/18/1997 14,000 Upper and lower 90-percent confidence intervals, in cubic feet per second, for indicated annual exceedance probability, in percent 1992 5/8/1992 5,840 6.55 2011 6/8/2011 13,700 10.67 266.7 50 42.9 20 10 4 2 1.0 0.5 0.2 1993 5/21/1993 11,100 8.61 2014 5/24/2014 13,100 10.00 11,200 12,900 13,700 17,400 21,000 26,300 30,600 35,200 40,200 47,700 1994 4/22/1994 5,990 7.30 2012 4/27/2012 12,900 9.96 8,120 9,560 10,200 12,600 14,300 16,200 17,300 18,200 18,900 19,700 1995 6/4/1995 14,100 9.29 2017 6/14/2017 12,800 9.97 1996 6/9/1996 18,700 10.07 2010 6/5/2010 12,400 9.73 6 1997 5/18/1997 14,000 2013 5/14/2013 11,800 9.68 6 1998 6,560 1991 6/6/1991 11,300 9.23 6 1999 4,380 1993 5/21/1993 11,100 8.61 6 2000 5,820 1989 6/16/1989 10,600 9.18 6 2001 5/16/2001 2,060 2002 5/31/2002 8,760 9.84 2002 5/31/2002 8,760 9.84 2016 5/9/2016 8,550 8.66 2003 5/31/2003 17,600 10.82 1988 6/5/1988 8,500 8.04 6 6 2004 6/6/2004 2,640 2008 5/20/2008 8,430 6 2005 5/20/2005 2,650 1990 5/30/1990 7,820 8.22 6 2006 5/20/2006 2,810 2015 5/17/2015 7,320 8.16 6 2007 3,890 1987 5/1/1987 6,980 7.52 6 6 2008 5/20/2008 8,430 1998 6,560 6 2009 4,630 1994 4/22/1994 5,990 7.30 2010 6/5/2010 12,400 9.73 1992 5/8/1992 5,840 6.55 6 2011 6/8/2011 13,700 10.67 2000 5,820 6 2012 4/27/2012 12,900 9.96 2009 4,630 6 2013 5/14/2013 11,800 9.68 1999 4,380 6 2014 5/24/2014 13,100 10.00 2007 3,890 6 2015 5/17/2015 7,320 8.16 2006 5/20/2006 2,810 6 2016 5/9/2016 8,550 8.66 2005 5/20/2005 2,650 6 2017 6/14/2017 12,800 9.97 2004 6/6/2004 2,640 6 2018 5/10/2018 15,100 10.44 2001 5/16/2001 2,060

Figure 1. Annual peak flows (probability plotting positions) and peak-flow frequency curve. EMA, Expected Moments Algorithm; Skew(G), analysis skew; PILF(LO), Potentially Influential Low Flow (low outlier).

Figure 2. Annual peak flows and perception thresholds.

1Definitions of types of PILF thresholds include: MGBT: PILF threshold calculated by using the multiple Grubbs-Beck Test as specified in Bulletin 17C (England and others, 2016); Manual: PILF threshold based on a systematic peak flow selected by the peak-flow frequency analyst. 2Flood-frequency results not reported because of too many values less than the PILF threshold used in the at-site analysis. 3Peak-flow data with a value of zero are not plotted in figures. 4In cases where the month, day, or both are not present in the date of a peak flow (as indicated by adjacent slash marks with no intervening values), the month, day, or both are unknown. 5Definitions of peak-flow designations used in analysis include: PT definition: The peak flow is used to define perception thresholds in ungaged historical periods; Opportunistic: The peak flow was excluded from the analysis because it is outside of the systematic record and was of insufficient magnitude to determine nonexceedance during an ungaged period; PILF: The peak flow was identified as a potentially influential low flow; Synthesized: The peak flow was synthesized using Maintenance of Variance Extension Type III record extension. 6The actual annual peak flow was greater than the indicated value. England, J.F., Jr., Cohn, T.A., Faber, B.A., Stedinger, J.R., Thomas, W.O., Jr., Veilleux, A.G., Kiang, J.E., and Mason, R.R., Jr., 2019, Guidelines for determining flood flow frequency—Bulletin 17C (ver. 1.1, May 2019): U.S. Geological Survey Techniques and Methods, book 4, chap. B5, 148 p., https://doi.org/10.3133/tm4B5. Note: Not all footnotes are applicable for each frequency analysis. Even if a footnote is not applicable for a given frequency analysis, it is retained in the worksheet for convenience and to maintain consistency among the various similarly structured frequency-analysis worksheets. Also, not all table columns are applicable for each frequency analysis. Even if a table column is not applicable for a given frequency analysis, it is retained in the worksheet for convenience and to maintain consistency among the various similarly structured frequency-analysis worksheets. 12350250 Bitterroot River at Bell Crossing near Victor ,Montana 12350250 Bitterroot River at Bell Crossing near Victor ,Montana Analysis for total period of record Analysis for total period of record Analysis period of record, water years: 1899 - 2018 Analysis period of record, water years: 1899 - 2018 Peak-flow frequency analysis conducted on recorded and synthesized data Peak-flow frequency analysis conducted on recorded and synthesized data Table 1-1 Table 1-2 Table 1-3 Table 1-4 Table 1-5 Table 1-6 Table 1-7 Table 1-8 [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. PILF; potentially influential low flow; MGBT, multiple Grubbs-Beck test; -- not applicable or not available] [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends; --, not applicable or not available.] Peak-flow data3 Ranked (largest to smallest) peak-flow data3 Number of Number of Total number Contributing PILF Type of peak- recorded Skew type peak flows of years in Type of PILF Peak flow, in Gage Peak-flow Peak flow, in Gage drainage area, threshold, in flow Water 4 Water 4 Peak-flow designation in peak flows used in 1 used for ungaged cubic feet per height, designation in cubic feet per height, in square threshold cubic feet per frequency Date Date 5 used in the analysis perception historical year 5 year analysis miles second analysis second in feet analysis second in feet analysis thresholds period(s) 1899 28,900 Synthesized 1899 28,900 Synthesized 1,944 94 Weighted MGBT 9,870 26 MOVE3 1900 14,400 Synthesized 1997 19,200 Peak flow, in cubic feet per second, for indicated annual exceedance probability (bold values), in percent 1901 14,500 Synthesized 1947 19,100 Synthesized 266.7 50 42.9 20 10 4 2 1.0 0.5 0.2 1903 15,500 Synthesized 1948 18,900 Synthesized 10,300 11,800 12,500 15,200 17,200 19,500 21,000 22,500 23,900 25,600 1904 14,400 Synthesized 1996 6/9/1996 18,700 10.07 Upper and lower 90-percent confidence intervals, in cubic feet per second, for indicated annual exceedance probability, in percent 1930 7,960 PILF , Synthesized 1974 18,600 Synthesized 266.7 50 42.9 20 10 4 2 1.0 0.5 0.2 1931 5,850 PILF , Synthesized 1956 17,900 Synthesized 11,565 13,245 14,021 17,322 19,984 23,511 26,390 29,540 33,034 38,204 1932 11,100 Synthesized 1972 17,900 Synthesized 8,102 10,239 10,997 13,450 15,051 16,722 17,726 18,506 19,136 19,762 1933 15,000 Synthesized 2003 5/31/2003 17,600 10.82 1934 11,000 Synthesized 1964 16,500 Synthesized 1935 9,000 PILF , Synthesized 1976 16,000 Synthesized 1936 11,500 Synthesized 1982 15,900 Synthesized 1937 5,650 PILF , Synthesized 1903 15,500 Synthesized 1938 10,800 Synthesized 1967 15,500 Synthesized 1939 9,970 Synthesized 1971 15,400 Synthesized 1940 7,430 PILF , Synthesized 2008 15,400 1941 5,780 PILF , Synthesized 1970 15,100 Synthesized 1942 11,500 Synthesized 2018 5/10/2018 15,100 10.44 1943 12,800 Synthesized 1933 15,000 Synthesized 1944 6,920 PILF , Synthesized 1953 14,800 Synthesized 1945 8,440 PILF , Synthesized 1901 14,500 Synthesized 1946 8,600 PILF , Synthesized 1900 14,400 Synthesized 1947 19,100 Synthesized 1904 14,400 Synthesized 1948 18,900 Synthesized 1958 14,300 Synthesized 1949 13,700 Synthesized 1975 14,200 Synthesized 1950 13,000 Synthesized 2009 14,200 1951 13,200 Synthesized 1995 6/4/1995 14,100 9.29 1952 10,900 Synthesized 1984 13,900 Synthesized 1953 14,800 Synthesized 1986 13,900 Synthesized 1954 13,700 Synthesized 2006 13,900 1955 12,700 Synthesized 1949 13,700 Synthesized 1956 17,900 Synthesized 1954 13,700 Synthesized 1957 11,700 Synthesized 2011 6/8/2011 13,700 10.67 1958 14,300 Synthesized 1978 13,300 Synthesized 1959 12,900 Synthesized 1951 13,200 Synthesized 1960 12,800 Synthesized 1965 13,200 Synthesized 1961 12,000 Synthesized 2014 5/24/2014 13,100 10.00 1962 9,100 PILF , Synthesized 1950 13,000 Synthesized 1963 11,000 Synthesized 1959 12,900 Synthesized 1964 16,500 Synthesized 2012 4/27/2012 12,900 9.96 1965 13,200 Synthesized 1943 12,800 Synthesized 1966 6,740 PILF , Synthesized 1960 12,800 Synthesized 1967 15,500 Synthesized 1969 12,800 Synthesized Figure 1. Annual peak flows (probability plotting positions) and peak-flow frequency curve. EMA, Expected Moments Algorithm; Skew(G), analysis 1968 11,300 Synthesized 2017 6/14/2017 12,800 6.97 skew; PILF(LO), Potentially Influential Low Flow (low outlier). 1969 12,800 Synthesized 1955 12,700 Synthesized 1970 15,100 Synthesized 1999 12,500 1971 15,400 Synthesized 2010 6/5/2010 12,400 9.73 1972 17,900 Synthesized 1961 12,000 Synthesized 1973 6,730 PILF , Synthesized 1983 11,900 Synthesized 1974 18,600 Synthesized 2013 5/14/2013 11,800 9.68 1975 14,200 Synthesized 1957 11,700 Synthesized 1976 16,000 Synthesized 1979 11,600 Synthesized 1977 6,500 PILF , Synthesized 1936 11,500 Synthesized 1978 13,300 Synthesized 1942 11,500 Synthesized 1979 11,600 Synthesized 1968 11,300 Synthesized 1980 7,550 PILF , Synthesized 1991 6/6/1991 11,300 9.23 1981 10,400 Synthesized 1932 11,100 Synthesized 1982 15,900 Synthesized 1993 5/21/1993 11,100 8.61 1983 11,900 Synthesized 1934 11,000 Synthesized 1984 13,900 Synthesized 1963 11,000 Synthesized 1985 7,620 PILF , Synthesized 1952 10,900 Synthesized 1986 13,900 Synthesized 1938 10,800 Synthesized 1987 5/1/1987 6,980 7.52 PILF 1989 6/16/1989 10,600 9.18 1988 6/5/1988 8,500 8.04 PILF 1981 10,400 Synthesized 1989 6/16/1989 10,600 9.18 1939 9,970 Synthesized 1990 5/30/1990 7,820 8.22 PILF 2007 9,870 1991 6/6/1991 11,300 9.23 1962 9,100 PILF , Synthesized 1992 5/8/1992 5,840 6.55 PILF 1935 9,000 PILF , Synthesized 1993 5/21/1993 11,100 8.61 2002 5/31/2002 8,760 9.84 PILF 1994 4/22/1994 5,990 7.30 PILF 1946 8,600 PILF , Synthesized 1995 6/4/1995 14,100 9.29 2016 5/9/2016 8,550 8.66 PILF Figure 2. Annual peak flows and perception thresholds. 1996 6/9/1996 18,700 10.07 1988 6/5/1988 8,500 8.04 PILF 1997 19,200 1945 8,440 PILF , Synthesized 1998 8,180 PILF 1998 8,180 PILF 1999 12,500 1930 7,960 PILF , Synthesized 2000 7,060 PILF 1990 5/30/1990 7,820 8.22 PILF 2001 7,110 PILF 1985 7,620 PILF , Synthesized 1Definitions of types of PILF thresholds include: 2002 5/31/2002 8,760 9.84 PILF 1980 7,550 PILF , Synthesized MGBT: PILF threshold calculated by using the multiple Grubbs-Beck Test as specified in Bulletin 17C (England and others, 2016); 2003 5/31/2003 17,600 10.82 2005 7,490 PILF Manual: PILF threshold based on a systematic peak flow selected by the peak-flow frequency analyst. 2004 7,280 PILF 1940 7,430 PILF , Synthesized 2Flood-frequency results not reported because of too many values less than the PILF threshold used in the at-site analysis. 2005 7,490 PILF 2015 5/17/2015 7,320 8.16 PILF 3Peak-flow data with a value of zero are not plotted in figures. 2006 13,900 2004 7,280 PILF 4In cases where the month, day, or both are not present in the date of a peak flow (as indicated by adjacent slash marks with no intervening 2007 9,870 2001 7,110 PILF values), the month, day, or both are unknown. 2008 15,400 2000 7,060 PILF 5Definitions of peak-flow designations used in analysis include: 2009 14,200 1987 5/1/1987 6,980 7.52 PILF PT definition: The peak flow is used to define perception thresholds in ungaged historical periods; 2010 6/5/2010 12,400 9.73 1944 6,920 PILF , Synthesized Opportunistic: The peak flow was excluded from the analysis because it is outside of the systematic record and was of insufficient magnitude to 2011 6/8/2011 13,700 10.67 1966 6,740 PILF , Synthesized determine nonexceedance during an ungaged period; 2012 4/27/2012 12,900 9.96 1973 6,730 PILF , Synthesized PILF: The peak flow was identified as a potentially influential low flow; 2013 5/14/2013 11,800 9.68 1977 6,500 PILF , Synthesized Synthesized: The peak flow was synthesized using Maintenance of Variance Extension Type III record extension. 2014 5/24/2014 13,100 10.00 1994 4/22/1994 5,990 7.30 PILF England, J.F., Jr., Cohn, T.A., Faber, B.A., Stedinger, J.R., Thomas, W.O., Jr., Veilleux, A.G., Kiang, J.E., and Mason, R.R., Jr., 2019, Guidelines 2015 5/17/2015 7,320 8.16 PILF 1931 5,850 PILF , Synthesized for determining flood flow frequency—Bulletin 17C (ver. 1.1, May 2019): U.S. Geological Survey Techniques and Methods, book 4, chap. B5, 148 2016 5/9/2016 8,550 8.66 PILF 1992 5/8/1992 5,840 6.55 PILF p., https://doi.org/10.3133/tm4B5. 2017 6/14/2017 12,800 6.97 1941 5,780 PILF , Synthesized 2018 5/10/2018 15,100 10.44 1937 5,650 PILF , Synthesized 12351200 Bitterroot River near Florence, Montana 12351200 Bitterroot River near Florence, Montana Analysis for unregulated period of record Analysis for unregulated period of record Analysis period of record, water years: 1958–65; 1972; 1974; 1982; 2003–11 Analysis period of record, water years: 1958–65; 1972; 1974; 1982; 2003–11 At-site peak-flow frequency analysis conducted on recorded data At-site peak-flow frequency analysis conducted on recorded data Table 1‐1 Table 1‐2 Table 1‐3 Table 1‐4 Table 1‐5 Table 1‐6 Table 1‐7 Table 1‐8 Table 1‐1 Table 1‐2 Table 1‐3 Table 1‐4 Table 1‐5 Table 1‐6 Table 1‐7 Table 1‐8 [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. PILF; potentially influential low flow; MGBT, multiple Grubbs-Beck test] [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends; --, not applicable or not available.] Peak-flow data Ranked (largest to smallest) peak-flow data Number of Number of Total number Contributing PILF Type of peak‐ recorded Skew type peak flows of years in Type of PILF flow Peak flow, in Gage Peak-flow Peak flow, in Gage Peak-flow drainage area, threshold, in Water 2 Water 2 peak flows used in 1 used for ungaged cubic feet per height, designation in cubic feet per height, designation in in square threshold cubic feet per frequency Date Date used in the analysis perception historical year 4 year 4 miles second second in feet analysis second in feet analysis analysis thresholds period(s) analysis 1958 05/26/1958 16,700 9.93 1974 06/20/1974 28,400 11.20 Historic 2,342 20 Weighted MGBT 11,400 At-site 1959 06/15/1959 15,900 9.46 1982 06/17/1982 25,000 10.72 Peak flow, in cubic feet per second, for indicated annual exceedance probability (bold values), in percent 1960 06/04/1960 14,700 10.10 1972 06/03/1972 21,400 10.80 Historic 66.7 50 42.9 20 10 4 2 1.0 0.5 0.2 1961 06/03/1961 13,600 9.50 1964 06/09/1964 20,300 10.82 Historic 13,600 15,100 15,800 18,700 20,900 23,600 25,600 27,500 29,400 31,800 1962 06/15/1962 11,400 8.30 2003 06/01/2003 19,200 13.43 Upper and lower 90-percent confidence intervals, in cubic feet per second, for indicated annual exceedance probability, in percent 1963 06/05/1963 13,100 8.98 1965 06/13/1965 17,600 10.15 66.7 50 42.9 20 10 4 2 1.0 0.5 0.2 1964 06/09/1964 20,300 10.82 Historic 2008 05/20/2008 17,300 12.89 15,100 16,800 17,600 20,900 23,700 27,800 31,400 35,300 39,900 47,100 1965 06/13/1965 17,600 10.15 1958 05/26/1958 16,700 9.93 11,400 13,300 14,000 16,600 18,600 20,800 22,300 23,600 24,800 26,300 1972 06/03/1972 21,400 10.80 Historic 2011 06/08/2011 16,500 12.74 1974 06/20/1974 28,400 11.20 Historic 2006 05/20/2006 16,100 12.56 1982 06/17/1982 25,000 10.72 1959 06/15/1959 15,900 9.46 2003 06/01/2003 19,200 13.43 2009 05/31/2009 15,600 12.41 2004 06/06/2004 8,100 9.91 PILF 1960 06/04/1960 14,700 10.10 2005 05/20/2005 8,570 10.09 PILF 1961 06/03/1961 13,600 9.50 2006 05/20/2006 16,100 12.56 1963 06/05/1963 13,100 8.98 2007 11/08/2006 12,700 11.54 2007 11/08/2006 12,700 11.54 2008 05/20/2008 17,300 12.89 2010 06/05/2010 12,500 11.67 2009 05/31/2009 15,600 12.41 1962 06/15/1962 11,400 8.30 2010 06/05/2010 12,500 11.67 2005 05/20/2005 8,570 10.09 PILF 2011 06/08/2011 16,500 12.74 2004 06/06/2004 8,100 9.91 PILF

