Hydrology Design Report

Home , Swan Lake (Montana), Swan River (Montana)

MONTANA DEPARTMENT OF NATURAL RESOURCES AND CONSERVATION

Upstream of Cold Creek Rd. Bridge, 9/18/14

Swan River Detailed Floodplain Study Missoula County, MT

By the Montana Department of Natural Resources and Conservation May 2015

Hydrology Design Report, Swan River Detailed Floodplain Study May 2015

HYDROLOGY DESIGN REPORT

SWAN RIVER Missoula County, MT Table of Contents 1.0 INTRODUCTION ...... 1 1.1 LiDAR Collection ...... 1 1.2 Watershed Description ...... 1 1.3 Effective FIS (Flood Insurance Study) Hydrology ...... 2 1.4 Historic Data ...... 3 2.0 HYDROLOGIC ANALYSIS ...... 5 2.1 Stream Gage Analyses ...... 5 2.1.1 Systematic Estimation ...... 6 2.1.2 Two Station Comparison & MOVE.1 ...... 7 2.1.3 Regional Regression Equations ...... 8 2.1.4 Weighted Estimation ...... 9 2.1.5 Results Comparison ...... 9 2.2 Transfer to Ungaged Sites ...... 12 2.2.1 Streamflow Change Locations ...... 12 2.2.2 Log Interpolation between 2 Gages ...... 14 3.0 HYDROLOGIC ANALYSIS SUMMARY & SELECTED DISCHARGES ...... 15

List of Tables Table 1: Flathead & Lake County Effective Floodplain Studies ...... 2 Table 2: Peak Flows used in Effective Studies ...... 3 Table 3: Highest Peaks Recorded at Swan River Stream Gages ...... 5 Table 4: Stream Gage Summary Table ...... 6 Table 5: Stream Gage Analysis Results ‐ Systematic ...... 7 Table 6: Record Extension Methods Discharge Estimates ‐ Stream Gage No. 12369200 ...... 8 Table 7: Regression Equation Estimates ‐ Stream Gage No. 12369200 ...... 8 Table 8: Weighted Estimates ‐ Stream Gage No. 12369200 ...... 9 Table 9: Flood Discharge Estimates at USGS Gage 12369200 ...... 10 Table 10: Gage 12370000 Trending Analysis Comparison ...... 11 Table 11: Hydraulic Model Stream Reaches ...... 14

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Table 12: Log Interpolation between 2 Gages ...... 15 Table 13: Selected Discharge Estimates ...... 15

List of Figures Figure 1: Study Reach Site Map ...... 4 Figure 2: Flood Discharge Estimates at USGS Gage 12369200 ...... 10 Figure 3: Drainage Basin Area ...... 13

Appendices Appendix A: USGS Stream Gage Data Appendix B: Hydrologic Calculations

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1.0 INTRODUCTION As part of the Missoula County Memorandum of Understanding (MOU) (Reference 9) Agreement activities, DNRC is assisting Missoula County to complete a new detailed riverine floodplain study, with floodway, for a section of the Swan River beginning at the Lake/Missoula County boundary and extending approximately 19.1 miles upstream near the Beaver Creek confluence. Per the MOU, DNRC has completed a new hydrologic analysis to be utilized for the new study.

The study reach is currently mapped by FEMA using approximate methods. This study will be completed satisfying current state and FEMA standards for Detailed/Enhanced floodplain studies. A Flood Insurance Study (FIS) has been completed for Missoula County and Incorporated Areas dated August 16, 1988; however, an update to the FIS has been completed and is slated to become effective on July 6, 2015. A summary of the Digital Flood Insurance Rate Map (DFIRM) panels covering this study reach is presented below (Reference 2):

Community Community No. FIRM Panels Effective Date Missoula County, MT 300048 30063C0200D 30063C0075D 08/16/1988 30063C0050D 30063C0200E 30063C0075E 07/06/2015 30063C0050E

This report summarizes the hydrologic analysis and results for the new detailed study stream reach described. The new study includes hydrologic analysis to estimate the 10-, 4-, 2-, 1-, and 0.2 percent-annual-chance flood discharges for the Swan River.

1.1 LiDAR Collection

In 2012, Missoula County contracted with Watershed Sciences, Inc. to collect Light Detection And Ranging (LiDAR) data for four areas of interest for the purpose of supporting floodplain mapping projects:

1. Rock Creek floodplain, 2. Bitterroot River floodplain, 3. Swan River floodplain, and 4. Clearwater River floodplain.

Accuracy of the topographic data meets FEMA standards for detailed level floodplain mapping. The LiDAR data covers the project reach for this study.

1.2 Watershed Description

The Swan River originates from Gray Wolf Lake in the Mission Mountain Range and flows generally in a northwesterly direction before emptying into Flathead Lake at the town of Bigfork.

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At its mouth, the river drains a watershed area of approximately 723 square miles. The Swan River valley bottom was formed by glacial melt waters and reworked by fluvial processes. The valley is bounded on the east by the Swan Mountain Range and on the west by the Mission Range. The vegetation in the valley is dominated by subalpine firs with slopes ranging from 0 to 20 percent.

1.3 Effective FIS (Flood Insurance Study) Hydrology

The effective floodplain mapping for the Swan River in Missoula County stretches from the Lake County boundary upstream to the Beaver Creek confluence, which matches the extents of the study at hand. The level of detail of the effective mapping is approximate, as it has been carried forward from the historic flood hazard boundary maps (FHBMs) developed in the 1970s. In Flathead and Lake Counties, the Swan River floodplain has both approximate level and detailed level mapping; information about the effective detailed studies is shown in Table 1.

Table 1: Flathead & Lake County Effective Floodplain Studies

Approx. Effective Level of Author Study Title County Length Description Date Detail (mi) Beginning at the Swan River Rd. Bridge Simons Li & Lower Swan Lake March 12 Detailed upstream of Bigfork Dam and extending Associates, Inc. River Study County 1986 upstream to the outlet of Swan Lake. Beginning approx. 6.5 miles upstream of Simons Li & Upper Swan Lake March 20 Detailed the Swan Lake inlet and extending Associates, Inc. River Study County 1986 upstream to the Missoula Co. boundary. Lake September Limited Analysis of the Swan Lake floodplain PBS&J Swan Lake 9.5 County 2010 Detail including 23 miles of shoreline. Beginning at the river mouth and Swan River Flathead December DNRC 0.5 Detailed Flathead Lake confluence and extending at Bigfork County 2012 upstream 0.5 miles.

Table 1 is a summary of the effective detailed level studies on the Swan River. There are also several reaches of the channel with approximate level mapping which are remnants of the FHBMs from the 1970s.

Each of the effective detailed studies included a hydrologic analysis including estimation of peak flows. The following is a description of the hydrologic analysis method selected for estimating peak flows for each study:

Lower Swan River Study – A Log-Pearson Type III analysis was performed on the annual maximum discharges to determine peak return flows (Reference 4).

Upper Swan River Study – A regional frequency method was used to develop peak flows for the Swan River above Swan Lake. The USGS developed this methodology for the entire state, and discharge estimates are a function of

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drainage areas, average annual precipitation, and location within the watershed (Reference 4).

Swan Lake – Procedures recommended in Bulletin 17B of the Interagency Committee on Water Data and the USGS Water Resources Investigation Report 03-4308 were used to develop peak flows (Reference 4).

Swan River at Bigfork - A Log-Pearson Type III analysis was performed on the annual maximum discharges to determine peak return flows (Reference 3).

As part of the hydrologic analyses for each of these studies, peak flows were calculated at various locations throughout the study reaches. Table 2 shows peak flow values at specified locations within the effective studies.

Table 2: Peak Flows used in Effective Studies

Discharges (cfs) Drainage Applicable FIS / Location 10% Annual 2% Annual 1% Annual 0.2% Annual Area (mi2) Effective Year Chance Chance Chance Chance

Flathead County / At mouth 727.2 7,420 9,080 9,760 11,300 2015 At USGS Gage Lake County / 671.0 7,200 8,500 9,000 10,000 12370000 Near Bigfork 2013 Lake County / At Swan Lake 659.0 6,860 8,460 9,120 10,700 2013 At Wildlife Refuge Lake County / 504.8 3,950 5,150 5,700 6,800 Boundary 2013 At confluence with Goat Lake County / 431.8 3,540 4,630 5,130 6,200 Creek 2013 At confluence with Lake County / 366.8 3,070 4,040 4,490 5,400 Piper Creek 2013 At Missoula County Lake County / 273.0 2,440 3,230 3,610 4,440 Line 2013

The downstream end of the study at hand will tie into the upstream end of the Upper Swan River Study; the peak flows used at the upstream end of the Upper Swan River Study are shown in the row titled “At Missoula County Line” of Table 2.

