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 Page i of viii Missoula-Granite PMR Missoula-Granite Hydrologic Analysis 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 .........................................................................