Figure 2. Annual peak flows and perception thresholds.

Peaks used in at-site analysis 1938 //1938 13,500 -- Synthesized 1949 //1949 18,100 -- Synthesized 1939 //1939 11,100 -- PILF; synthesized 1996 //1996 18,100 -- Synthesized Potentially influential low flow (PILF) 1940 //1940 7,800 -- PILF; synthesized 1975 //1975 18,000 -- Synthesized 1941 //1941 7,650 -- PILF; synthesized 1903 //1903 17,600 -- Synthesized Weighted peak-flow frequency 1942 //1942 14,300 -- Synthesized 1965 06/13/1965 17,600 10.15 curve 1943 //1943 14,800 -- Synthesized 1978 //1978 17,500 -- Synthesized Upper and lower 95-percent 1944 //1944 9,450 -- PILF; synthesized 2018 //2018 17,400 -- Synthesized confidence intervals 1945 //1945 9,560 -- PILF; synthesized 2008 05/20/2008 17,300 12.89 1946 //1946 11,100 -- PILF; synthesized 1971 //1971 17,100 -- Synthesized 1947 //1947 20,800 -- Synthesized 1970 //1970 17,000 -- Synthesized 1948 //1948 21,100 -- Synthesized 1958 05/26/1958 16,700 9.93 1949 //1949 18,100 -- Synthesized 1901 //1901 16,600 -- Synthesized 10,000 1950 //1950 16,000 -- Synthesized 1951 //1951 16,600 -- Synthesized 1951 //1951 16,600 -- Synthesized 1904 //1904 16,500 -- Synthesized 1952 //1952 12,500 -- Synthesized 2011 06/08/2011 16,500 12.74 Flow, in cubicfeetsecond per 1953 //1953 15,400 -- Synthesized 1900 //1900 16,400 -- Synthesized 1954 //1954 14,300 -- Synthesized 1986 //1986 16,400 -- Synthesized 1955 //1955 12,200 -- Synthesized 1967 //1967 16,200 -- Synthesized 1956 //1956 18,400 -- Synthesized 2006 05/20/2006 16,100 12.56 1957 //1957 14,500 -- Synthesized 2014 //2014 16,100 -- Synthesized 1958 05/26/1958 16,700 9.93 1950 //1950 16,000 -- Synthesized 1959 06/15/1959 15,900 9.46 1959 06/15/1959 15,900 9.46 1960 06/04/1960 14,700 10.10 2012 //2012 15,900 -- Synthesized 1961 06/03/1961 13,600 9.50 1969 //1969 15,700 -- Synthesized 1962 06/15/1962 11,400 8.30 PILF 2009 05/31/2009 15,600 12.41 1963 06/05/1963 13,100 8.98 1953 //1953 15,400 -- Synthesized 1,000 1964 06/09/1964 20,300 10.82 1979 //1979 15,400 -- Synthesized 99.5 99 98 95 90 80 70 60 50 40 30 20 10 5 2 1 0.5 0.2 1965 06/13/1965 17,600 10.15 1983 //1983 15,400 -- Synthesized 1966 //1966 8,490 -- PILF; synthesized 2017 //2017 15,100 -- Synthesized Exceedance probability, in percent 1967 //1967 16,200 -- Synthesized 1943 //1943 14,800 -- Synthesized Figure 1. Annual peak flows (plotting positions determined using the Cunnane formulation; Helsel and HIrsch, 2002) and peak-flow frequencycurve 1968 //1968 12,900 -- Synthesized 1960 06/04/1960 14,700 10.10 1969 //1969 15,700 -- Synthesized 1957 //1957 14,500 -- Synthesized 1970 //1970 17,000 -- Synthesized 1942 //1942 14,300 -- Synthesized 1971 //1971 17,100 -- Synthesized 1954 //1954 14,300 -- Synthesized 1972 06/03/1972 21,400 10.80 1999 //1999 14,300 -- Synthesized 1Peak flows with a value of zero are not plotted in figure 1 . 1973 //1973 13,400 -- Synthesized 1981 //1981 14,100 -- Synthesized 2In cases where the month, day, or both are not present in the date of a peak flow (as indicated by adjacent slash marks with no intervening values), 1974 06/20/1974 28,400 11.20 1991 //1991 13,900 -- Synthesized the month, day, or both are unknown. 1975 //1975 18,000 -- Synthesized 1936 //1936 13,800 -- Synthesized 3Flood-frequency results not reported because of too many values less than the PILF threshold used in the at-site analysis. 1976 //1976 20,500 -- Synthesized 1961 06/03/1961 13,600 9.50 4Definitions of peak-flow designations used in analysis include: 1977 //1977 8,640 -- PILF; synthesized 2002 //2002 13,600 -- Synthesized PT definition: The peak flow is used to define perception thresholds in ungaged historical periods; 1978 //1978 17,500 -- Synthesized 2013 //2013 13,600 -- Synthesized Opportunistic: The peak flow was excluded from the analysis because it is outside of the systematic record and was of insufficient magnitude to 1979 //1979 15,400 -- Synthesized 1938 //1938 13,500 -- Synthesized determine nonexceedance during an ungaged period; 1980 //1980 9,440 -- PILF; synthesized 1973 //1973 13,400 -- Synthesized PILF: The peak flow was identified as a potentially influential low flow; Synthesized: The peak flow was synthesized using Maintenance of Variance Extension Type III record extension. 1981 //1981 14,100 -- Synthesized 1932 //1932 13,300 -- Synthesized England, J.F., Jr., Cohn, T.A., Faber, B.A., Stedinger, J.R., Thomas, W.O., Jr., Veilleux, A.G., Kiang, J.E., and Mason, R.R., Jr., 2019, Guidelines 1982 06/17/1982 25,000 10.72 1934 //1934 13,300 -- Synthesized for determining flood flow frequency—Bulletin 17C (ver. 1.1, May 2019): U.S. Geological Survey Techniques and Methods, book 4, chap. B5, 148 1983 //1983 15,400 -- Synthesized 1995 //1995 13,200 -- Synthesized p., https://doi.org/10.3133/tm4B5. 1984 //1984 18,700 -- Synthesized 1963 06/05/1963 13,100 8.98 1985 //1985 8,440 -- PILF; synthesized 1968 //1968 12,900 -- Synthesized 1986 //1986 16,400 -- Synthesized 2007 11/08/2006 12,700 11.54 Helsel,D.R.,andHirsch,R.M.,2002,Statisticalmethodsinwaterresources:U.S.GeologicalSur-veyTechniquesofWater- 1987 //1987 4,270 -- PILF; synthesized 1952 //1952 12,500 -- Synthesized ResourcesInvestigations,book4,chap.A3,522p.,accessedOctober30,2017,athttps://pubs.usgs.gov/twri/twri4a3/. 1988 //1988 9,670 -- PILF; synthesized 2010 06/05/2010 12,500 11.67 1989 //1989 12,400 -- Synthesized 1989 //1989 12,400 -- Synthesized 1990 //1990 9,620 -- PILF; synthesized 1955 //1955 12,200 -- Synthesized 1991 //1991 13,900 -- Synthesized 1962 06/15/1962 11,400 8.30 PILF 1992 //1992 6,230 -- PILF; synthesized 1939 //1939 11,100 -- PILF; synthesi 1993 //1993 11,000 -- PILF; synthesized 1946 //1946 11,100 -- PILF; synthesi 1994 //1994 7,600 -- PILF; synthesized 1993 //1993 11,000 -- PILF; synthesi 1995 //1995 13,200 -- Synthesized 1935 //1935 10,600 -- PILF; synthesi 1996 //1996 18,100 -- Synthesized 1988 //1988 9,670 -- PILF; synthesi 1997 //1997 21,800 -- Synthesized 1990 //1990 9,620 -- PILF; synthesi 1998 //1998 9,440 -- PILF; synthesized 1945 //1945 9,560 -- PILF; synthesi 1999 //1999 14,300 -- Synthesized 1944 //1944 9,450 -- PILF; synthesi 2000 //2000 8,180 -- PILF; synthesized 1980 //1980 9,440 -- PILF; synthesi 2001 //2001 8,230 -- PILF; synthesized 1998 //1998 9,440 -- PILF; synthesi 2002 //2002 13,600 -- Synthesized 1930 //1930 9,280 -- PILF; synthesi 2003 06/01/2003 19,200 13.43 2016 //2016 9,080 -- PILF; synthesi 2004 06/06/2004 8,100 9.91 PILF 1977 //1977 8,640 -- PILF; synthesi 2005 05/20/2005 8,570 10.09 PILF 2005 05/20/2005 8,570 10.09 PILF 2006 05/20/2006 16,100 12.56 1966 //1966 8,490 -- PILF; synthesi 2007 11/08/2006 12,700 11.54 1985 //1985 8,440 -- PILF; synthesi 2008 05/20/2008 17,300 12.89 2001 //2001 8,230 -- PILF; synthesi 2009 05/31/2009 15,600 12.41 2000 //2000 8,180 -- PILF; synthesi 2010 06/05/2010 12,500 11.67 2004 06/06/2004 8,100 9.91 PILF 2011 06/08/2011 16,500 12.74 2015 //2015 7,940 -- PILF; synthesi 2012 //2012 15,900 -- Synthesized 1940 //1940 7,800 -- PILF; synthesi 2013 //2013 13,600 -- Synthesized 1941 //1941 7,650 -- PILF; synthesi 2014 //2014 16,100 -- Synthesized 1994 //1994 7,600 -- PILF; synthesi 2015 //2015 7,940 -- PILF; synthesized 1937 //1937 7,550 -- PILF; synthesi 2016 //2016 9,080 -- PILF; synthesized 1931 //1931 6,650 -- PILF; synthesi 2017 //2017 15,100 -- Synthesized 1992 //1992 6,230 -- PILF; synthesi 2018 //2018 17,400 -- Synthesized 1987 //1987 4,270 -- PILF; synthesi 12352500 Bitterroot River near Missoula, Montana 12352500 Bitterroot River near Missoula, Montana Analysis for unregulated period of record Analysis for unregulated period of record Analysis period of record, water years: 1899–1901; 1903–4; 1990–2018 Analysis period of record, water years: 1899–1901; 1903–4; 1990–2018 At-site peak-flow frequency analysis conducted on recorded data At-site peak-flow frequency analysis conducted on recorded data Table 1‐1 Table 1‐2 Table 1‐3 Table 1‐4 Table 1‐5 Table 1‐6 Table 1‐7 Table 1‐8 Table 1‐1 Table 1‐2 Table 1‐3 Table 1‐4 Table 1‐5 Table 1‐6 Table 1‐7 Table 1‐8 [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. PILF; potentially influential low flow; MGBT, multiple Grubbs-Beck test; -- not applicable or not available] [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends; --, not applicable or not available.] Peak-flow data Ranked (largest to smallest) peak-flow data Number of Number of Total number Contributing PILF Type of peak‐ recorded Skew type peak flows of years in Type of PILF flow Peak flow, in Gage Peak-flow Peak flow, in Gage Peak-flow drainage area, threshold, in Water 2 Water 2 peak flows used in 1 used for ungaged cubic feet per height, designation in cubic feet per height, designation in in square threshold cubic feet per frequency Date Date used in the analysis perception historical year 4 year 4 miles second second in feet analysis second in feet analysis analysis thresholds period(s) analysis 1899 06/20/1899 38,300 11.55 Historic 1899 06/20/1899 38,300 11.55 Historic 2,824 34 Weighted MGBT -- At-site 1900 05/13/1900 18,200 -- 1997 05/18/1997 24,800 13.11 Peak flow, in cubic feet per second, for indicated annual exceedance probability (bold values), in percent 1901 05/30/1901 18,400 -- 2003 06/01/2003 21,600 12.65 66.7 50 42.9 20 10 4 2 1.0 0.5 0.2 1903 06/05/1903 19,700 -- 1996 06/10/1996 20,300 11.83 12,200 14,400 15,400 19,700 23,100 27,200 30,100 32,900 35,700 39,300 1904 05/25/1904 18,300 -- 1903 06/05/1903 19,700 -- Upper and lower 90-percent confidence intervals, in cubic feet per second, for indicated annual exceedance probability, in percent 1990 05/31/1990 10,200 8.13 2008 05/21/2008 19,600 11.97 66.7 50 42.9 20 10 4 2 1.0 0.5 0.2 1991 06/07/1991 15,200 9.76 2018 05/11/2018 19,400 12.23 14,100 16,600 17,800 22,800 26,800 32,100 36,400 41,000 46,100 53,300 1992 05/01/1992 6,370 6.54 2011 06/09/2011 18,600 11.37 10,400 12,500 13,400 17,200 20,000 23,000 25,000 26,800 28,300 30,200 1993 05/22/1993 11,800 8.71 1901 05/30/1901 18,400 -- 1994 04/23/1994 7,900 7.22 1904 05/25/1904 18,300 -- 1995 06/04/1995 14,400 9.53 1900 05/13/1900 18,200 -- 1996 06/10/1996 20,300 11.83 2009 06/01/2009 17,900 11.38 1997 05/18/1997 24,800 13.11 2014 05/25/2014 17,900 11.11 1998 05/27/1998 10,000 8.40 2012 04/28/2012 17,600 11.09 1999 06/04/1999 15,700 10.65 2006 05/21/2006 17,500 11.24 2000 05/29/2000 8,550 7.60 2017 06/14/2017 16,700 10.80 2001 05/15/2001 8,610 7.68 1999 06/04/1999 15,700 10.65 2002 06/01/2002 14,900 10.33 1991 06/07/1991 15,200 9.76 2003 06/01/2003 21,600 12.65 2002 06/01/2002 14,900 10.33 2004 06/07/2004 8,830 7.92 2013 05/15/2013 14,900 10.24 2005 05/20/2005 9,100 7.94 1995 06/04/1995 14,400 9.53 2006 05/21/2006 17,500 11.24 2010 06/06/2010 13,900 9.94 2007 11/08/2006 12,200 9.27 2007 11/08/2006 12,200 9.27 2008 05/21/2008 19,600 11.97 1993 05/22/1993 11,800 8.71 2009 06/01/2009 17,900 11.38 1990 05/31/1990 10,200 8.13 2010 06/06/2010 13,900 9.94 1998 05/27/1998 10,000 8.40 2011 06/09/2011 18,600 11.37 2016 05/09/2016 9,580 8.29 2012 04/28/2012 17,600 11.09 2005 05/20/2005 9,100 7.94 2013 05/15/2013 14,900 10.24 2004 06/07/2004 8,830 7.92 2014 05/25/2014 17,900 11.11 2001 05/15/2001 8,610 7.68 2015 06/03/2015 8,280 7.75 2000 05/29/2000 8,550 7.60 2016 05/09/2016 9,580 8.29 2015 06/03/2015 8,280 7.75 2017 06/14/2017 16,700 10.80 1994 04/23/1994 7,900 7.22 2018 05/11/2018 19,400 12.23 1992 05/01/1992 6,370 6.54