1.4 Historic Data

The most severe flooding on the Swan River typically occurs in the spring and early summer as a result of snowmelt and/or rainfall runoff. Localized flooding can occasionally result from a long sustained rainfall or ice or log jams (Reference 4).

There are two USGS stream flow gages present on the Swan River (see Figure 1): 1) Gage 12369200 is an inactive gage located at the upstream end of the river near the town of Condon, MT with 20 years of record (1973-1992); 2) Gage 12370000 is an active gage near the town of Bigfork with 92 years of record (1922-2013). The five highest annual peak discharges recorded at each gage are listed in Table 3.

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Figure 1: Study Reach Site Map

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Table 3: Highest Peaks Recorded at Swan River Stream Gages

USGS Gage No. 12369200 USGS Gage No. 12370000 near Condon (69.1 mi2) near Bigfork (671.0 mi2) Ranking Peak Peak Date Discharge Date Discharge (cfs) (cfs)

1 June 18, 1974 1,540 June 20, 1974 8,890

2 June 16, 1975 1,020 May 18, 1997 8,520

3 May 26, 1980 1,090 May 24, 1948 8,400

4 June 17, 1982 1,060 June 18, 1933 8,280

5 May 31, 1987 1,090 June 10, 1964 8,100

Comparing the dates of the peak flows at the two stations, only one of the top five peaks at the downstream gage (12370000) occur within the operation period of the upstream gage (12369200), which is the 1974 event and is the highest recorded flow at both stations. Statistically, the fact that the top ranked event at the downstream gage is captured within the period of record at the upstream gage is significant; the magnitude of the event will aid in increasing the confidence of the upper end of the Log Pearson Type III curve of the Systematic analysis (discussed in Section 2.1.1).

For the purpose of documenting historic flood records, county officials were contacted; the only records of historic flood events through the project reach are aerial photographs of the 1997 flood event. These photos should be referenced for comparison purposes during the hydraulic analysis and floodplain mapping phases of this project; the Missoula County Floodplain Administrator has these files on record.

2.0 HYDROLOGIC ANALYSIS The new detailed study project covers approximately 19.1 miles of the Swan River, beginning at the Missoula/Lake County boundary and extending upstream to the Beaver Creek confluence (see Figure 1). This section of the report describes the various hydrologic analysis methods evaluated and the results of the flood discharges estimated for the study reach. The selected method(s) and estimated discharges are discussed in Section 3.0 .

2.1 Stream Gage Analyses

As previously mentioned, there are two USGS stream flow gages present on the Swan River (see Figure 1): Gage 12369200 is located at the upstream end of the study reach and gage 12370000 is located near the mouth of the river. A summary of the Swan River gages is provided in Table 4. Based on USGS WRIR 03-4308 (Reference 11), recurrence interval discharge estimates at gaging stations can be transferred to ungaged sites, such as at selected

5 Hydrology Design Report, Swan River Detailed Floodplain Study May 2015 flow change locations throughout the study reach. The transfer methodology will be discussed in Section 2.2 of this report.

Table 4: Stream Gage Summary Table

Highest Peak of Gage Basin Area Period of Number of Gage Name Record / Year Number (mi2) Record Annual Peaks Recorded Swan River near 12369200 69.1 1973 - 1992 20 1,540 / 1974 Condon, MT

Swan River near 12370000 671.0 1922 - 2013 92 8,890 / 1997 Bigfork, MT

Prior to transferring discharges to the study reach, recurrence interval flows at the stream gages must be estimated. There are multiple ways to calculate peak flow estimates at the stream gages, and the following methods for estimating recurrence interval peaks at USGS gages 12370000 and 12369200 were analyzed:

Gage 12370000 Swan River near Bigfork, MT  Systematic estimation

Gage 12369200 Swan River near Condon, MT  Systematic estimation,  2 Station Comparison (record extension based on 12370000),  MOVE.1 analysis (record extension based on 12370000),  Regional Regression, and  Weighted estimation.

Estimation methods differ between the two gages due to the difference in reliability of the periods of record. Gage 12370000 has a substantial period with 92 years while gage 12369200 is less reliable with 20 years. To aid in the evaluation of the reliability of the 20 years of record at gage 12369200, the five peak flow estimation methods listed above were compared. The following sections provide a description of each estimation method.

2.1.1 Systematic Estimation The Systematic analysis involves development of a flood flow frequency curve using a Log- Pearson Type III (LP3) distribution based on recorded annual stream flow peaks following Bulletin 17B guidelines (Reference 7). The flood frequency analysis was performed using the USGS’s PeakFQ v 7.1 program. The annual peak flow record was downloaded from the USGS website for the stream gage in the WATSTORE text format for PeakFQ input and annual peak flow frequency analysis. See Appendix B for PeakFQ output files.

Prior to performing the new analysis, a baseline analysis was run for each gage in an effort to reproduce previously published flows. For gage 12370000, the most recent analysis was reported in the Swan River at Bigfork Hydrology Design Report (Reference 8) based on data

6 Hydrology Design Report, Swan River Detailed Floodplain Study May 2015 through 2011; and for gage 12369200, the most recent analysis known is that of WRIR 03-4308 through water year 1998. The results were successfully duplicated using a Generalized Skew Standard Error value of 0.64, the weighted skew method, and no defined outliers.

The new LP3 flood frequency analysis for gage 12370000 incorporated the current 92 year (1922 – 2013) period of record for the gage and used the same options and parameters as the baseline analysis: a Generalized Skew Standard Error of 0.64, the weighted skew method, and no defined outliers. USGS personnel from the state office in Helena are currently recommending the value of 0.64 for Generalized Skew Standard Error. An updated analysis of gage 12369200 is unnecessary since the gage was discontinued after 1973 and the entire period of record was captured in the previous analysis.

In accordance with FEMA guidelines (Reference 1), no expected probability adjustments were made to the Bulletin 17B frequency curve. Results of the flood frequency analysis are presented in Table 5.

Table 5: Stream Gage Analysis Results - Systematic

Period of Percent Annual Chance Peak Discharge Gage Description Record 10% 4% 2% 1% 0.2% 1922 – 2011 Baseline Analysis 6,690 7,870 8,540 9,190 10,700 (90yrs) 12370000 Updated / Current 1922 – 2013 6,960 7,860 8,500 9,130 10,600 Analysis (92yrs)

1973 – 1992 12369200 Current Analysis 1,180 1,360 1,480 1,600 1,890 (20yrs)

2.1.2 Two Station Comparison & MOVE.1 Comparing the dates of the five highest peak flows at each stream gage (Table 3), it is likely that some of the largest flood events on the Swan River were not captured in the 20 years of record at gage 12369200; four of the highest top five peaks at gage 12370000 are outside of the gage 12369200 period of record. Therefore, the peak discharges estimated using the Systematic analysis method may not be reflective of a longer period of record; which provides greater reliability.

To strengthen the period of record of gage 12369200 there are several methods which can be used to extend the record by utilizing gages with long, overlapping periods of record on the same stream. The two record extension methods compared in this analysis were the Two Station Comparison (Reference 7), and a Maintenance Of Variance Extension Type 1 (MOVE.1) analysis (Reference 6). The method of the Two Station Comparison is to perform a regression on the concurrent peak flows measured at both gages and use the correlation to improve estimates of the mean and standard deviation of the short-term gage record. Similar to the Two Station Comparison, the MOVE.1 analysis also results in modifications to the sample mean and variance. However, rather than directly changing these parameters based on correlation of the data between the two gages, the procedure of the MOVE.1 analysis is to synthesize the

7 Hydrology Design Report, Swan River Detailed Floodplain Study May 2015 individual peaks of the target station based on a correlation analysis of the concurrent data between the two gages; and the distributional properties of the final dataset reflect adjustment of the mean and variance. Discharge estimates based on these two record extension methods at gage 12369200 are shown in Table 6.

Table 6: Record Extension Methods Discharge Estimates - Stream Gage No. 12369200

Discharge Estimates (cfs) Recurrence Two Station Interval MOVE.1 Comparison

10yr 1,374 1,210

25yr 1,648 1,373

50yr 1,856 1,489

100yr 2,068 1,603

500yr 2,584 1,862

The MOVE.1 analysis was completed by local USGS personnel, and all calculations were performed in the USGS FORTRAN program. Calculations of the Two Station Comparison and output files for the MOVE.1 analysis are provided in Appendix B.