Figure 2. Annual peak flows and perception thresholds.

Potentially influential low flow (PILF) 1940 //1940 8,940 -- PILF; synthesized 1967 //1967 19,500 -- Synthesized 1941 //1941 6,840 -- PILF; synthesized 1971 //1971 19,400 -- Synthesized Weighted peak-flow frequency 1942 //1942 14,200 -- Synthesized 2018 43231 19,400 12.23 curve 1943 //1943 15,900 -- Synthesized 1970 //1970 19,000 -- Synthesized Upper and lower 95-percent 1944 //1944 8,280 -- PILF; synthesized 1933 //1933 18,600 -- Synthesized confidence intervals 1945 //1945 10,200 -- PILF; synthesized 2011 06/09/2011 18,600 11.37 1946 //1946 10,400 -- PILF; synthesized 1953 //1953 18,500 -- Synthesized 1947 //1947 24,400 -- Synthesized 1958 //1958 18,500 -- Synthesized 1948 //1948 24,000 -- Synthesized 1901 05/30/1901 18,400 -- 1949 //1949 17,100 -- Synthesized 1904 05/25/1904 18,300 -- 10,000 1950 //1950 16,100 -- Synthesized 1900 05/13/1900 18,200 -- 1951 //1951 16,400 -- Synthesized 2009 06/01/2009 17,900 11.38 1952 //1952 13,400 -- Synthesized 2014 05/25/2014 17,900 11.11 Flow, in cubic feet per second per feet cubic in Flow, 1953 //1953 18,500 -- Synthesized 1975 //1975 17,800 -- Synthesized 1954 //1954 17,100 -- Synthesized 2012 04/28/2012 17,600 11.09 1955 //1955 15,800 -- Synthesized 1959 //1959 17,500 -- Synthesized 1956 //1956 22,600 -- Synthesized 2006 05/21/2006 17,500 11.24 1957 //1957 14,400 -- Synthesized 1984 //1984 17,300 -- Synthesized 1958 //1958 18,500 -- Synthesized 1986 //1986 17,300 -- Synthesized 1959 //1959 17,500 -- Synthesized 1949 //1949 17,100 -- Synthesized 1960 //1960 16,100 -- Synthesized 1954 //1954 17,100 -- Synthesized 1961 //1961 14,800 -- Synthesized 2017 06/14/2017 16,700 10.80 1962 //1962 12,200 -- Synthesized 1978 //1978 16,600 -- Synthesized 1963 //1963 14,200 -- Synthesized 1951 //1951 16,400 -- Synthesized 1,000 1964 //1964 22,900 -- Synthesized 1950 //1950 16,100 -- Synthesized 99.5 0.5 0.2 99 98 95 90 80 70 60 50 40 30 20 10 5 2 1 1965 //1965 19,600 -- Synthesized 1960 //1960 16,100 -- Synthesized 1966 //1966 8,060 -- PILF; synthesized 1943 //1943 15,900 -- Synthesized Exceedance probability, in percent 1967 //1967 19,500 -- Synthesized 1955 //1955 15,800 -- Synthesized Figure 1. Annual peak flows (plotting positions determined using the Cunnane formulation; Helsel and HIrsch, 2002) and peak-flow frequencycurve 1968 //1968 14,000 -- Synthesized 1969 //1969 15,800 -- Synthesized 1969 //1969 15,800 -- Synthesized 1999 06/04/1999 15,700 10.65 1970 //1970 19,000 -- Synthesized 1991 06/07/1991 15,200 9.76 1971 //1971 19,400 -- Synthesized 2002 06/01/2002 14,900 10.33 1972 //1972 24,200 -- Synthesized 2013 05/15/2013 14,900 10.24 1Peak flows with a value of zero are not plotted in figure 1 . 1973 //1973 8,040 -- PILF; synthesized 1961 //1961 14,800 -- Synthesized 2In cases where the month, day, or both are not present in the date of a peak flow (as indicated by adjacent slash marks with no intervening values), 1974 //1974 32,900 -- Synthesized 1983 //1983 14,700 -- Synthesized the month, day, or both are unknown. 1975 //1975 17,800 -- Synthesized 1957 //1957 14,400 -- Synthesized 3Flood-frequency results not reported because of too many values less than the PILF threshold used in the at-site analysis. 1976 //1976 20,100 -- Synthesized 1995 06/04/1995 14,400 9.53 4Definitions of peak-flow designations used in analysis include: 1977 //1977 7,750 -- PILF; synthesized 1979 //1979 14,300 -- Synthesized PT definition: The peak flow is used to define perception thresholds in ungaged historical periods; 1978 //1978 16,600 -- Synthesized 1942 //1942 14,200 -- Synthesized Opportunistic: The peak flow was excluded from the analysis because it is outside of the systematic record and was of insufficient magnitude to 1979 //1979 14,300 -- Synthesized 1963 //1963 14,200 -- Synthesized determine nonexceedance during an ungaged period; 1980 //1980 9,090 -- PILF; synthesized 1936 //1936 14,000 -- Synthesized PILF: The peak flow was identified as a potentially influential low flow; Synthesized: The peak flow was synthesized using Maintenance of Variance Extension Type III record extension. 1981 //1981 12,800 -- Synthesized 1968 //1968 14,000 -- Synthesized England, J.F., Jr., Cohn, T.A., Faber, B.A., Stedinger, J.R., Thomas, W.O., Jr., Veilleux, A.G., Kiang, J.E., and Mason, R.R., Jr., 2019, Guidelines 1982 //1982 28,700 -- Synthesized 2010 06/06/2010 13,900 9.94 for determining flood flow frequency—Bulletin 17C (ver. 1.1, May 2019): U.S. Geological Survey Techniques and Methods, book 4, chap. B5, 148 1983 //1983 14,700 -- Synthesized 1932 //1932 13,500 -- Synthesized p., https://doi.org/10.3133/tm4B5. 1984 //1984 17,300 -- Synthesized 1934 //1934 13,500 -- Synthesized 1985 //1985 9,170 -- PILF; synthesized 1952 //1952 13,400 -- Synthesized 1986 //1986 17,300 -- Synthesized 1938 //1938 13,300 -- Synthesized Helsel,D.R.,andHirsch,R.M.,2002,Statisticalmethodsinwaterresources:U.S.GeologicalSur-veyTechniquesofWater- 1987 //1987 7,610 -- PILF; synthesized 1981 //1981 12,800 -- Synthesized ResourcesInvestigations,book4,chap.A3,522p.,accessedOctober30,2017,athttps://pubs.usgs.gov/twri/twri4a3/. 1988 //1988 9,770 -- PILF; synthesized 1939 //1939 12,200 -- Synthesized 1989 //1989 11,100 -- PILF; synthesized 1962 //1962 12,200 -- Synthesized 1990 05/31/1990 10,200 8.13 PILF 2007 11/08/2006 12,200 9.27 1991 06/07/1991 15,200 9.76 1993 05/22/1993 11,800 8.71 1992 05/01/1992 6,370 6.54 PILF 1989 //1989 11,100 -- PILF; synthesi 1993 05/22/1993 11,800 8.71 1935 //1935 10,800 -- PILF; synthesi 1994 04/23/1994 7,900 7.22 PILF 1946 //1946 10,400 -- PILF; synthesi 1995 06/04/1995 14,400 9.53 1945 //1945 10,200 -- PILF; synthesi 1996 06/10/1996 20,300 11.83 1990 05/31/1990 10,200 8.13 PILF 1997 05/18/1997 24,800 13.11 1998 05/27/1998 10,000 8.40 PILF 1998 05/27/1998 10,000 8.40 PILF 1988 //1988 9,770 -- PILF; synthesi 1999 06/04/1999 15,700 10.65 2016 05/09/2016 9,580 8.29 PILF 2000 05/29/2000 8,550 7.60 PILF 1930 //1930 9,520 -- PILF; synthesi 2001 05/15/2001 8,610 7.68 PILF 1985 //1985 9,170 -- PILF; synthesi 2002 06/01/2002 14,900 10.33 2005 05/20/2005 9,100 7.94 PILF 2003 06/01/2003 21,600 12.65 1980 //1980 9,090 -- PILF; synthesi 2004 06/07/2004 8,830 7.92 PILF 1940 //1940 8,940 -- PILF; synthesi 2005 05/20/2005 9,100 7.94 PILF 2004 06/07/2004 8,830 7.92 PILF 2006 05/21/2006 17,500 11.24 2001 05/15/2001 8,610 7.68 PILF 2007 11/08/2006 12,200 9.27 2000 05/29/2000 8,550 7.60 PILF 2008 05/21/2008 19,600 11.97 1944 //1944 8,280 -- PILF; synthesi 2009 06/01/2009 17,900 11.38 2015 06/03/2015 8,280 7.75 PILF 2010 06/06/2010 13,900 9.94 1966 //1966 8,060 -- PILF; synthesi 2011 06/09/2011 18,600 11.37 1973 //1973 8,040 -- PILF; synthesi 2012 04/28/2012 17,600 11.09 1994 04/23/1994 7,900 7.22 PILF 2013 05/15/2013 14,900 10.24 1977 //1977 7,750 -- PILF; synthesi 2014 05/25/2014 17,900 11.11 1987 //1987 7,610 -- PILF; synthesi 2015 06/03/2015 8,280 7.75 PILF 1931 //1931 6,870 -- PILF; synthesi 2016 05/09/2016 9,580 8.29 PILF 1941 //1941 6,840 -- PILF; synthesi 2017 06/14/2017 16,700 10.80 1937 //1937 6,620 -- PILF; synthesi 2018 05/11/2018 19,400 12.23 1992 05/01/1992 6,370 6.54 PILF 12353000 Clark Fork below Missoula, Montana 12353000 Clark Fork below Missoula, Montana Analysis for unregulated period of record Analysis for unregulated period of record Analysis period of record, water years: 1930–2018 Analysis period of record, water years: 1930–2018 At-site peak-flow frequency analysis conducted on recorded data At-site peak-flow frequency analysis conducted on recorded data Table 1‐1 Table 1‐2 Table 1‐3 Table 1‐4 Table 1‐5 Table 1‐6 Table 1‐7 Table 1‐8 Table 1‐1 Table 1‐2 Table 1‐3 Table 1‐4 Table 1‐5 Table 1‐6 Table 1‐7 Table 1‐8 [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. PILF; potentially influential low flow; MGBT, multiple Grubbs-Beck test] [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends; --, not applicable or not available.] Peak-flow data Ranked (largest to smallest) peak-flow data Number of Number of Total number Contributing PILF Type of peak‐ recorded Skew type peak flows of years in Type of PILF flow Peak flow, in Gage Peak-flow Peak flow, in Gage Peak-flow drainage area, threshold, in Water 2 Water 2 peak flows used in 1 used for ungaged cubic feet per height, designation in cubic feet per height, designation in in square threshold cubic feet per frequency Date Date used in the analysis perception historical year 4 year 4 miles second second in feet analysis second in feet analysis analysis thresholds period(s) analysis 1930 04/26/1930 17,500 6.32 PILF 1997 05/18/1997 55,100 12.18 9,017 89 Weighted MGBT 22,000 At-site 1931 05/18/1931 12,200 5.08 PILF 1948 05/23/1948 52,800 12.08 Historic Peak flow, in cubic feet per second, for indicated annual exceedance probability (bold values), in percent 1932 05/23/1932 25,800 8.12 1972 06/03/1972 52,200 11.71 66.7 50 42.9 20 10 4 2 1.0 0.5 0.2 1933 06/11/1933 36,800 10.14 2018 05/11/2018 52,200 12.14 23,700 28,200 30,200 38,200 44,000 50,400 54,800 58,700 62,300 66,700 1934 04/26/1934 25,700 8.00 1964 06/10/1964 50,100 11.45 Upper and lower 90-percent confidence intervals, in cubic feet per second, for indicated annual exceedance probability, in percent 1935 05/25/1935 20,200 6.80 PILF 1975 06/21/1975 49,200 11.67 66.7 50 42.9 20 10 4 2 1.0 0.5 0.2 1936 05/16/1936 26,900 8.34 1974 06/18/1974 47,900 11.50 25,800 30,600 32,800 41,700 48,500 56,800 63,000 69,400 75,900 85,000 1937 05/27/1937 11,700 5.09 PILF 2011 06/09/2011 47,000 11.33 19,900 25,400 27,500 35,200 40,400 46,000 49,400 52,300 54,800 57,500 1938 05/30/1938 35,700 10.00 1947 05/10/1947 45,900 11.18 1939 05/05/1939 22,000 7.29 1956 05/25/1956 44,600 10.75 1940 05/13/1940 14,200 5.56 PILF 1976 05/15/1976 43,600 10.96 1941 06/02/1941 10,400 4.61 PILF 1981 05/24/1981 42,900 10.61 1942 05/27/1942 30,500 8.96 1982 06/18/1982 41,000 10.37 1943 06/20/1943 33,200 9.48 1953 06/04/1953 40,100 10.10 1944 06/18/1944 14,400 5.58 PILF 1967 06/08/1967 39,900 10.09 1945 06/02/1945 19,100 6.66 PILF 1950 06/18/1950 39,100 10.17 1946 05/29/1946 19,700 6.72 PILF 2003 06/01/2003 38,700 10.24 1947 05/10/1947 45,900 11.18 1996 06/10/1996 38,200 10.12 1948 05/23/1948 52,800 12.08 Historic 1933 06/11/1933 36,800 10.14 1949 05/17/1949 34,300 9.55 2008 05/21/2008 36,400 9.93 1950 06/18/1950 39,100 10.17 1971 05/14/1971 36,200 9.53 1951 05/25/1951 32,500 9.09 1970 06/07/1970 36,100 9.54 1952 05/16/1952 27,100 8.17 1938 05/30/1938 35,700 10.00 1953 06/04/1953 40,100 10.10 1959 06/07/1959 35,500 9.50 1954 05/21/1954 34,200 9.37 2009 06/01/2009 35,400 9.79 1955 06/16/1955 27,500 8.19 2012 04/28/2012 34,900 9.71 1956 05/25/1956 44,600 10.75 1949 05/17/1949 34,300 9.55 1957 05/21/1957 31,500 8.88 1954 05/21/1954 34,200 9.37 1958 05/23/1958 32,900 9.34 1965 06/13/1965 33,500 9.38 1959 06/07/1959 35,500 9.50 2014 05/26/2014 33,500 9.52 1960 06/05/1960 27,100 8.14 1943 06/20/1943 33,200 9.48 1961 05/28/1961 29,300 8.51 1958 05/23/1958 32,900 9.34 1962 05/25/1962 25,800 7.84 1951 05/25/1951 32,500 9.09 1963 06/06/1963 24,600 7.54 2017 06/14/2017 32,500 9.37 1964 06/10/1964 50,100 11.45 1986 05/31/1986 32,300 9.27 1965 06/13/1965 33,500 9.38 1979 05/27/1979 31,700 9.14 1966 05/11/1966 16,800 6.31 PILF 1957 05/21/1957 31,500 8.88 1967 06/08/1967 39,900 10.09 2006 05/21/2006 31,500 9.21 1968 06/13/1968 24,700 7.79 1999 06/04/1999 31,300 9.25 1969 05/21/1969 28,700 8.45 1978 06/09/1978 30,700 9.10 1970 06/07/1970 36,100 9.54 1942 05/27/1942 30,500 8.96 1971 05/14/1971 36,200 9.53 1961 05/28/1961 29,300 8.51 1972 06/03/1972 52,200 11.71 1984 06/01/1984 28,800 8.72 1973 05/20/1973 15,000 6.37 PILF 1969 05/21/1969 28,700 8.45 1974 06/18/1974 47,900 11.50 1983 05/30/1983 28,500 8.67 1975 06/21/1975 49,200 11.67 1955 06/16/1955 27,500 8.19 1976 05/15/1976 43,600 10.96 2002 05/31/2002 27,500 8.56 Figure 1. Annual peak flows (probability plotting positions) and peak-flow frequency curve. EMA, Expected Moments Algorithm; Skew(G), analysis 1977 06/08/1977 9,100 4.76 PILF 1980 05/26/1980 27,400 8.41 skew; PILF(LO), potentially Influencial low flow (low outlier). 1978 06/09/1978 30,700 9.10 1991 06/08/1991 27,200 8.46 1979 05/27/1979 31,700 9.14 1952 05/16/1952 27,100 8.17 1980 05/26/1980 27,400 8.41 1960 06/05/1960 27,100 8.14 1981 05/24/1981 42,900 10.61 1936 05/16/1936 26,900 8.34 1982 06/18/1982 41,000 10.37 2013 05/15/2013 26,900 8.42 1983 05/30/1983 28,500 8.67 1989 05/11/1989 26,300 8.32 1984 06/01/1984 28,800 8.72 1932 05/23/1932 25,800 8.12 1985 05/25/1985 21,100 7.38 PILF 1962 05/25/1962 25,800 7.84 1986 05/31/1986 32,300 9.27 1934 04/26/1934 25,700 8.00 1987 05/01/1987 15,800 6.28 PILF 1995 06/07/1995 25,500 8.17 1988 06/05/1988 14,300 5.94 PILF 1968 06/13/1968 24,700 7.79 1989 05/11/1989 26,300 8.32 1963 06/06/1963 24,600 7.54 1990 05/31/1990 22,200 7.58 2010 06/06/2010 23,500 7.87 1991 06/08/1991 27,200 8.46 1993 05/16/1993 23,400 7.80 1992 05/01/1992 12,400 5.45 PILF 1990 05/31/1990 22,200 7.58 1993 05/16/1993 23,400 7.80 1939 05/05/1939 22,000 7.29 1994 04/25/1994 16,900 6.52 PILF 1985 05/25/1985 21,100 7.38 PILF 1995 06/07/1995 25,500 8.17 1998 06/20/1998 21,000 7.53 PILF 1996 06/10/1996 38,200 10.12 1935 05/25/1935 20,200 6.80 PILF 1997 05/18/1997 55,100 12.18 1946 05/29/1946 19,700 6.72 PILF 1998 06/20/1998 21,000 7.53 PILF 1945 06/02/1945 19,100 6.66 PILF 1999 06/04/1999 31,300 9.25 2007 05/14/2007 19,100 7.01 PILF 2000 05/23/2000 13,500 5.87 PILF 2005 05/20/2005 19,000 6.69 PILF 2001 05/16/2001 15,900 6.28 PILF 2016 05/09/2016 17,600 6.59 PILF 2002 05/31/2002 27,500 8.56 1930 04/26/1930 17,500 6.32 PILF 2003 06/01/2003 38,700 10.24 1994 04/25/1994 16,900 6.52 PILF 2004 06/07/2004 15,600 6.25 PILF 1966 05/11/1966 16,800 6.31 PILF 2005 05/20/2005 19,000 6.69 PILF 2001 05/16/2001 15,900 6.28 PILF 2006 05/21/2006 31,500 9.21 1987 05/01/1987 15,800 6.28 PILF 2007 05/14/2007 19,100 7.01 PILF 2004 06/07/2004 15,600 6.25 PILF 2008 05/21/2008 36,400 9.93 2015 06/03/2015 15,300 6.05 PILF 2009 06/01/2009 35,400 9.79 1973 05/20/1973 15,000 6.37 PILF Figure 2. Annual peak flows and perception thresholds. 2010 06/06/2010 23,500 7.87 1944 06/18/1944 14,400 5.58 PILF 2011 06/09/2011 47,000 11.33 1988 06/05/1988 14,300 5.94 PILF 2012 04/28/2012 34,900 9.71 1940 05/13/1940 14,200 5.56 PILF 2013 05/15/2013 26,900 8.42 2000 05/23/2000 13,500 5.87 PILF 2014 05/26/2014 33,500 9.52 1992 05/01/1992 12,400 5.45 PILF 1Definitions of types of PILF thresholds include: 2015 06/03/2015 15,300 6.05 PILF 1931 05/18/1931 12,200 5.08 PILF MGBT: PILF threshold calculated by using the multiple Grubbs-Beck Test as specifed in Bulletin 17C (England and others, 2016); 2016 05/09/2016 17,600 6.59 PILF 1937 05/27/1937 11,700 5.09 PILF Manual: PILF threshold based on a systematic peak flow selected by the peak-flow frequency analyst. 2017 06/14/2017 32,500 9.37 1941 06/02/1941 10,400 4.61 PILF 2018 05/11/2018 52,200 12.14 1977 06/08/1977 9,100 4.76 PILF 2Peak-flow data with a value of zero are not plotted in figures 1 or 2. 3In cases where the month, day, or both are not present in the date of a peak flow (as indicated by adjacent slash marks with no intervening values), the month, day, or both are unknown. 4Definitions of peak-flow designations used in analysis include: PT definition: The peak flow is used to define perception thresholds in ungaged historical periods; Opportunistic: The peak flow was excluded from the analysis because it is outside of the systematic record and was of insufficient magnitude to determine nonexceedance during an ungaged period; PILF: The peak flow was identified as a potentially influential low flow; Synthesized: The peak flow was synthesized using Maintenance of Variance Extension Type III record extension. England, J.F., Jr., Cohn, T.A., Faber, B.A., Stedinger, J.R., Thomas, W.O., Jr., Veilleux, A.G., Kiang, J.E., and Mason, R.R., Jr., 2019, Guidelines for determining flood flow frequency—Bulletin 17C (ver. 1.1, May 2019): U.S. Geological Survey Techniques and Methods, book 4, chap. B5, 148 p., https://doi.org/10.3133/tm4B5. 12353000 Clark Fork below Missoula, Montana 12353000 Clark Fork below Missoula, Montana Analysis for unregulated period of record Analysis for unregulated period of record Analysis period of record, water years: 1899–1908; 1911–23; 1929–2018 Analysis period of record, water years: 1899–1908; 1911–23; 1929–2018 Peak-flow frequency analysis conducted on recorded and synthesized data Peak-flow frequency analysis conducted on recorded and synthesized data Table 1‐1 Table 1‐2 Table 1‐3 Table 1‐4 Table 1‐5 Table 1‐6 Table 1‐7 Table 1‐8 Table 1‐1 Table 1‐2 Table 1‐3 Table 1‐4 Table 1‐5 Table 1‐6 Table 1‐7 Table 1‐8 [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. PILF; potentially influential low flow; MGBT, multiple Grubbs-Beck test] [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends; --, not applicable or not available.] Peak-flow data Ranked (largest to smallest) peak-flow data Contributing Type of peak‐ Peak flow, in Gage Peak-flow Peak flow, in Gage Peak-flow drainage area, flow Water Water 2 cubic feet per height, designation in 2 cubic feet per height, designation in in square frequency year Date year Date second in feet 4 second in feet 4 miles analysis analysis analysis 1899 //1899 61,700 -- Synthesized 1908 //1908 78,400 -- Synthesized 9,017 MOVE.3 1900 //1900 45,400 -- Synthesized 1899 //1899 61,700 -- Synthesized Peak flow, in cubic feet per second, for indicated annual exceedance probability (bold values), in percent 1901 //1901 37,000 -- Synthesized 1997 05/18/1997 55,100 12.18 66.7 50 42.9 20 10 4 2 1.0 0.5 0.2 1902 //1902 54,200 -- Synthesized 1902 //1902 54,200 -- Synthesized 24,400 29,300 31,500 40,500 47,200 54,900 60,200 65,000 69,600 75,200 1903 //1903 44,500 -- Synthesized 1948 05/23/1948 52,800 12.08 Historic Upper and lower 90-percent confidence intervals, in cubic feet per second, for indicated annual exceedance probability, in percent 1904 //1904 33,100 -- Synthesized 1972 06/03/1972 52,200 11.71 66.7 50 42.9 20 10 4 2 1.0 0.5 0.2 1905 //1905 17,400 -- PILF; synthesized 2018 05/11/2018 52,200 12.14 26,500 31,800 34,200 44,200 52,000 62,100 70,000 78,200 86,900 99,200 1906 //1906 16,100 -- PILF; synthesized 1913 //1913 51,000 -- Synthesized 21,500 26,700 28,900 37,300 43,400 50,000 54,200 57,600 60,600 64,000 1907 //1907 34,200 -- Synthesized 1964 06/10/1964 50,100 11.45 1908 //1908 78,400 -- Synthesized 1975 06/21/1975 49,200 11.67 1911 //1911 26,900 -- Synthesized 1974 06/18/1974 47,900 11.50 100,000 EXPLANATION 1912 //1912 33,000 -- Synthesized 1917 //1917 47,400 -- Synthesized 1913 //1913 51,000 -- Synthesized 2011 06/09/2011 47,000 11.33 Peaks used in at-site analysis 1914 //1914 27,900 -- Synthesized 1947 05/10/1947 45,900 11.18 1915 //1915 13,700 -- PILF; synthesized 1900 //1900 45,400 -- Synthesized Potentially influential low flow (PILF) 1916 //1916 42,400 -- Synthesized 1956 05/25/1956 44,600 10.75 1917 //1917 47,400 -- Synthesized 1903 //1903 44,500 -- Synthesized Weighted peak-flow frequency 1918 //1918 38,700 -- Synthesized 1976 05/15/1976 43,600 10.96 curve 1919 //1919 23,300 -- Synthesized 1981 05/24/1981 42,900 10.61 Upper and lower 95-percent 1920 //1920 25,500 -- Synthesized 1916 //1916 42,400 -- Synthesized confidence intervals 1921 //1921 33,400 -- Synthesized 1982 06/18/1982 41,000 10.37 1922 //1922 37,700 -- Synthesized 1953 06/04/1953 40,100 10.10 1923 //1923 26,300 -- Synthesized 1967 06/08/1967 39,900 10.09 1929 //1929 25,900 -- Synthesized 1950 06/18/1950 39,100 10.17 1930 04/26/1930 17,500 6.32 PILF 1918 //1918 38,700 -- Synthesized 10,000 1931 05/18/1931 12,200 5.08 PILF 2003 06/01/2003 38,700 10.24 1932 05/23/1932 25,800 8.12 1996 06/10/1996 38,200 10.12 1933 06/11/1933 36,800 10.14 1922 //1922 37,700 -- Synthesized Flow, in cubicfeetsecond per 1934 04/26/1934 25,700 8.00 1901 //1901 37,000 -- Synthesized 1935 05/25/1935 20,200 6.80 PILF 1933 06/11/1933 36,800 10.14 1936 05/16/1936 26,900 8.34 2008 05/21/2008 36,400 9.93 1937 05/27/1937 11,700 5.09 PILF 1971 05/14/1971 36,200 9.53 1938 05/30/1938 35,700 10.00 1970 06/07/1970 36,100 9.54 1939 05/05/1939 22,000 7.29 1938 05/30/1938 35,700 10.00 1940 05/13/1940 14,200 5.56 PILF 1959 06/07/1959 35,500 9.50 1941 06/02/1941 10,400 4.61 PILF 2009 06/01/2009 35,400 9.79 1942 05/27/1942 30,500 8.96 2012 04/28/2012 34,900 9.71 1943 06/20/1943 33,200 9.48 1949 05/17/1949 34,300 9.55 1944 06/18/1944 14,400 5.58 PILF 1907 //1907 34,200 -- Synthesized 1,000 1945 06/02/1945 19,100 6.66 PILF 1954 05/21/1954 34,200 9.37 99.5 99 98 95 90 80 70 60 50 40 30 20 10 5 2 1 0.5 0.2 1946 05/29/1946 19,700 6.72 PILF 1965 06/13/1965 33,500 9.38