2.1.3 Regional Regression Equations Flood frequency discharges were estimated using USGS published regional regression equations derived from basin characteristics (Reference 11). The basin characteristics equations have been developed using characteristics of drainage basins within designated regions throughout the state with similar hydrologic properties. The study reach for this project is located in the “West” region, and the regression equations for the region are based on drainage basin characteristics variables of 1) area, 2) average precipitation, and 3) percent of the basin that is forested. All of the data needed to perform the regression analysis at USGS gage 12369200 has been provided by USGS in WRIR 03-4308 (Reference 11). Results of the basin characteristics regression analysis are provided in Table 7.

Table 7: Regression Equation Estimates - Stream Gage No. 12369200

Discharge Estimates (cfs) Recurrence Basin Active Bankfull Interval Characteristics Channel Width 10yr 1,671 3,160 3,030

25yr 1,972 3,780 3,640

50yr 2,260 4,290 4,130 100yr 2,580 4,820 4,650 500yr 3,437 6,140 5,950

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Similar to the regression equations derived from basin characteristics, USGS WRIR 03-4308 provides regression equations based on the stream active channel width and bankfull width for each region. Channel measurements representative of the stream gage locations are published in WRIR 03-4308, and the discharge estimates based on the channel characteristics regression equations are shown in Table 7. The Standard Error of Prediction (SEP) of the basin characteristics equations is significantly lower than the channel characteristics equations, and therefore, the channel characteristics estimates were not included in the final comparison of discharge estimates at gaging station 12369200.

2.1.4 Weighted Estimation The final methods employed for estimating flood discharges at gage 12369200 are two procedures used to combine various estimation methods. The first procedure combines the Systematic estimates with the Basin Characteristics regression estimates. According to WRIR 03-4308 (Reference 11), the Systematic discharge estimates may have large errors due to time- sampling, especially at gages with shorter periods of record. To improve the Systematic estimates, Bulletin 17B (Reference 7) states that the Systematic estimates can be weighted with the regression estimates. Using equation 22 in WRIR 03-4308, the discharges in Table 8 were estimated.

Table 8: Weighted Estimates - Stream Gage No. 12369200

Discharge Estimates (cfs) Recurrence Systematic + Basin Basin Characteristics Interval Characteristics Regression + Channel Regression Characteristics Regression

10yr 1,216 2,160

25yr 1,421 2,550

50yr 1,567 2,850

100yr 1,715 3,170

500yr 2,075 4,010

The second procedure combines the estimates of all of the regression equations (basin characteristics and channel characteristics). This procedure was performed through the use of an online calculator developed by the USGS (Reference 16). These results are also shown in Table 8.

2.1.5 Results Comparison Due to the reliability of the 92 years of record at gage 12370000, the Systematic method was the only method selected for estimating discharges at gage 12370000. Provided in Table 9 and Figure 2 are the results of the five methods used to estimate discharges at gage 12369200.

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Table 9: Flood Discharge Estimates at USGS Gage 12369200

Discharge Estimates (cfs) Recurrence Basin Two Station Interval Systematic MOVE.1 Weighted 1* Weighted 2** Characteristics Comparison Regression

10yr 1,180 1,374 1,210 1,216 2,160 1,671

25yr 1,360 1,648 1,373 1,421 2,550 1,972

50yr 1,480 1,856 1,489 1,567 2,850 2,260

100yr 1,600 2,068 1,603 1,715 3,170 2,580

500yr 1,890 2,584 1,862 2,075 4,010 3,437

* Weighted 1 – Systematic weighted with Basin Characteristics regression ** Weighted 2 – Basin Characteristics regression weighted with Channel Characteristics regressions (active channel & bankfull width)

Figure 2: Flood Discharge Estimates at USGS Gage 12369200

Estimates from the Systematic computations and the MOVE.1 analysis are nearly identical, while the Two Station Comparison, Basin Characteristics Regression, and Weighted regressions estimates are significantly higher. As one would intuitively suspect, the Systematic

10 Hydrology Design Report, Swan River Detailed Floodplain Study May 2015 weighted with the Basin Characteristics regression values are between the Systematic and Basin Characteristics Regression estimates.

The initial step in both of the record extension methods (Two Station Comparison and MOVE.1) is calculation of a correlation coefficient for the purpose of determining whether the analysis is a reliable method for improving the dataset. A satisfying coefficient was calculated for both methods. Therefore, the record extension methods were assumed to be more reliable than the Systematic method. Furthermore, given the reliability of the downstream gaging station (12370000) with 92 years of record, and the ability to improve the record of the upstream gage (12369200) using gage 12370000 as a base station, the record extension methods were also determined to be more reliable than the regression equation and weighted estimates for estimating peak flows at gage 12369200.

The MOVE.1 and Two Station Comparison analyses are both industry-accepted record extension methods, and neither method is preferred over the other. Comparing the 100-year event estimated using each method, the Two Station Comparison estimates are approximately 25 percent greater. In order to determine which method is more appropriate for this study, the following trending analysis was performed on gage 12370000 near Bigfork, as recommended by USGS personnel:

The 92-year period of record was truncated to match the 20-year record at gage 12369200 near Condon. The Systematic analysis was then rerun using the truncated period of record. Comparison of the truncated Systematic results to the unmodified period of record Systematic results may provide aid in selection of the appropriate record extension method.

The results of the truncated and unmodified periods of record Systematic analysis are shown in Table 10. Table 10: Gage 12370000 Trending Analysis Comparison

Period of Percent Annual Chance Peak Discharge Description Record 10% 4% 2% 1% 0.2% Complete Period of 1922 – 2013 6,960 7,860 8,500 9,130 10,600 Record (92yrs)

Truncated Period of 1973 – 1992 6,713 7,615 8,276 8,930 10,450 Record (20yrs)

The results of the truncated period of record Systematic analysis are slightly (~2.5 percent) less than the results of the unmodified period of record Systematic analysis. In other words, the additional 72 years of record only slightly increases the flow estimates at this station. Assuming that this trend would also be present at the upstream gaging station if the additional 72 years of record were included in the Systematic analysis, there would be a minimal increase in the discharge estimates. Due to the much smaller flow magnitudes at the upstream gage compared to the downstream gage, the flow increases at the upstream gage would be even smaller than the increases at the downstream gage.

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Since the discharge estimates of the MOVE.1 analysis are more similar to the results of the Systematic analysis at gage 12369200 and based on the results of this trending analysis, the discharge estimates of the MOVE.1 analysis have been selected for use at gaging station 12369200.

Also noteworthy, the USGS is currently under contract with the Montana Department of Transportation (MDT) and the Montana DNRC to perform an update to the recommended methods for estimating flood frequency in Montana based on data through water year 2011. One deliverable of the project is an updated analysis of all the gaging stations throughout the state; this deliverable includes flood frequency estimates at all gages based on Systematic analysis, record extension methods, and Systematic estimates weighted with regression estimates, as deemed necessary. In the draft report, the flood frequency flows recommended at station 12369200 are much closer to the MOVE.1 results than the Two Station Comparison results included in this report.

2.2 Transfer to Ungaged Sites

As described, the new study reach is located shortly downstream of USGS gage 12369200 and upstream of USGS gage 12370000. As documented in WRIR 03-4308, the recommended procedure for estimating discharges in between two gaging stations located on the same stream is a log interpolation between the two gages (see Section 2.2.2). Identification of the appropriate transfer locations throughout the study reach is discussed in the following section.

2.2.1 Streamflow Change Locations There are several significant tributaries to the Swan River throughout the study reach. For the purpose of developing a one-dimensional “steady flow” hydraulic model, the stream study reach is divided into separate sections based on the flow estimated to be in the floodplain. The study reach was divided into reaches which span from immediately upstream of a tributary confluence to immediately upstream of the adjacent tributary confluence. This methodology allows the additional flow resulting from the tributary hydrology to be applied in the hydraulic model at the appropriate location. The model may also be divided at other locations to reflect changes in stream flow. The major tributaries throughout the study reach include:  Holland Creek,  Buck Creek,  Glacier Creek, and  Cold Creek. Drainage basins for each of the tributaries are shown in Figure 3 and a summary of the different model reaches is provided in Table 11. The following section describes the method used to transfer the peak flows estimated at the gaging stations to the individual reaches throughout the study reach.