Exceedance probability, in percent 1947 05/10/1947 45,900 11.18 2014 05/26/2014 33,500 9.52 1948 05/23/1948 52,800 12.08 Historic 1921 //1921 33,400 -- Synthesized Figure 1. Annual peak flows (plotting positions determined using the Cunnane formulation; Helsel and HIrsch, 2002) and peak-flow frequencycurve 1949 05/17/1949 34,300 9.55 1943 06/20/1943 33,200 9.48 1950 06/18/1950 39,100 10.17 1904 //1904 33,100 -- Synthesized 1951 05/25/1951 32,500 9.09 1912 //1912 33,000 -- Synthesized 1952 05/16/1952 27,100 8.17 1958 05/23/1958 32,900 9.34 1953 06/04/1953 40,100 10.10 1951 05/25/1951 32,500 9.09 1Peak flows with a value of zero are not plotted in figure 1 . 1954 05/21/1954 34,200 9.37 2017 06/14/2017 32,500 9.37 2In cases where the month, day, or both are not present in the date of a peak flow (as indicated by adjacent slash marks with no intervening values), 1955 06/16/1955 27,500 8.19 1986 05/31/1986 32,300 9.27 the month, day, or both are unknown. 1956 05/25/1956 44,600 10.75 1979 05/27/1979 31,700 9.14 3Flood-frequency results not reported because of too many values less than the PILF threshold used in the at-site analysis. 1957 05/21/1957 31,500 8.88 1957 05/21/1957 31,500 8.88 4Definitions of peak-flow designations used in analysis include: 1958 05/23/1958 32,900 9.34 2006 05/21/2006 31,500 9.21 PT definition: The peak flow is used to define perception thresholds in ungaged historical periods; 1959 06/07/1959 35,500 9.50 1999 06/04/1999 31,300 9.25 Opportunistic: The peak flow was excluded from the analysis because it is outside of the systematic record and was of insufficient magnitude to 1960 06/05/1960 27,100 8.14 1978 06/09/1978 30,700 9.10 determine nonexceedance during an ungaged period; 1961 05/28/1961 29,300 8.51 1942 05/27/1942 30,500 8.96 PILF: The peak flow was identified as a potentially influential low flow; Synthesized: The peak flow was synthesized using Maintenance of Variance Extension Type III record extension. 1962 05/25/1962 25,800 7.84 1961 05/28/1961 29,300 8.51 England, J.F., Jr., Cohn, T.A., Faber, B.A., Stedinger, J.R., Thomas, W.O., Jr., Veilleux, A.G., Kiang, J.E., and Mason, R.R., Jr., 2019, Guidelines 1963 06/06/1963 24,600 7.54 1984 06/01/1984 28,800 8.72 for determining flood flow frequency—Bulletin 17C (ver. 1.1, May 2019): U.S. Geological Survey Techniques and Methods, book 4, chap. B5, 148 1964 06/10/1964 50,100 11.45 1969 05/21/1969 28,700 8.45 p., https://doi.org/10.3133/tm4B5. 1965 06/13/1965 33,500 9.38 1983 05/30/1983 28,500 8.67 1966 05/11/1966 16,800 6.31 PILF 1914 //1914 27,900 -- Synthesized 1967 06/08/1967 39,900 10.09 1955 06/16/1955 27,500 8.19 Helsel,D.R.,andHirsch,R.M.,2002,Statisticalmethodsinwaterresources:U.S.GeologicalSur-veyTechniquesofWater- 1968 06/13/1968 24,700 7.79 2002 05/31/2002 27,500 8.56 ResourcesInvestigations,book4,chap.A3,522p.,accessedOctober30,2017,athttps://pubs.usgs.gov/twri/twri4a3/. 1969 05/21/1969 28,700 8.45 1980 05/26/1980 27,400 8.41 1970 06/07/1970 36,100 9.54 1991 06/08/1991 27,200 8.46 1971 05/14/1971 36,200 9.53 1952 05/16/1952 27,100 8.17 1972 06/03/1972 52,200 11.71 1960 06/05/1960 27,100 8.14 1973 05/20/1973 15,000 6.37 PILF 1911 //1911 26,900 -- Synthesized 1974 06/18/1974 47,900 11.50 1936 05/16/1936 26,900 8.34 1975 06/21/1975 49,200 11.67 2013 05/15/2013 26,900 8.42 1976 05/15/1976 43,600 10.96 1923 //1923 26,300 -- Synthesized 1977 06/08/1977 9,100 4.76 PILF 1989 05/11/1989 26,300 8.32 1978 06/09/1978 30,700 9.10 1929 //1929 25,900 -- Synthesized 1979 05/27/1979 31,700 9.14 1932 05/23/1932 25,800 8.12 1980 05/26/1980 27,400 8.41 1962 05/25/1962 25,800 7.84 1981 05/24/1981 42,900 10.61 1934 04/26/1934 25,700 8.00 1982 06/18/1982 41,000 10.37 1920 //1920 25,500 -- Synthesized 1983 05/30/1983 28,500 8.67 1995 06/07/1995 25,500 8.17 1984 06/01/1984 28,800 8.72 1968 06/13/1968 24,700 7.79 1985 05/25/1985 21,100 7.38 1963 06/06/1963 24,600 7.54 1986 05/31/1986 32,300 9.27 2010 06/06/2010 23,500 7.87 1987 05/01/1987 15,800 6.28 PILF 1993 05/16/1993 23,400 7.80 1988 06/05/1988 14,300 5.94 PILF 1919 //1919 23,300 -- Synthesized 1989 05/11/1989 26,300 8.32 1990 05/31/1990 22,200 7.58 1990 05/31/1990 22,200 7.58 1939 05/05/1939 22,000 7.29 1991 06/08/1991 27,200 8.46 1985 05/25/1985 21,100 7.38 1992 05/01/1992 12,400 5.45 PILF 1998 06/20/1998 21,000 7.53 1993 05/16/1993 23,400 7.80 1935 05/25/1935 20,200 6.80 PILF 1994 04/25/1994 16,900 6.52 PILF 1946 05/29/1946 19,700 6.72 PILF 1995 06/07/1995 25,500 8.17 1945 06/02/1945 19,100 6.66 PILF 1996 06/10/1996 38,200 10.12 2007 05/14/2007 19,100 7.01 PILF 1997 05/18/1997 55,100 12.18 2005 05/20/2005 19,000 6.69 PILF 1998 06/20/1998 21,000 7.53 2016 05/09/2016 17,600 6.59 PILF 1999 06/04/1999 31,300 9.25 1930 04/26/1930 17,500 6.32 PILF 2000 05/23/2000 13,500 5.87 PILF 1905 //1905 17,400 -- PILF; synthesi 2001 05/16/2001 15,900 6.28 PILF 1994 04/25/1994 16,900 6.52 PILF 2002 05/31/2002 27,500 8.56 1966 05/11/1966 16,800 6.31 PILF 2003 06/01/2003 38,700 10.24 1906 //1906 16,100 -- PILF; synthesi 2004 06/07/2004 15,600 6.25 PILF 2001 05/16/2001 15,900 6.28 PILF 2005 05/20/2005 19,000 6.69 PILF 1987 05/01/1987 15,800 6.28 PILF 2006 05/21/2006 31,500 9.21 2004 06/07/2004 15,600 6.25 PILF 2007 05/14/2007 19,100 7.01 PILF 2015 06/03/2015 15,300 6.05 PILF 2008 05/21/2008 36,400 9.93 1973 05/20/1973 15,000 6.37 PILF 2009 06/01/2009 35,400 9.79 1944 06/18/1944 14,400 5.58 PILF 2010 06/06/2010 23,500 7.87 1988 06/05/1988 14,300 5.94 PILF 2011 06/09/2011 47,000 11.33 1940 05/13/1940 14,200 5.56 PILF 2012 04/28/2012 34,900 9.71 1915 //1915 13,700 -- PILF; synthesi 2013 05/15/2013 26,900 8.42 2000 05/23/2000 13,500 5.87 PILF 2014 05/26/2014 33,500 9.52 1992 05/01/1992 12,400 5.45 PILF 2015 06/03/2015 15,300 6.05 PILF 1931 05/18/1931 12,200 5.08 PILF 12354500 Clark Fork at St. Regis, Montana 12354500 Clark Fork at St. Regis, Montana Analysis for unregulated period of record Analysis for unregulated period of record Analysis period of record, water years: 1911–23; 1929–2018 Analysis period of record, water years: 1911–23; 1929–2018 At-site peak-flow frequency analysis conducted on recorded data At-site peak-flow frequency analysis conducted on recorded data Table 1‐1 Table 1‐2 Table 1‐3 Table 1‐4 Table 1‐5 Table 1‐6 Table 1‐7 Table 1‐8 Table 1‐1 Table 1‐2 Table 1‐3 Table 1‐4 Table 1‐5 Table 1‐6 Table 1‐7 Table 1‐8 [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. PILF; potentially influential low flow; MGBT, multiple Grubbs-Beck test] [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends; --, not applicable or not available.] Peak-flow data Ranked (largest to smallest) peak-flow data Number of Number of Total number Contributing PILF Type of peak‐ recorded Skew type peak flows of years in Type of PILF flow Peak flow, in Gage Peak-flow Peak flow, in Gage Peak-flow drainage area, threshold, in Water 2 Water 2 peak flows used in 1 used for ungaged cubic feet per height, designation in cubic feet per height, designation in in square threshold cubic feet per frequency Date Date used in the analysis perception historical year 4 year 4 miles second second in feet analysis second in feet analysis analysis thresholds period(s) analysis 1911 06/14/1911 33,800 13.90 1948 05/24/1948 68,900 19.96 Historic 10,728 103 Weighted MGBT 27,400 At-site 1912 06/12/1912 41,400 15.50 1997 05/18/1997 68,900 20.27 Peak flow, in cubic feet per second, for indicated annual exceedance probability (bold values), in percent 1913 05/30/1913 63,500 19.20 1972 06/03/1972 63,900 19.50 66.7 50 42.9 20 10 4 2 1.0 0.5 0.2 1914 05/15/1914 35,000 14.20 1913 05/30/1913 63,500 19.20 30,700 36,200 38,600 48,400 55,300 63,100 68,300 73,000 77,300 82,500 1915 06/21/1915 17,300 10.00 PILF 1974 06/18/1974 63,100 19.39 Upper and lower 90-percent confidence intervals, in cubic feet per second, for indicated annual exceedance probability, in percent 1916 06/20/1916 53,000 17.50 1956 05/24/1956 62,000 18.98 66.7 50 42.9 20 10 4 2 1.0 0.5 0.2 1917 05/29/1917 59,100 18.50 1964 06/10/1964 60,900 18.54 33,200 39,000 41,600 52,300 60,500 70,500 78,100 85,800 93,700 105,000 1918 06/14/1918 48,400 16.70 2018 05/12/2018 60,500 18.90 27,000 33,200 35,700 44,900 51,200 58,000 62,100 65,500 68,300 71,500 1919 05/31/1919 29,300 13.00 1917 05/29/1917 59,100 18.50 1920 05/18/1920 32,100 13.60 1975 06/21/1975 55,900 18.21 1921 05/28/1921 41,800 15.50 2011 06/10/2011 55,500 18.16 1922 06/08/1922 47,200 16.50 1976 05/16/1976 55,000 18.07 1923 06/12/1923 33,100 13.80 1947 05/10/1947 54,600 17.91 1929 05/26/1929 32,600 13.70 1916 06/20/1916 53,000 17.50 1930 04/26/1930 22,200 11.30 PILF 1950 06/19/1950 51,200 17.39 1931 05/17/1931 15,900 9.55 PILF 1933 06/11/1933 50,000 16.85 1932 05/23/1932 32,700 13.70 1971 05/14/1971 49,200 16.92 1933 06/11/1933 50,000 16.85 1954 05/22/1954 49,000 16.85 1934 04/26/1934 35,700 14.27 1967 06/08/1967 48,800 16.98 1935 05/25/1935 27,400 12.36 1982 06/18/1982 48,600 17.09 1936 05/17/1936 34,000 14.16 1918 06/14/1918 48,400 16.70 1937 05/28/1937 17,100 10.06 PILF 1981 05/25/1981 48,000 17.00 1938 05/31/1938 46,400 16.00 1949 05/17/1949 47,900 16.84 1939 05/05/1939 26,100 12.81 PILF 1922 06/08/1922 47,200 16.50 1940 05/13/1940 17,200 10.38 PILF 1970 06/07/1970 46,900 16.66 1941 06/03/1941 12,000 8.82 PILF 1996 06/11/1996 46,800 16.57 1942 05/28/1942 35,800 14.22 1938 05/31/1938 46,400 16.00 1943 06/21/1943 41,300 15.69 2008 05/21/2008 46,300 16.72 1944 06/18/1944 16,000 9.94 PILF 1953 06/05/1953 45,800 16.48 1945 06/02/1945 23,900 12.05 PILF 1958 05/24/1958 44,800 16.30 1946 05/30/1946 23,900 12.03 PILF 2003 06/02/2003 44,300 16.39 1947 05/10/1947 54,600 17.91 1959 06/08/1959 43,000 15.98 1948 05/24/1948 68,900 19.96 Historic 2009 06/01/2009 42,700 16.13 1949 05/17/1949 47,900 16.84 2012 04/28/2012 42,700 16.13 1950 06/19/1950 51,200 17.39 1921 05/28/1921 41,800 15.50 1951 05/26/1951 41,700 15.76 1951 05/26/1951 41,700 15.76 1952 05/17/1952 34,400 14.32 1912 06/12/1912 41,400 15.50 1953 06/05/1953 45,800 16.48 1943 06/21/1943 41,300 15.69 1954 05/22/1954 49,000 16.85 2014 05/26/2014 41,300 15.88 1955 06/14/1955 36,400 14.73 1965 06/20/1965 40,400 15.54 1956 05/24/1956 62,000 18.98 1999 05/28/1999 39,800 15.61 1957 05/21/1957 39,300 15.30 1961 05/28/1961 39,600 15.39 1958 05/24/1958 44,800 16.30 2006 05/21/2006 39,600 15.59 1959 06/08/1959 43,000 15.98 1957 05/21/1957 39,300 15.30 1960 06/05/1960 33,600 14.28 1986 06/01/1986 38,800 15.35 1961 05/28/1961 39,600 15.39 2002 06/01/2002 38,700 15.43 1962 05/26/1962 33,500 14.25 1979 05/28/1979 38,600 15.37 Figure 1. Annual peak flows (probability plotting positions) and peak-flow frequency curve. EMA, Expected Moments Algorithm; Skew(G), analysis 1963 06/06/1963 28,600 13.28 1978 06/09/1978 37,400 15.12 skew; PILF(LO), potentially Influencial low flow (low outlier). 1964 06/10/1964 60,900 18.54 1969 05/22/1969 36,600 14.84 1965 06/20/1965 40,400 15.54 1983 05/31/1983 36,600 14.94 1966 05/09/1966 24,700 12.46 PILF 1955 06/14/1955 36,400 14.73 1967 06/08/1967 48,800 16.98 1942 05/28/1942 35,800 14.22 1968 06/04/1968 30,200 13.46 1934 04/26/1934 35,700 14.27 1969 05/22/1969 36,600 14.84 1914 05/15/1914 35,000 14.20 1970 06/07/1970 46,900 16.66 1984 06/01/1984 34,900 14.60 1971 05/14/1971 49,200 16.92 2017 06/03/2017 34,900 14.72 1972 06/03/1972 63,900 19.50 1991 05/20/1991 34,800 14.57 1973 05/20/1973 18,500 10.98 PILF 1952 05/17/1952 34,400 14.32 1974 06/18/1974 63,100 19.39 1980 05/27/1980 34,400 14.50 1975 06/21/1975 55,900 18.21 1936 05/17/1936 34,000 14.16 1976 05/16/1976 55,000 18.07 1911 06/14/1911 33,800 13.90 1977 06/08/1977 11,300 8.94 PILF 1960 06/05/1960 33,600 14.28 1978 06/09/1978 37,400 15.12 1962 05/26/1962 33,500 14.25 1979 05/28/1979 38,600 15.37 1923 06/12/1923 33,100 13.80 1980 05/27/1980 34,400 14.50 2013 05/15/2013 33,100 14.39 1981 05/25/1981 48,000 17.00 1989 05/12/1989 32,900 14.19 1982 06/18/1982 48,600 17.09 1932 05/23/1932 32,700 13.70 1983 05/31/1983 36,600 14.94 1929 05/26/1929 32,600 13.70 1984 06/01/1984 34,900 14.60 1920 05/18/1920 32,100 13.60 1985 05/26/1985 28,200 13.28 1993 05/17/1993 30,700 13.82 1986 06/01/1986 38,800 15.35 1968 06/04/1968 30,200 13.46 1987 05/01/1987 23,000 12.19 PILF 1995 06/07/1995 29,600 13.53 1988 05/26/1988 17,600 10.88 PILF 1919 05/31/1919 29,300 13.00 1989 05/12/1989 32,900 14.19 1963 06/06/1963 28,600 13.28 1990 06/01/1990 27,800 13.19 1985 05/26/1985 28,200 13.28 1991 05/20/1991 34,800 14.57 1990 06/01/1990 27,800 13.19 1992 05/09/1992 16,100 10.50 PILF 2010 06/06/2010 27,500 13.25 1993 05/17/1993 30,700 13.82 1935 05/25/1935 27,400 12.36 1994 04/26/1994 20,800 11.66 PILF 1939 05/05/1939 26,100 12.81 PILF 1995 06/07/1995 29,600 13.53 1966 05/09/1966 24,700 12.46 PILF Figure 2. Annual peak flows and perception thresholds. 1996 06/11/1996 46,800 16.57 1998 06/21/1998 24,100 12.50 PILF 1997 05/18/1997 68,900 20.27 1945 06/02/1945 23,900 12.05 PILF 1998 06/21/1998 24,100 12.50 PILF 1946 05/30/1946 23,900 12.03 PILF 1999 05/28/1999 39,800 15.61 2007 05/14/2007 23,600 12.39 PILF 2000 05/24/2000 18,700 11.18 PILF 1987 05/01/1987 23,000 12.19 PILF 1Definitions of types of PILF thresholds include: 2001 05/16/2001 19,900 11.47 PILF 2005 05/21/2005 22,700 12.17 PILF MGBT: PILF threshold calculated by using the multiple Grubbs-Beck Test as specifed in Bulletin 17C (England and others, 2016); 2002 06/01/2002 38,700 15.43 1930 04/26/1930 22,200 11.30 PILF Manual: PILF threshold based on a systematic peak flow selected by the peak-flow frequency analyst. 2003 06/02/2003 44,300 16.39 2016 05/10/2016 21,100 11.80 PILF 2004 06/07/2004 19,600 11.42 PILF 1994 04/26/1994 20,800 11.66 PILF 2Peak-flow data with a value of zero are not plotted in figures 1 or 2. 2005 05/21/2005 22,700 12.17 PILF 2001 05/16/2001 19,900 11.47 PILF 3In cases where the month, day, or both are not present in the date of a peak flow (as indicated by adjacent slash marks with no intervening values), 2006 05/21/2006 39,600 15.59 2004 06/07/2004 19,600 11.42 PILF the month, day, or both are unknown. 2007 05/14/2007 23,600 12.39 PILF 2000 05/24/2000 18,700 11.18 PILF 4Definitions of peak-flow designations used in analysis include: 2008 05/21/2008 46,300 16.72 1973 05/20/1973 18,500 10.98 PILF PT definition: The peak flow is used to define perception thresholds in ungaged historical periods; 2009 06/01/2009 42,700 16.13 1988 05/26/1988 17,600 10.88 PILF Opportunistic: The peak flow was excluded from the analysis because it is outside of the systematic record and was of insufficient magnitude to 2010 06/06/2010 27,500 13.25 1915 06/21/1915 17,300 10.00 PILF determine nonexceedance during an ungaged period; 2011 06/10/2011 55,500 18.16 2015 06/03/2015 17,300 10.85 PILF PILF: The peak flow was identified as a potentially influential low flow; Synthesized: The peak flow was synthesized using Maintenance of Variance Extension Type III record extension. 2012 04/28/2012 42,700 16.13 1940 05/13/1940 17,200 10.38 PILF England, J.F., Jr., Cohn, T.A., Faber, B.A., Stedinger, J.R., Thomas, W.O., Jr., Veilleux, A.G., Kiang, J.E., and Mason, R.R., Jr., 2019, Guidelines 2013 05/15/2013 33,100 14.39 1937 05/28/1937 17,100 10.06 PILF for determining flood flow frequency—Bulletin 17C (ver. 1.1, May 2019): U.S. Geological Survey Techniques and Methods, book 4, chap. B5, 148 2014 05/26/2014 41,300 15.88 1992 05/09/1992 16,100 10.50 PILF p., https://doi.org/10.3133/tm4B5. 2015 06/03/2015 17,300 10.85 PILF 1944 06/18/1944 16,000 9.94 PILF 2016 05/10/2016 21,100 11.80 PILF 1931 05/17/1931 15,900 9.55 PILF 2017 06/03/2017 34,900 14.72 1941 06/03/1941 12,000 8.82 PILF 2018 05/12/2018 60,500 18.90 1977 06/08/1977 11,300 8.94 PILF 12354500 Clark Fork at St. Regis, Montana 12354500 Clark Fork at St. Regis, Montana Analysis for unregulated period of record Analysis for unregulated period of record Analysis period of record, water years: 1899–1908; 1911–23; 1929–2018 Analysis period of record, water years: 1899–1908; 1911–23; 1929–2018 Peak-flow frequency analysis conducted on recorded and synthesized data Peak-flow frequency analysis conducted on recorded and synthesized data Table 1‐1 Table 1‐2 Table 1‐3 Table 1‐4 Table 1‐5 Table 1‐6 Table 1‐7 Table 1‐8 Table 1‐1 Table 1‐2 Table 1‐3 Table 1‐4 Table 1‐5 Table 1‐6 Table 1‐7 Table 1‐8 [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. PILF; potentially influential low flow; MGBT, multiple Grubbs-Beck test] [Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends; --, not applicable or not available.] Peak-flow data Ranked (largest to smallest) peak-flow data Contributing Type of peak‐ Peak flow, in Gage Peak-flow Peak flow, in Gage Peak-flow drainage area, flow Water Water 2 cubic feet per height, designation in 2 cubic feet per height, designation in in square frequency year Date year Date second in feet 4 second in feet 4 miles analysis analysis analysis 1899 //1899 76,500 -- Synthesized 1908 //1908 97,100 -- Synthesized 10,728 MOVE.3 1900 //1900 56,500 -- Synthesized 1899 //1899 76,500 -- Synthesized Peak flow, in cubic feet per second, for indicated annual exceedance probability (bold values), in percent 1901 //1901 46,000 -- Synthesized 1948 05/24/1948 68,900 19.96 Historic 66.7 50 42.9 20 10 4 2 1.0 0.5 0.2 1902 //1902 67,300 -- Synthesized 1997 05/18/1997 68,900 20.27 30,500 36,700 39,500 50,800 59,000 68,300 74,500 80,200 85,500 91,900 1903 //1903 55,300 -- Synthesized 1902 //1902 67,300 -- Synthesized Upper and lower 90-percent confidence intervals, in cubic feet per second, for indicated annual exceedance probability, in percent 1904 //1904 41,300 -- Synthesized 1972 06/03/1972 63,900 19.50 66.7 50 42.9 20 10 4 2 1.0 0.5 0.2 1905 //1905 21,800 -- PILF; synthesized 1913 05/30/1913 63,500 19.20 33,300 39,900 42,900 55,300 64,900 77,000 86,100 95,600 105,000 120,000 1906 //1906 20,100 -- PILF; synthesized 1974 06/18/1974 63,100 19.39 26,700 33,500 36,300 46,800 54,300 62,400 67,300 71,300 74,700 78,300 1907 //1907 42,700 -- Synthesized 1956 05/24/1956 62,000 18.98 1908 //1908 97,100 -- Synthesized 1964 06/10/1964 60,900 18.54 1911 06/14/1911 33,800 13.90 2018 05/12/2018 60,500 18.90 1,000,000 EXPLANATION 1912 06/12/1912 41,400 15.50 1917 05/29/1917 59,100 18.50 1913 05/30/1913 63,500 19.20 1900 //1900 56,500 -- Synthesized Peaks used in at-site analysis 1914 05/15/1914 35,000 14.20 1975 06/21/1975 55,900 18.21 1915 06/21/1915 17,300 10.00 PILF 2011 06/10/2011 55,500 18.16 Potentially influential low flow (PILF) 1916 06/20/1916 53,000 17.50 1903 //1903 55,300 -- Synthesized 1917 05/29/1917 59,100 18.50 1976 05/16/1976 55,000 18.07 Weighted peak-flow frequency 1918 06/14/1918 48,400 16.70 1947 05/10/1947 54,600 17.91 curve 1919 05/31/1919 29,300 13.00 1916 06/20/1916 53,000 17.50 Upper and lower 95-percent 1920 05/18/1920 32,100 13.60 1950 06/19/1950 51,200 17.39 confidence intervals 1921 05/28/1921 41,800 15.50 1933 06/11/1933 50,000 16.85 1922 06/08/1922 47,200 16.50 1971 05/14/1971 49,200 16.92 1923 06/12/1923 33,100 13.80 1954 05/22/1954 49,000 16.85 1929 05/26/1929 32,600 13.70 1967 06/08/1967 48,800 16.98 1930 04/26/1930 22,200 11.30 PILF 1982 06/18/1982 48,600 17.09 100,000 1931 05/17/1931 15,900 9.55 PILF 1918 06/14/1918 48,400 16.70 1932 05/23/1932 32,700 13.70 1981 05/25/1981 48,000 17.00