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Figure 3: Drainage Basin Area

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Table 11: Hydraulic Model Stream Reaches

Cumulative River Station1 Reach Subbasin Reach Subbasin Name Basin Area (ft) Length (ft) Area (mi2) (mi2) Cold Creek 1 0 – 11,050 11,050 40.2 271.1 (downstream study limit) 2 11,050 – 37,800 26,750 Glacier Creek 93.3 230.9

3 37,800 – 56,140 18,340 Buck Creek 27.9 137.6

4 56,140 – 86,670 30,530 Holland Creek 39.1 109.7

5 86,670 – 99,370 12,700 Upstream Study Extents 70.6 70.6

1 Distance above Missoula/Lake County boundary.

2.2.2 Log Interpolation between 2 Gages

As previously mentioned, WRIR 03-4308 documents an interpolation estimation method for ungaged sites located between two gaged sites on the same stream, which is the case for this study. The WRIR 03-4308 equation is as follows (Reference 11, equation 21):

As mentioned in Section 2.1.5, the most reliable flood frequency estimates at gage 12370000 are the Systematic estimates, and the most reliable estimates at gage 12369200 are the MOVE.1 estimates. Therefore, based on these estimates at each gaging station, peak flow estimates in Table 12 were calculated using the interpolation equation.

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Table 12: Log Interpolation between 2 Gages

Discharge Estimates (cfs) Recurrence Upstream Cold Creek Glacier Creek Buck Creek Holland Creek Interval Study Extents Subbasin Subbasin Subbasin Subbasin Subbasin 10yr 3,465 3,062 2,056 1,727 1,230 25yr 3,920 3,465 2,329 1,958 1,396 50yr 4,245 3,754 2,524 2,122 1,514 100yr 4,564 4,036 2,716 2,283 1,630 500yr 5,287 4,677 3,150 2,650 1,893

3.0 HYDROLOGIC ANALYSIS SUMMARY & SELECTED DISCHARGES

Stream flow gage records are typically the most reliable hydrologic information when the periods of record are of sufficient length. There are two gages on the Swan River; one near the upstream end of the study reach and one significantly downstream of the study reach. Transfer of the recurrence interval peak flow estimates from the gaging stations to the study reach is considered to be the most reliable estimation method, and interpolation of the flows between the two gages on the stream is the selected transfer methodology. Due to the 92 years of record, the Systematic estimation method was selected for the purpose of estimating flows at gage 12370000 Swan River near Bigfork. With only 20 years of record, several methods were analyzed for estimating flows at gage 12369200 Swan River near Condon; ultimately, the most appropriate method for estimating flows at gage 12369200 was the MOVE.1 record extension method using gage 12370000 as a base gage for the extension. The selected flows for this study reach have been rounded to the nearest 10 cfs and are shown in Table 13.

Table 13: Selected Discharge Estimates

River Station Drainage Upstream Limit of Peak Discharge Estimates (cfs) Applicability Area Applicability Limits1 (ft) (mi2) 10yr 25yr 50yr 100yr 500yr At Missoula / Lake County 0 – 11,050 271.1 3,470 3,920 4,250 4,560 5,290 Boundary Immediately upstream of Cold 11,050 – 37,800 230.9 3,060 3,470 3,750 4,040 4,680 Creek confluence Immediately upstream of Glacier 37,800 – 56,140 137.6 2,060 2,330 2,520 2,720 3,150 Creek confluence Immediately upstream of Buck 56,140 – 86,670 109.7 1,730 1,960 2,120 2,280 2,650 Creek confluence Immediately upstream of 86,670 – 99,370 70.6 1,230 1,400 1,510 1,630 1,890 Holland Creek confluence 1 Distance above Missoula/Lake County boundary.

The downstream end of this study reach ties into an effective detailed study in Lake County titled the “Upper Swan River Study” which became effective in March of 1986. The 100-year

15 Hydrology Design Report, Swan River Detailed Floodplain Study May 2015 flow in the “Upper Swan River Study” (Table 2) is 3,610 cfs, significantly lower than the flow recommended for use in the Cold Creek Subbasin reach of this study, which is 4,560 cfs. This study will result in an increase in the predicted flow of the 100-year event through this reach of 26 percent. Differences in these flow estimates can likely be attributed to the difference in hydrologic analysis methods and additional years of record at the analyzed gages. Peak flows in the “Upper Swan River Study” were estimated using regional frequency methods, and the gages used in that analysis may have had up to 27 years of record less than the downstream gage (12370000) used in this analysis.

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REFERENCES 1. Federal Emergency Management Agency (FEMA), 2009, Guidelines and Specifications for Flood Hazard Mapping Partners, Appendix C: Guidance for Riverine Flooding Analyses and Mapping, 81 p.

2. Federal Emergency Management Agency (FEMA), Digital Flood Insurance Rate Maps:

Community: Community No.: FIRM Panels: Effective Date: Missoula County, MT 300048 30063C0200D 30063C0075D 08/16/1988 30063C0050D 30063C0200E 30063C0075E 07/06/2015 30063C0050E

3. Flood Insurance Study Flathead County, Montana and Incorporated Areas. (Preliminary). (2014). Federal Emergency Management Agency.

4. Flood Insurance Study Lake County, Montana and Incorporated Areas. (2013). Federal Emergency Management Agency.

5. Flood Insurance Study Missoula County, Montana and Incorporated Areas. (1988). Federal Emergency Management Agency.

6. Hirsch, R. (1982). A Comparison of Four Streamflow Record Extension Techniques. Water Resources Research, 18, 1081-1088.

7. Interagency Advisory Committee on Water Data. (1982). Guidelines for Determining Flood Flow Frequency: Bulletin #17B of the Hydrology Committee (Rev. Sept. 1981 ; ed.). Reston, Va.: U.S. Dept. of the Interior, Geological Survey, Office of Water Data Coordination.

8. Montana DNRC. (2012). Swan River at Bigfork Hydrology Design Report.

9. Montana DNRC, & Missoula County. (2014). Memorandum of Understanding.

10. PBS&J. (2010). Swan Lake Hydrologic Analysis Technical Support Data Notebook. 4-4.

11. Parrett, C., & Johnson, D. (2004). Methods for estimating flood frequency in Montana based on data through water year 1998. Helena, Mont.: U.S. Dept. of the Interior, U.S. Geological Survey.

12. Simons, LI and Associates, Inc., Lower Swan River Flood Study, (1986, March).

13. Simons, LI and Associates, Inc., Upper Swan River Flood Study, (1986, March).

17 Hydrology Design Report, Swan River Detailed Floodplain Study May 2015

14. USGS Peak Streamflow for Montana. (n.d.). Retrieved October 22, 2014, from http://nwis.waterdata.usgs.gov/mt/nwis/peak?search_criteria=search_site_no&submitted _form=introduction

15. Watershed Sciences Inc., 2013, 2-foot contours – derived from ground-classified LiDAR point data (collected Oct. 2012), Missoula County, Montana.

16. (n.d.). Retrieved April 24, 2015, from http://wy-mt.water.usgs.gov/freq

18 Appendix A

USGS Stream Gage Data

 USGS Gage 12370000 (#105) Swan River near Bigfork, MT o Systematic Peak Streamflow – Table o Systematic Peak Streamflow – Graph o LP3 Analysis through WY 1998

 USGS Gage 12369200 (#102) Swan River near Condon, MT o Systematic Peak Streamflow – Table o Systematic Peak Streamflow – Graph o LP3 Analysis through WY 1998

USGS Surface Water for Montana: Peak Streamflow http://nwis.waterdata.usgs.gov/mt/nwis/peak?site_no=12370000&agenc...

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Peak Streamflow for Montana

Available data for this site Lake County, Montana Hydrologic Unit Code 17010211 Latitude 48°01'27.23", Longitude 113°58'43.75" NAD83 Drainage area 671 square miles Gage datum 3,062.6 feet above NGVD29 Output formats Table

Graph

Tab-separated file

peakfq (watstore) format

Reselect output format

Gage Stream- Water Date Height flow Year (feet) (cfs)

1922 Jun. 08, 1922 5,500

1923 Jun. 14, 1923 4,780

1924 May 19, 1924 5,380

1925 May 23, 1925 6,760

1926 Apr. 21, 1926 3,860

1 of 5 11/26/2014 2:02 PM USGS Surface Water for Montana: Peak Streamflow http://nwis.waterdata.usgs.gov/mt/nwis/peak?site_no=12370000&agenc...

Gage Stream- Water Date Height flow Year (feet) (cfs)

1927 Jun. 13, 1927 7,060

1928 May 28, 1928 7,820

1929 May 26, 1929 5,300

1930 Jun. 01, 1930 2,920

1931 May 18, 1931 5.57 5,140

1932 May 24, 1932 5.92 5,710

1933 Jun. 18, 1933 7.00 8,280

1934 Dec. 25, 1933 5.42 4,750

1935 May 25, 1935 5.33 4,650

1936 May 17, 1936 5.97 6,180

1937 May 29, 1937 4.71 3,380

1938 May 30, 1938 5.26 4,350

1939 May 06, 1939 5.24 4,350

1940 May 27, 1940 4.82 3,650

1941 Jun. 03, 1941 3.88 2,120

1942 May 28, 1942 5.06 3,980

1943 Jun. 21, 1943 5.82 5,570

1944 May 20, 1944 4.53 3,120

1945 Jun. 04, 1945 5.01 3,940

1946 May 30, 1946 5.34 4,610

1947 May 11, 1947 6.11 5,960

1948 May 24, 1948 7.12 8,400

1949 May 17, 1949 5.64 5,130

1950 Jun. 23, 1950 6.01 5,900

1951 May 14, 1951 5.45 4,750

1952 Apr. 30, 1952 5.37 4,590

1953 Jun. 05, 1953 5.87 5,610

1954 May 22, 1954 5.88 5,630

1955 Jun. 16, 1955 5.60 5,060

1956 Jun. 04, 1956 6.42 6,790

1957 May 08, 1957 5.55 4,960

2 of 5 11/26/2014 2:02 PM USGS Surface Water for Montana: Peak Streamflow http://nwis.waterdata.usgs.gov/mt/nwis/peak?site_no=12370000&agenc...

Gage Stream- Water Date Height flow Year (feet) (cfs)

1958 May 27, 1958 6.05 5,920

1959 Jun. 22, 1959 6.72 7,520

1960 Jun. 06, 1960 5.75 5,500

1961 May 29, 1961 6.38 6,810

1962 May 31, 1962 5.26 4,530

1963 Jun. 02, 1963 4.84 3,740

1964 Jun. 10, 1964 6.98 8,100

1965 Jun. 20, 1965 6.00 6,010

1966 Jun. 02, 1966 5.25 4,520

1967 May 25, 1967 5.77 5,540

1968 Jun. 25, 1968 5.23 4,480

1969 Jun. 01, 1969 5.18 4,380

1970 Jun. 08, 1970 6.58 7,230

1971 May 31, 1971 5.72 5,440

1972 Jun. 12, 1972 6.25 6,540

1973 May 21, 1973 4.86 3,770

1974 Jun. 20, 1974 7.34 8,890

1975 Jun. 17, 1975 5.74 5,410

1976 May 16, 1976 5.45 4,860

1977 May 04, 1977 4.69 3,430

1978 Jun. 10, 1978 5.69 5,320

1979 May 28, 1979 5.95 5,840

1980 May 27, 1980 5.88 5,810

1981 May 27, 1981 5.75 5,520

1982 Jun. 19, 1982 5.83 5,710

1983 Jun. 01, 1983 5.45 4,790

1984 Jun. 26, 1984 5.43 4,820

1985 Jun. 10, 1985 5.59 5,160

1986 Jun. 01, 1986 6.15 6,340

1987 May 03, 1987 5.14 4,220

1988 Jun. 09, 1988 4.57 3,170

3 of 5 11/26/2014 2:02 PM USGS Surface Water for Montana: Peak Streamflow http://nwis.waterdata.usgs.gov/mt/nwis/peak?site_no=12370000&agenc...

Gage Stream- Water Date Height flow Year (feet) (cfs)

1989 May 12, 1989 5.49 4,940

1990 Jun. 28, 1990 5.36 4,670

1991 May 21, 1991 5.31 4,570

1992 May 10, 1992 4.55 3,140

1993 May 18, 1993 5.47 5,050

1994 May 14, 1994 4.90 3,760

1995 Jun. 07, 1995 4.98 3,910

1996 Jun. 11, 1996 5.88 5,810

1997 May 18, 1997 7.27 8,520

1998 May 29, 1998 4.99 3,860

1999 May 29, 1999 5.06 3,820

2000 Jun. 17, 2000 4.82 3,580

2001 May 30, 2001 5.08 3,910

2002 Jun. 03, 2002 5.27 4,480

2003 Jun. 02, 2003 5.65 5,290

2004 Jun. 08, 2004 4.58 3,150

2005 Jun. 05, 2005 5.45 4,860

2006 Jun. 18, 2006 6.06 6,170

2007 Jun. 07, 2007 4.76 3,510

2008 May 22, 2008 5.94 5,930

2009 Jun. 02, 2009 5.51 4,990

2010 Jun. 06, 2010 5.41 4,770

2011 Jul. 02, 2011 6.20 6,710

2012 Apr. 29, 2012 5.73 5,710

2013 May 15, 2013 6.08 6,210

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Peak Streamflow for Montana

Available data for this site

Lake County, Montana Hydrologic Unit Code 17010211 Latitude 48°01'27.23", Longitude 113°58'43.75" NAD83 Drainage area 671 square miles Gage datum 3,062.6 feet above NGVD29 Output formats Table

Graph

Tab-separated file

peakfq (watstore) format

Reselect output format

1 of 2 11/26/2014 2:05 PM USGS Surface Water for Montana: Peak Streamflow http://nwis.waterdata.usgs.gov/mt/nwis/peak?site_no=12370000&agenc...

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2 of 2 11/26/2014 2:05 PM Montana Flood-Frequency and Basin-Characteristic Data http://wy-mt.water.usgs.gov/freq?page_type=site&site_no=12370000

Flood-frequency data are based on recorded annual peak discharges through 1998. Peak discharges for specified frequencies (exceedance probabilities) were determined by fitting a log-Pearson Type 3 probability distribution to base 10 logarithms of recorded annual peak discharges as described by the Interagency Advisory Committee on Water Data (1982, Guidelines for Determining Flood Flow Frequency--Bulletin 17-B of the Hydrology Subcommittee: U.S. Geological Survey, Office of Water Data Coordination). Note: Data are provisional and user is responsible for assessment and interpretation of flood-frequency data.

Most of the basin characteristic data were measured in the 1970s from the best-scale topographic maps available at the time. Some data, such as mean annual precipitation, soil index data, and mean January minimum temperatures, were compiled from maps prepared by other agencies. Channel widths were measured in the field by USGS personnel.

The flood-frequency and basin characteristics data were used in a new flood-frequency report just published by the USGS, entitled "Methods for estimating Flood Frequency in Montana Based on Data through Water Year 1998" (Water-Resources Investigations Report 03-4308). Information about the equations described in that report can be found at the following link.

For more detailed information contact Wayne Berkas: Phone: 406-457-5903 or by e-mail.

Annual peak discharge, in cubic feet per second (top line), for indicated exceedance probability, in percent (bottom line):

-- -- 3370 3680 4110 5080 6300 7070 8000 8670 9330 9980 10800 99.5999590805020104 2 10.50.2

1 of 3 11/26/2014 2:41 PM Montana Flood-Frequency and Basin-Characteristic Data http://wy-mt.water.usgs.gov/freq?page_type=site&site_no=12370000

NOTE: Systematic peaks are those that are recorded within the period of gaged record. The computed systematic flood-frequency curve is based only on the systematic peaks. The computed Bulletin 17-B flood-frequency curve often is different from the systematic flood-frequency curve because of differences between station skew and regional skew, low- or high-outlier adjustments, or the presence of one or more historical peaks outside the systematic record. Historical peaks also result in historical adjusted plotting positions (exceedance probabilities) for all peaks.