Flow, in cubicfeetsecond per 1933 06/11/1933 50,000 16.85 1949 05/17/1949 47,900 16.84 1934 04/26/1934 35,700 14.27 1922 06/08/1922 47,200 16.50 1935 05/25/1935 27,400 12.36 1970 06/07/1970 46,900 16.66 1936 05/17/1936 34,000 14.16 1996 06/11/1996 46,800 16.57 1937 05/28/1937 17,100 10.06 PILF 1938 05/31/1938 46,400 16.00 1938 05/31/1938 46,400 16.00 2008 05/21/2008 46,300 16.72 1939 05/05/1939 26,100 12.81 1901 //1901 46,000 -- Synthesized 1940 05/13/1940 17,200 10.38 PILF 1953 06/05/1953 45,800 16.48 1941 06/03/1941 12,000 8.82 PILF 1958 05/24/1958 44,800 16.30 1942 05/28/1942 35,800 14.22 2003 06/02/2003 44,300 16.39 1943 06/21/1943 41,300 15.69 1959 06/08/1959 43,000 15.98 1944 06/18/1944 16,000 9.94 PILF 1907 //1907 42,700 -- Synthesized 10,000

99 98 95 90 80 70 60 50 40 30 20 10 5 2 1 1945 06/02/1945 23,900 12.05 PILF 2009 06/01/2009 42,700 16.13 99.5 0.5 0.2 1946 05/30/1946 23,900 12.03 PILF 2012 04/28/2012 42,700 16.13 Exceedance probability, in percent 1947 05/10/1947 54,600 17.91 1921 05/28/1921 41,800 15.50 1948 05/24/1948 68,900 19.96 Historic 1951 05/26/1951 41,700 15.76 Figure 1. Annual peak flows (plotting positions determined using the Cunnane formulation; Helsel and HIrsch, 2002) and peak-flow frequencycurve 1949 05/17/1949 47,900 16.84 1912 06/12/1912 41,400 15.50 1950 06/19/1950 51,200 17.39 1904 //1904 41,300 -- Synthesized 1951 05/26/1951 41,700 15.76 1943 06/21/1943 41,300 15.69 1952 05/17/1952 34,400 14.32 2014 05/26/2014 41,300 15.88 1953 06/05/1953 45,800 16.48 1965 06/20/1965 40,400 15.54 1Peak flows with a value of zero are not plotted in figure 1 . 1954 05/22/1954 49,000 16.85 1999 05/28/1999 39,800 15.61 2In cases where the month, day, or both are not present in the date of a peak flow (as indicated by adjacent slash marks with no intervening values), 1955 06/14/1955 36,400 14.73 1961 05/28/1961 39,600 15.39 the month, day, or both are unknown. 1956 05/24/1956 62,000 18.98 2006 05/21/2006 39,600 15.59 3Flood-frequency results not reported because of too many values less than the PILF threshold used in the at-site analysis. 1957 05/21/1957 39,300 15.30 1957 05/21/1957 39,300 15.30 4Definitions of peak-flow designations used in analysis include: 1958 05/24/1958 44,800 16.30 1986 06/01/1986 38,800 15.35 PT definition: The peak flow is used to define perception thresholds in ungaged historical periods; 1959 06/08/1959 43,000 15.98 2002 06/01/2002 38,700 15.43 Opportunistic: The peak flow was excluded from the analysis because it is outside of the systematic record and was of insufficient magnitude to 1960 06/05/1960 33,600 14.28 1979 05/28/1979 38,600 15.37 determine nonexceedance during an ungaged period; 1961 05/28/1961 39,600 15.39 1978 06/09/1978 37,400 15.12 PILF: The peak flow was identified as a potentially influential low flow; Synthesized: The peak flow was synthesized using Maintenance of Variance Extension Type III record extension. 1962 05/26/1962 33,500 14.25 1969 05/22/1969 36,600 14.84 England, J.F., Jr., Cohn, T.A., Faber, B.A., Stedinger, J.R., Thomas, W.O., Jr., Veilleux, A.G., Kiang, J.E., and Mason, R.R., Jr., 2019, Guidelines 1963 06/06/1963 28,600 13.28 1983 05/31/1983 36,600 14.94 for determining flood flow frequency—Bulletin 17C (ver. 1.1, May 2019): U.S. Geological Survey Techniques and Methods, book 4, chap. B5, 148 1964 06/10/1964 60,900 18.54 1955 06/14/1955 36,400 14.73 p., https://doi.org/10.3133/tm4B5. 1965 06/20/1965 40,400 15.54 1942 05/28/1942 35,800 14.22 1966 05/09/1966 24,700 12.46 PILF 1934 04/26/1934 35,700 14.27 1967 06/08/1967 48,800 16.98 1914 05/15/1914 35,000 14.20 Helsel,D.R.,andHirsch,R.M.,2002,Statisticalmethodsinwaterresources:U.S.GeologicalSur-veyTechniquesofWater- 1968 06/04/1968 30,200 13.46 1984 06/01/1984 34,900 14.60 ResourcesInvestigations,book4,chap.A3,522p.,accessedOctober30,2017,athttps://pubs.usgs.gov/twri/twri4a3/. 1969 05/22/1969 36,600 14.84 2017 06/03/2017 34,900 14.72 1970 06/07/1970 46,900 16.66 1991 05/20/1991 34,800 14.57 1971 05/14/1971 49,200 16.92 1952 05/17/1952 34,400 14.32 1972 06/03/1972 63,900 19.50 1980 05/27/1980 34,400 14.50 1973 05/20/1973 18,500 10.98 PILF 1936 05/17/1936 34,000 14.16 1974 06/18/1974 63,100 19.39 1911 06/14/1911 33,800 13.90 1975 06/21/1975 55,900 18.21 1960 06/05/1960 33,600 14.28 1976 05/16/1976 55,000 18.07 1962 05/26/1962 33,500 14.25 1977 06/08/1977 11,300 8.94 PILF 1923 06/12/1923 33,100 13.80 1978 06/09/1978 37,400 15.12 2013 05/15/2013 33,100 14.39 1979 05/28/1979 38,600 15.37 1989 05/12/1989 32,900 14.19 1980 05/27/1980 34,400 14.50 1932 05/23/1932 32,700 13.70 1981 05/25/1981 48,000 17.00 1929 05/26/1929 32,600 13.70 1982 06/18/1982 48,600 17.09 1920 05/18/1920 32,100 13.60 1983 05/31/1983 36,600 14.94 1993 05/17/1993 30,700 13.82 1984 06/01/1984 34,900 14.60 1968 06/04/1968 30,200 13.46 1985 05/26/1985 28,200 13.28 1995 06/07/1995 29,600 13.53 1986 06/01/1986 38,800 15.35 1919 05/31/1919 29,300 13.00 1987 05/01/1987 23,000 12.19 PILF 1963 06/06/1963 28,600 13.28 1988 05/26/1988 17,600 10.88 PILF 1985 05/26/1985 28,200 13.28 1989 05/12/1989 32,900 14.19 1990 06/01/1990 27,800 13.19 1990 06/01/1990 27,800 13.19 2010 06/06/2010 27,500 13.25 1991 05/20/1991 34,800 14.57 1935 05/25/1935 27,400 12.36 1992 05/09/1992 16,100 10.50 PILF 1939 05/05/1939 26,100 12.81 1993 05/17/1993 30,700 13.82 1966 05/09/1966 24,700 12.46 PILF 1994 04/26/1994 20,800 11.66 PILF 1998 06/21/1998 24,100 12.50 PILF 1995 06/07/1995 29,600 13.53 1945 06/02/1945 23,900 12.05 PILF 1996 06/11/1996 46,800 16.57 1946 05/30/1946 23,900 12.03 PILF 1997 05/18/1997 68,900 20.27 2007 05/14/2007 23,600 12.39 PILF 1998 06/21/1998 24,100 12.50 PILF 1987 05/01/1987 23,000 12.19 PILF 1999 05/28/1999 39,800 15.61 2005 05/21/2005 22,700 12.17 PILF 2000 05/24/2000 18,700 11.18 PILF 1930 04/26/1930 22,200 11.30 PILF 2001 05/16/2001 19,900 11.47 PILF 1905 //1905 21,800 -- PILF; synthesi 2002 06/01/2002 38,700 15.43 2016 05/10/2016 21,100 11.80 PILF 2003 06/02/2003 44,300 16.39 1994 04/26/1994 20,800 11.66 PILF 2004 06/07/2004 19,600 11.42 PILF 1906 //1906 20,100 -- PILF; synthesi 2005 05/21/2005 22,700 12.17 PILF 2001 05/16/2001 19,900 11.47 PILF 2006 05/21/2006 39,600 15.59 2004 06/07/2004 19,600 11.42 PILF 2007 05/14/2007 23,600 12.39 PILF 2000 05/24/2000 18,700 11.18 PILF 2008 05/21/2008 46,300 16.72 1973 05/20/1973 18,500 10.98 PILF 2009 06/01/2009 42,700 16.13 1988 05/26/1988 17,600 10.88 PILF 2010 06/06/2010 27,500 13.25 1915 06/21/1915 17,300 10.00 PILF 2011 06/10/2011 55,500 18.16 2015 06/03/2015 17,300 10.85 PILF 2012 04/28/2012 42,700 16.13 1940 05/13/1940 17,200 10.38 PILF 2013 05/15/2013 33,100 14.39 1937 05/28/1937 17,100 10.06 PILF 2014 05/26/2014 41,300 15.88 1992 05/09/1992 16,100 10.50 PILF 2015 06/03/2015 17,300 10.85 PILF 1944 06/18/1944 16,000 9.94 PILF