Basin Characteristics:

Value Abbrev Explanation 36.3 SLOPE Main channel slope, in ft per mile 84.5 LENGTH Total stream length, miles 5020.0 ELEV Mean basin elevation, ft above msl 26.0 EL6000 Percent of basin above 6,000 ft, msl 1.85 STORAGE Percent of basin in lakes, ponds, and swamps 89.8 FOREST Percent of basin in forest 8.9 SOIL_INF Soil index, in inches 48.02444444 LAT_GAGE Latitude of gage, in decimal degrees 113.97888889 LNG_GAGE Longitude of gage, in decimal degrees

2 of 3 11/26/2014 2:41 PM Montana Flood-Frequency and Basin-Characteristic Data http://wy-mt.water.usgs.gov/freq?page_type=site&site_no=12370000

23.0 PRECIP Mean annual precipitation, in inches Precipitation intensity for a 24-hour storm 1.7 I24_2 having a 2-year recurrence interval, in inches per hour Mean minimum January temperature, in 14.0 JANMIN degrees F 165.0 WAC Width of active channel, in feet 1.0 W2 Mean depth for active channel, in feet 185.0 WBF Width of bankfull channel, in feet 4.0 W4 Mean depth of bankfull channel, in feet

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Peak Streamflow for Montana

Available data for this site Missoula County, Montana Hydrologic Unit Code 17010211 Latitude 47°25'20.58", Longitude 113°40'15.50" NAD83 Drainage area 69.1 square miles Gage datum 4,015 feet above NGVD29 Output formats Table

Graph

Tab-separated file

peakfq (watstore) format

Reselect output format

Gage Stream- Water Date Height flow Year (feet) (cfs)

1973 May 20, 1973 3.45 560

1974 Jun. 18, 1974 4.88 1,540

1975 Jun. 16, 1975 4.22 1,020

1976 May 15, 1976 4.19 994

1977 May 03, 1977 3.50 616

1 of 2 11/26/2014 3:13 PM USGS Surface Water for Montana: Peak Streamflow http://nwis.waterdata.usgs.gov/mt/nwis/peak?site_no=12369200&agenc...

Gage Stream- Water Date Height flow Year (feet) (cfs)

1978 Jun. 09, 1978 3.87 821

1979 May 27, 1979 4.10 960

1980 May 26, 1980 4.29 1,090

1981 Jun. 07, 1981 3.85 876

1982 Jun. 17, 1982 4.25 1,060

1983 May 30, 1983 3.63 691

1984 Jun. 22, 1984 3.91 846

1985 Jun. 08, 1985 3.98 864

1986 May 31, 1986 4.37 1,090

1987 May 01, 1987 3.61 665

1988 Jun. 08, 1988 3.41 591

1989 May 11, 1989 4.09 942

1990 Jun. 26, 1990 3.82 793

1991 May 19, 1991 3.74 720

1992 May 09, 1992 3.29 511

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Peak Streamflow for Montana

Available data for this site

Missoula County, Montana Hydrologic Unit Code 17010211 Latitude 47°25'20.58", Longitude 113°40'15.50" NAD83 Drainage area 69.1 square miles Gage datum 4,015 feet above NGVD29 Output formats Table

Graph

Tab-separated file

peakfq (watstore) format

Reselect output format

1 of 2 11/26/2014 3:13 PM USGS Surface Water for Montana: Peak Streamflow http://nwis.waterdata.usgs.gov/mt/nwis/peak?site_no=12369200&agenc...

Download a presentation-quality graph

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2 of 2 11/26/2014 3:13 PM Montana Flood-Frequency and Basin-Characteristic Data http://wy-mt.water.usgs.gov/freq?page_type=site&site_no=12369200

For more detailed information contact Wayne Berkas: Phone: 406-457-5903 or by e-mail.

Annual peak discharge, in cubic feet per second (top line), for indicated exceedance probability, in percent (bottom line):

426 452 537 590 661 828 1050 1180 1360 1480 1600 1730 1890 99.5999590805020104 2 10.50.2

1 of 3 11/26/2014 2:43 PM Montana Flood-Frequency and Basin-Characteristic Data http://wy-mt.water.usgs.gov/freq?page_type=site&site_no=12369200

Basin Characteristics:

Value Abbrev Explanation 179.0 SLOPE Main channel slope, in ft per mile 19.0 LENGTH Total stream length, miles 5830.0 ELEV Mean basin elevation, ft above msl 39.0 EL6000 Percent of basin above 6,000 ft, msl 4.53 STORAGE Percent of basin in lakes, ponds, and swamps 61.0 FOREST Percent of basin in forest -- SOIL_INF Soil index, in inches 47.4225 LAT_GAGE Latitude of gage, in decimal degrees 113.67 LNG_GAGE Longitude of gage, in decimal degrees

2 of 3 11/26/2014 2:43 PM Montana Flood-Frequency and Basin-Characteristic Data http://wy-mt.water.usgs.gov/freq?page_type=site&site_no=12369200

54.0 PRECIP Mean annual precipitation, in inches Precipitation intensity for a 24-hour storm 2.0 I24_2 having a 2-year recurrence interval, in inches per hour Mean minimum January temperature, in 9.0 JANMIN degrees F 72.0 WAC Width of active channel, in feet 1.3 W2 Mean depth for active channel, in feet 84.0 WBF Width of bankfull channel, in feet 3.8 W4 Mean depth of bankfull channel, in feet

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3 of 3 11/26/2014 2:43 PM

Appendix B

Hydrologic Calculations

 Stream Gage Analysis ‐ USGS Gage No. 12370000 o Input – PeakFQ WATSTORE data format o Output through WY 2013  Stream Gage Analysis ‐ USGS Gage No. 12369200 o Input – PeakFQ WATSTORE data format o Output through WY 1992  Weighted Regional Regression Results – USGS Online Calculator Output  Stream Gage Analysis Summary & Log Interpolation between Gages  Two Station Comparison Record Extension – USGS Gage No. 12369200  USGS MOVE.1 Fortran

12370000peak.txt Z12370000 USGS H12370000 4801271135843003030047SW17010211671 3062.6 N12370000 Swan River near Bigfork, MT Y12370000 312370000 19220608 5500 312370000 19230614 4780 312370000 19240519 5380 312370000 19250523 6760 312370000 19260421 3860 312370000 19270613 7060 312370000 19280528 7820 312370000 19290526 5300 312370000 19300601 2920 312370000 19310518 5140 5.57 312370000 19320524 5710 5.92 312370000 19330618 8280 7.00 312370000 19331225 4750 5.42 312370000 19350525 4650 5.33 312370000 19360517 6180 5.97 312370000 19370529 3380 4.71 312370000 19380530 4350 5.26 312370000 19390506 4350 5.24 312370000 19400527 3650 4.82 312370000 19410603 2120 3.88 312370000 19420528 3980 5.06 312370000 19430621 5570 5.82 312370000 19440520 3120 4.53 312370000 19450604 3940 5.01 312370000 19460530 4610 5.34 312370000 19470511 5960 6.11 312370000 19480524 8400 7.12 312370000 19490517 5130 5.64 312370000 19500623 5900 6.01 312370000 19510514 4750 5.45 312370000 19520430 4590 5.37 312370000 19530605 5610 5.87 312370000 19540522 5630 5.88 312370000 19550616 5060 5.60 312370000 19560604 6790 6.42 312370000 19570508 4960 5.55 312370000 19580527 5920 6.05 312370000 19590622 7520 6.72 312370000 19600606 5500 5.75 312370000 19610529 6810 6.38 312370000 19620531 4530 5.26 312370000 19630602 3740 4.84 312370000 19640610 8100 6.98 312370000 19650620 6010 6.00 312370000 19660602 4520 5.25 312370000 19670525 5540 5.77 312370000 19680625 4480 5.23 312370000 19690601 4380 5.18 312370000 19700608 7230 6.58 312370000 19710531 5440 5.72 312370000 19720612 6540 6.25 312370000 19730521 3770 4.86 312370000 19740620 8890 7.34 312370000 19750617 5410 5.74 312370000 19760516 4860 5.45 312370000 19770504 3430 4.69 312370000 19780610 5320 5.69 312370000 19790528 5840 5.95 312370000 19800527 5810 5.88 Page 1 12370000peak.txt 312370000 19810527 5520 5.75 312370000 19820619 5710 5.83 312370000 19830601 4790 5.45 312370000 19840626 4820 5.43 312370000 19850610 5160 5.59 312370000 19860601 6340 6.15 312370000 19870503 4220 5.14 312370000 19880609 3170 4.57 312370000 19890512 4940 5.49 312370000 19900628 4670 5.36 312370000 19910521 4570 5.31 312370000 19920510 3140 4.55 312370000 19930518 5050 5.47 312370000 19940514 3760 4.90 312370000 19950607 3910 4.98 312370000 19960611 5810 5.88 312370000 19970518 8520 7.27 312370000 19980529 3860 4.99 312370000 19990529 3820 5.06 312370000 20000617 3580 4.82 312370000 20010530 3910 5.08 312370000 20020603 4480 5.27 312370000 20030602 5290 5.65 312370000 20040608 3150 4.58 312370000 20050605 4860 5.45 312370000 20060618 6170 6.06 312370000 20070607 3510 4.76 312370000 20080522 5930 5.94 312370000 20090602 4990 5.51 312370000 20100606 4770 5.41 312370000 20110702 6710 6.20 312370000 20120429 5710 5.73 312370000 20130515 6210 6.08

Page 2 12370000PEAK.PRT 1 Program PeakFq U. S. GEOLOGICAL SURVEY Seq.002.000 Version 7.1 Annual peak flow frequency analysis Run Date / Time 3/14/2014 11/26/2014 14:51 --- PROCESSING OPTIONS --- Plot option = None Basin char output = None Print option = Yes Debug print = No Input peaks listing = Long Input peaks format = WATSTORE peak file Input files used: peaks (ascii) - C:\Users\cn0077\Desktop\12370000PEAK.TXT

specifications - C:\Users\cn0077\Desktop\PKFQWPSF.TMP

Output file(s): main - C:\Users\cn0077\Desktop\12370000PEAK.PRT

Program PeakFq U. S. GEOLOGICAL SURVEY Seq.001.001 Version 7.1 Annual peak flow frequency analysis Run Date / Time 3/14/2014 11/26/2014 14:51

Station - 12370000 Swan River near Bigfork, MT

I N P U T D A T A S U M M A R Y

Number of peaks in record = 92 Peaks not used in analysis = 0 Systematic peaks in analysis = 92 Historic peaks in analysis = 0 Beginning Year = 1922 Ending Year = 2013 Historical Period Length = 0 Generalized skew = 0.314 Standard error = 0.640 Mean Square error = 0.410 Skew option = WEIGHTED Gage base discharge = 0.0 User supplied high outlier threshold = -- User supplied PILF (LO) criterion = -- Plotting position parameter = 0.00 Type of analysis BULL.17B PILF (LO) Test Method GBT Perception Thresholds = Not Applicable Interval Data = Not Applicable

********* NOTICE -- Preliminary machine computations. ********* ********* User responsible for assessment and interpretation. *********

WCF134I-NO SYSTEMATIC PEAKS WERE BELOW GAGE BASE. 0.0 WCF198I-LOW OUTLIERS BELOW FLOOD BASE WERE DROPPED. 1 2269.5 WCF163I-NO HIGH OUTLIERS OR HISTORIC PEAKS EXCEEDED HHBASE. 11033.7 Page 1 12370000PEAK.PRT

Kendall's Tau Parameters MEDIAN No. of TAU P-VALUE SLOPE PEAKS ------SYSTEMATIC RECORD -0.055 0.438 -4.344 92

Program PeakFq U. S. GEOLOGICAL SURVEY Seq.001.002 Version 7.1 Annual peak flow frequency analysis Run Date / Time 3/14/2014 11/26/2014 14:51

Station - 12370000 Swan River near Bigfork, MT

ANNUAL FREQUENCY CURVE PARAMETERS -- LOG-PEARSON TYPE III

FLOOD BASE LOGARITHMIC ------EXCEEDANCE STANDARD DISCHARGE PROBABILITY MEAN DEVIATION SKEW ------SYSTEMATIC RECORD 0.0 1.0000 3.6996 0.1150 -0.268 BULL.17B ESTIMATE 2269.5 0.9891 3.7021 0.1092 0.056

BULL.17B ESTIMATE OF MSE OF AT-SITE SKEW 0.0589

ANNUAL FREQUENCY CURVE -- DISCHARGES AT SELECTED EXCEEDANCE PROBABILITIES

ANNUAL <-- FOR BULLETIN 17B ESTIMATES --> EXCEEDANCE BULL.17B SYSTEMATIC VARIANCE 95% CONFIDENCE INTERVALS PROBABILITY ESTIMATE RECORD OF EST. LOWER UPPER

0.9950 2369. ------0.9900 2569. ------0.9500 3344. 3178. ---- 3109.0 3553.0 0.9000 3655. 3543. ---- 3429.0 3858.0 0.8000 4074. 4024. ---- 3859.0 4272.0 0.6667 4511. 4513. ---- 4303.0 4713.0 0.5000 5025. 5067. ---- 4811.0 5248.0 0.4292 5256. 5309. ---- 5034.0 5494.0 0.2000 6219. 6275. ---- 5930.0 6563.0 0.1000 6961. 6971. ---- 6594.0 7421.0 0.0400 7858. 7761. ---- 7376.0 8486.0 0.0200 8503. 8297. ---- 7927.0 9267.0 0.0100 9132. 8795. ---- 8459.0 10040.0 0.0050 9751. 9265. ---- 8978.0 10800.0 0.0020 10560. 9850. ---- 9650.0 11820.0 1

Program PeakFq U. S. GEOLOGICAL SURVEY Seq.001.003 Version 7.1 Annual peak flow frequency analysis Run Date / Time 3/14/2014 11/26/2014 14:51

Station - 12370000 Swan River near Bigfork, MT Page 2 12370000PEAK.PRT

I N P U T D A T A L I S T I N G

WATER PEAK PEAKFQ YEAR VALUE CODES REMARKS 1922 5500.0 1923 4780.0 1924 5380.0 1925 6760.0 1926 3860.0 1927 7060.0 1928 7820.0 1929 5300.0 1930 2920.0 1931 5140.0 1932 5710.0 1933 8280.0 1934 4750.0 1935 4650.0 1936 6180.0 1937 3380.0 1938 4350.0 1939 4350.0 1940 3650.0 1941 2120.0 1942 3980.0 1943 5570.0 1944 3120.0 1945 3940.0 1946 4610.0 1947 5960.0 1948 8400.0 1949 5130.0 1950 5900.0 1951 4750.0 1952 4590.0 1953 5610.0 1954 5630.0 1955 5060.0 1956 6790.0 1957 4960.0 1958 5920.0 1959 7520.0 1960 5500.0 1961 6810.0 1962 4530.0 1963 3740.0 1964 8100.0 1965 6010.0 1966 4520.0 1967 5540.0 1968 4480.0 1969 4380.0 1970 7230.0 1971 5440.0 1972 6540.0 1973 3770.0 1974 8890.0 1975 5410.0 1976 4860.0 1977 3430.0 Page 3 12370000PEAK.PRT 1978 5320.0 1979 5840.0 1980 5810.0 1981 5520.0 1982 5710.0 1983 4790.0 1984 4820.0 1985 5160.0 1986 6340.0 1987 4220.0 1988 3170.0 1989 4940.0 1990 4670.0 1991 4570.0 1992 3140.0 1993 5050.0 1994 3760.0 1995 3910.0 1996 5810.0 1997 8520.0 1998 3860.0 1999 3820.0 2000 3580.0 2001 3910.0 2002 4480.0 2003 5290.0 2004 3150.0 2005 4860.0 2006 6170.0 2007 3510.0 2008 5930.0 2009 4990.0 2010 4770.0 2011 6710.0 2012 5710.0 2013 6210.0

Explanation of peak discharge qualification codes

PeakFQ NWIS CODE CODE DEFINITION

D 3 Dam failure, non-recurrent flow anomaly G 8 Discharge greater than stated value X 3+8 Both of the above L 4 Discharge less than stated value K 6 OR C Known effect of regulation or urbanization H 7 Historic peak - Minus-flagged discharge -- Not used in computation -8888.0 -- No discharge value given - Minus-flagged water year -- Historic peak used in computation

Program PeakFq U. S. GEOLOGICAL SURVEY Seq.001.004 Version 7.1 Annual peak flow frequency analysis Run Date / Time 3/14/2014 11/26/2014 14:51