Appendix B. Ungaged Flow Node Flood Frequency Calculations

Clark Fork Log Interpolation Calculations Log Interpolation of Extended Record Gage Analysis Estimated Discharge (cfs) Cumulative 50% Annual Chance 10% Annual Chance 4% Annual Chance 2% Annual Chance 1% Annual Chance 0.2% Annual Chance 1% + Annual Chance Node/USGS Station ID Location Description Basin Area (mi2) 2‐year 10‐year 25‐year 50‐year 100‐year 500‐year 100‐year + 12324680* Clark Fork at Goldcreek MT 1,789 2,730 6,260 8,300 9,900 11,600 15,600 17,200 2700 Perkins Creek 1,875 2,800 6,410 8,540 10,200 12,000 16,400 18,200 2600 Lower Flint Creek 1,900 2,820 6,450 8,610 10,300 12,200 16,700 18,500 12331600 Clark Fork at Drummond MT 2,383 3,190 7,220 9,880 12,100 14,600 21,300 24,400 2400 Rattler Gulch 2,434 3,220 7,300 10,000 12,200 14,800 21,800 25,100 2300 Mulkey Gulch 2,455 3,240 7,330 10,100 12,300 14,900 22,000 25,400 12331800* Clark Fork near Drummond MT 2,513 3,280 7,420 10,200 12,500 15,200 22,600 26,100 2100 Harvey Creek 2,537 3,330 7,510 10,300 12,600 15,300 22,700 26,200 2000 Tyler Creek 2,589 3,450 7,710 10,500 12,800 15,600 22,900 26,300 1900 Dry Gulch 2,608 3,500 7,780 10,600 12,900 15,600 23,000 26,300 12331900 Clark Fork near Clinton MT 2,646 3,580 7,930 10,800 13,100 15,800 23,200 26,400 1700 Rock Creek‐Kitchen Gulch 2,682 3,670 8,060 10,900 13,300 16,000 23,300 26,500 1600 Schwartz Creek 3,580 6,020 11,700 14,800 17,300 20,000 26,700 28,400 1500 Wallace Creek 3,633 6,180 11,900 15,100 17,600 20,300 26,900 28,400 12334550* Clark Fork at Turah Bridge near Bonner MT 3,661 6,260 12,000 15,200 17,700 20,400 27,000 28,500 1300 Clark Fork upstream of Blackfoot River 3,691 6,350 12,200 15,400 17,900 20,600 27,300 28,700 12340500* Clark Fork above Missoula MT 6,013 15,000 26,600 32,100 36,100 39,900 48,400 46,600 1200 Lower Rattlesnake Creek 6,018 15,000 26,600 32,100 36,100 39,900 48,400 46,600 1100 Grant Creek 6,087 15,300 27,100 32,600 36,700 40,500 49,000 47,200 1000 Clark Fork upstream of Bitteroot River 6,149 15,600 27,500 33,100 37,100 41,000 49,600 47,800 12353000* Clark Fork below Missoula MT 9,007 29,300 47,200 54,900 60,200 65,000 75,200 73,500 800 Deep Creek 9,052 29,500 47,500 55,200 60,600 65,400 75,600 73,900 700 Rock Creek 9,123 29,800 48,000 55,800 61,200 66,000 76,300 74,600 600 Mill Creek 9,165 30,000 48,300 56,100 61,500 66,400 76,700 75,000 500 Roman Creek 9,210 30,200 48,600 56,500 61,900 66,800 77,200 75,400 400 SixmileCreek 9,238 30,300 48,800 56,700 62,100 67,000 77,400 75,700 300 Ninemile Creek 9,269 30,400 49,000 56,900 62,400 67,300 77,700 76,000 200 Petty Creek 9,463 31,200 50,300 58,400 64,000 69,000 79,600 77,900 100 Missoula‐Mineral County Boundary 9,548 31,600 50,900 59,100 64,700 69,800 80,400 78,700 12354500* Clark Fork at St. Regis MT 10,715 36,700 59,000 68,300 74,500 80,200 91,900 90,100

For locations that are ungaged and located between two gaged locations with reliable period of record (10 yrs) Equation recommended by SIR 2015‐5019‐F. Equation utilizes drainage areas and flows. * Values calculated with MOVE.3 extended record analysis. Bitterroot Mainstem Log Interpolation Calculations Log Interpolation of Extended Record Gage Analysis Estimated Discharge (cfs) Cumulative 50% Annual Chance 10% Annual Chance 4% Annual Chance 2% Annual Chance 1% Annual Chance 0.2% Annual Chance 1% + Annual Chance Node/USGS Station ID Location Description Basin Area (mi2) 2‐year 10‐year 25‐year 50‐year 100‐year 500‐year 100‐year + 12351200* Bitterroot River near Florence MT 2,340 14,100 17,800 20,100 23,000 25,100 32,000 32,400 400 Bitterroot River‐North Woodchuck Creek 2,414 14,200 18,500 21,000 23,900 26,100 32,800 33,100 300 Lower Lolo Creek 2,742 14,600 21,900 25,300 27,900 30,400 36,100 35,900 12352500* Bitterroot River near Missoula MT 2,821 14,700 22,700 26,400 28,900 31,400 36,900 36,600

For locations that are ungaged and located betweeno tw gaged locations with reliable period of record (10 yrs) Equation recommended by SIR 2015‐5019‐F. Equation utilizes drainage areas and flows. * Values calculated with MOVE.3 extended record analysis. Bitterroot River Mainstem Drainage Area Gage Transfer Calculations Drainage Area Gage Transfer Estimated Discharge (cfs) Cumulative Estimated Discharge (cfs) Node/USGS Station Basin Area 50% Annual Chance 10% Annual Chance 4% Annual Chance 2% Annual Chance 1% Annual Chance 0.2% Annual Chance 1% + Annual Chance 2 ID Location Description (mi ) DAU/DAG 2‐year 10‐year 25‐year 50‐year 100‐year 500‐year 100‐year + 400 Bitterroot River‐North Woodchuck Creek 2414 0.9 12,900 20,100 23,400 25,600 27,900 32,900 32,500 300 Lower Lolo Creek 2742 1.0 14,400 22,200 25,800 28,300 30,700 36,100 35,800 12352500* Bitterroot River near Missoula MT. 2821 ‐ 14,700 22,700 26,400 28,900 31,400 36,900 36,600 100 Bitterroot River at junction with Clark Fork River. 2857 1.0 15,200 23,500 27,300 29,800 32,400 38,000 37,800 Regression Coefficient expAEP = 0.843 0.794 0.777 0.766 0.755 0.735 0.755

For an ungaged site that is not located between two gaged locations. Recommended that the drainage ratio be within 0.5 to 1.5. Equation recommended by SIR 2015‐5019‐F. * Values calculated with MOVE.3 extended record analysis. **West Region regression coefficient. Rock Creek Mainstem Log Interpolation Calculations Log Interpolation of At‐Site and Extended Record Gaged Analysis Estimated Discharge (cfs) Cumulative Basin 50% Annual Chance 10% Annual Chance 4% Annual Chance 2% Annual Chance 1% Annual Chance 0.2% Annual Chance 1% + Annual Chance Node/USGS Station ID Location Description Area (mi2) 2‐year 10‐year 25‐year 50‐year 100‐year 500‐year 100‐year + 12332000* Middle Fork Rock Creek near Philipsburg, Montana 121 902 1,500 1,750 1,920 2,070 2,390 2,370 1200 Mallard Creek 390 1,940 3,310 3,920 4,340 4,720 5,570 5,520 1100 Upper Willow Creek 431 2,080 3,540 4,190 4,650 5,070 5,990 5,930 1000 Rock Creek‐Flat Gulch 543 2,420 4,140 4,920 5,460 5,960 7,070 7,010 900 Wyman Gulch 601 2,580 4,430 5,270 5,860 6,400 7,610 7,540 800 Hogback Creek 619 2,640 4,530 5,380 5,980 6,540 7,780 7,710 700 Rock Creek‐Hutsinpilar Creek 662 2,750 4,740 5,640 6,270 6,860 8,170 8,090 600 Rock Creek‐Wahlquist Creek 702 2,860 4,930 5,870 6,530 7,150 8,520 8,440 500 Welcome Creek 731 2,940 5,060 6,030 6,710 7,350 8,770 8,680 12333500 Rock Creek near Quigley MT 756 3,000 5,180 6,170 6,870 7,530 8,990 8,900 300 Brewster Creek 811 3,150 5,440 6,480 7,220 7,920 9,460 9,370 200 Gilbert Creek 854 3,250 5,630 6,710 7,480 8,210 9,810 9,720 12334510* Rock Creek near Clinton MT 888 3,340 5,780 6,900 7,690 8,440 10,100 10,000

For locations that are ungaged and located between two gaged locations with reliable period of record (10 yrs) Equation recommended by SIR 2015‐5019‐F. Equation utilizes drainage areas and flows. * Values calculated with MOVE.3 extended record analysis. Rock Creek Tribs Regression Calculations Southwest Region Regression Estimated Peak Flow (cfs) Cumulative Basin Area 50% Annual Chance 10% Annual Chance 4% Annual Chance 2% Annual Chance 1% Annual Chance 0.2% Annual Chance 1% + Node/USGS Station ID Location Description (mi2)F (%) P (in) 2‐year 10‐year 25‐year 50‐year 100‐year 500‐year 100‐year 900 Maukey Gulch at junction with West Fork Rock Creek 3 47.6 20.40 13 39 56 72 89 134 139 800 Ross Fork at junction with West Fork Rock Creek 85 80.2 28.87 355 723 908 1,060 1,230 1,590 1,920 700 Lower West Fork Ross Creek 93 87.0 29.36 374 749 935 1,090 1,250 1,620 1,950 600 West Fork Rock Creek at junction with Rock Creek 178 83.8 29.12 688 1,330 1,640 1,900 2,160 2,750 3,370 500 Middle Fork Rock Creek at junction with Rock Creek 203 78.5 28.54 786 1,520 1,880 2,180 2,490 3,170 3,880 400 Upper Upper Willow Creek 46 89.4 27.06 159 348 448 533 622 828 970 300 Middle Upper Willow Creek 76 81.0 24.98 229 509 657 784 917 1,230 1,430 200 Willow Creek at junction with Rock Creek 95 67.8 23.90 297 668 868 1,040 1,220 1,640 1,900 100 Ranch Creek at junction with Rock Creek 43 91.4 31.56 206 418 524 613 705 911 1,100

Equation recommended by SIR 2015‐5019‐F Cumulative Basin Areas Derived from ArcGIS Percent Forest and Annual Percipiation Derived from Streamstats