Page 4 12370000PEAK.PRT Station - 12370000 Swan River near Bigfork, MT

EMPIRICAL FREQUENCY CURVES -- WEIBULL PLOTTING POSITIONS WATER RANKED SYSTEMATIC B17B YEAR DISCHARGE RECORD ESTIMATE 1974 8890.0 0.0108 0.0108 1997 8520.0 0.0215 0.0215 1948 8400.0 0.0323 0.0323 1933 8280.0 0.0430 0.0430 1964 8100.0 0.0538 0.0538 1928 7820.0 0.0645 0.0645 1959 7520.0 0.0753 0.0753 1970 7230.0 0.0860 0.0860 1927 7060.0 0.0968 0.0968 1961 6810.0 0.1075 0.1075 1956 6790.0 0.1183 0.1183 1925 6760.0 0.1290 0.1290 2011 6710.0 0.1398 0.1398 1972 6540.0 0.1505 0.1505 1986 6340.0 0.1613 0.1613 2013 6210.0 0.1720 0.1720 1936 6180.0 0.1828 0.1828 2006 6170.0 0.1935 0.1935 1965 6010.0 0.2043 0.2043 1947 5960.0 0.2151 0.2151 2008 5930.0 0.2258 0.2258 1958 5920.0 0.2366 0.2366 1950 5900.0 0.2473 0.2473 1979 5840.0 0.2581 0.2581 1980 5810.0 0.2688 0.2688 1996 5810.0 0.2796 0.2796 1932 5710.0 0.2903 0.2903 1982 5710.0 0.3011 0.3011 2012 5710.0 0.3118 0.3118 1954 5630.0 0.3226 0.3226 1953 5610.0 0.3333 0.3333 1943 5570.0 0.3441 0.3441 1967 5540.0 0.3548 0.3548 1981 5520.0 0.3656 0.3656 1922 5500.0 0.3763 0.3763 1960 5500.0 0.3871 0.3871 1971 5440.0 0.3978 0.3978 1975 5410.0 0.4086 0.4086 1924 5380.0 0.4194 0.4194 1978 5320.0 0.4301 0.4301 1929 5300.0 0.4409 0.4409 2003 5290.0 0.4516 0.4516 1985 5160.0 0.4624 0.4624 1931 5140.0 0.4731 0.4731 1949 5130.0 0.4839 0.4839 1955 5060.0 0.4946 0.4946 1993 5050.0 0.5054 0.5054 2009 4990.0 0.5161 0.5161 1957 4960.0 0.5269 0.5269 1989 4940.0 0.5376 0.5376 1976 4860.0 0.5484 0.5484 2005 4860.0 0.5591 0.5591 1984 4820.0 0.5699 0.5699 1983 4790.0 0.5806 0.5806 1923 4780.0 0.5914 0.5914 2010 4770.0 0.6022 0.6022 Page 5 12370000PEAK.PRT 1934 4750.0 0.6129 0.6129 1951 4750.0 0.6237 0.6237 1990 4670.0 0.6344 0.6344 1935 4650.0 0.6452 0.6452 1946 4610.0 0.6559 0.6559 1952 4590.0 0.6667 0.6667 1991 4570.0 0.6774 0.6774 1962 4530.0 0.6882 0.6882 1966 4520.0 0.6989 0.6989 1968 4480.0 0.7097 0.7097 2002 4480.0 0.7204 0.7204 1969 4380.0 0.7312 0.7312 1938 4350.0 0.7419 0.7419 1939 4350.0 0.7527 0.7527 1987 4220.0 0.7634 0.7634 1942 3980.0 0.7742 0.7742 1945 3940.0 0.7849 0.7849 1995 3910.0 0.7957 0.7957 2001 3910.0 0.8065 0.8065 1926 3860.0 0.8172 0.8172 1998 3860.0 0.8280 0.8280 1999 3820.0 0.8387 0.8387 1973 3770.0 0.8495 0.8495 1994 3760.0 0.8602 0.8602 1963 3740.0 0.8710 0.8710 1940 3650.0 0.8817 0.8817 2000 3580.0 0.8925 0.8925 2007 3510.0 0.9032 0.9032 1977 3430.0 0.9140 0.9140 1937 3380.0 0.9247 0.9247 1988 3170.0 0.9355 0.9355 2004 3150.0 0.9462 0.9462 1992 3140.0 0.9570 0.9570 1944 3120.0 0.9677 0.9677 1930 2920.0 0.9785 0.9785 1941 2120.0 0.9892 0.9892 1

End PeakFQ analysis. Stations processed : 1 Number of errors : 0 Stations skipped : 0 Station years : 92

Data records may have been ignored for the stations listed below. (Card type must be Y, Z, N, H, I, 2, 3, 4, or *.) (2, 4, and * records are ignored.)

For the station below, the following records were ignored:

FINISHED PROCESSING STATION: 12370000 USGS Swan River near Bigfork, MT

For the station below, the following records were ignored:

FINISHED PROCESSING STATION:

Page 6 12369200peak.txt Z12369200 USGS H12369200 4725201134015003030063SW1701021169.1 4015 N12369200 Swan River near Condon MT Y12369200 312369200 19730520 560 3.45 312369200 19740618 1540 4.88 312369200 19750616 1020 4.22 312369200 19760515 994 4.19 312369200 19770503 616 3.50 312369200 19780609 821 3.87 312369200 19790527 960 4.10 312369200 19800526 1090 4.29 312369200 19810607 876 3.85 312369200 19820617 1060 4.25 312369200 19830530 691 3.63 312369200 19840622 846 3.91 312369200 19850608 864 3.98 312369200 19860531 1090 4.37 312369200 19870501 665 3.61 312369200 19880608 591 3.41 312369200 19890511 942 4.09 312369200 19900626 793 3.82 312369200 19910519 720 3.74 312369200 19920509 511 3.29

Page 1 12369200PEAK.PRT 1 Program PeakFq U. S. GEOLOGICAL SURVEY Seq.002.000 Version 7.1 Annual peak flow frequency analysis Run Date / Time 3/14/2014 11/26/2014 14:53 --- PROCESSING OPTIONS --- Plot option = None Basin char output = None Print option = Yes Debug print = No Input peaks listing = Long Input peaks format = WATSTORE peak file Input files used: peaks (ascii) - C:\Users\cn0077\Desktop\12369200PEAK.TXT

specifications - C:\Users\cn0077\Desktop\PKFQWPSF.TMP

Output file(s): main - C:\Users\cn0077\Desktop\12369200PEAK.PRT

Program PeakFq U. S. GEOLOGICAL SURVEY Seq.001.001 Version 7.1 Annual peak flow frequency analysis Run Date / Time 3/14/2014 11/26/2014 14:53

Station - 12369200 Swan River near Condon MT

I N P U T D A T A S U M M A R Y

Number of peaks in record = 20 Peaks not used in analysis = 0 Systematic peaks in analysis = 20 Historic peaks in analysis = 0 Beginning Year = 1973 Ending Year = 1992 Historical Period Length = 0 Generalized skew = 0.125 Standard error = 0.640 Mean Square error = 0.410 Skew option = WEIGHTED Gage base discharge = 0.0 User supplied high outlier threshold = -- User supplied PILF (LO) criterion = -- Plotting position parameter = 0.00 Type of analysis BULL.17B PILF (LO) Test Method GBT Perception Thresholds = Not Applicable Interval Data = Not Applicable

********* NOTICE -- Preliminary machine computations. ********* ********* User responsible for assessment and interpretation. *********

WCF134I-NO SYSTEMATIC PEAKS WERE BELOW GAGE BASE. 0.0 WCF195I-NO LOW OUTLIERS WERE DETECTED BELOW CRITERION. 435.0 WCF163I-NO HIGH OUTLIERS OR HISTORIC PEAKS EXCEEDED HHBASE. 1593.0 Page 1 12369200PEAK.PRT

Kendall's Tau Parameters MEDIAN No. of TAU P-VALUE SLOPE PEAKS ------SYSTEMATIC RECORD -0.268 0.105 -16.651 20

Program PeakFq U. S. GEOLOGICAL SURVEY Seq.001.002 Version 7.1 Annual peak flow frequency analysis Run Date / Time 3/14/2014 11/26/2014 14:53

Station - 12369200 Swan River near Condon MT

ANNUAL FREQUENCY CURVE PARAMETERS -- LOG-PEARSON TYPE III

FLOOD BASE LOGARITHMIC ------EXCEEDANCE STANDARD DISCHARGE PROBABILITY MEAN DEVIATION SKEW ------SYSTEMATIC RECORD 0.0 1.0000 2.9204 0.1182 0.110 BULL.17B ESTIMATE 0.0 1.0000 2.9204 0.1182 0.116

BULL.17B ESTIMATE OF MSE OF AT-SITE SKEW 0.2538

ANNUAL FREQUENCY CURVE -- DISCHARGES AT SELECTED EXCEEDANCE PROBABILITIES

ANNUAL <-- FOR BULLETIN 17B ESTIMATES --> EXCEEDANCE BULL.17B SYSTEMATIC VARIANCE 95% CONFIDENCE INTERVALS PROBABILITY ESTIMATE RECORD OF EST. LOWER UPPER

0.9950 425.4 424.8 ---- 324.6 503.1 0.9900 452.4 451.9 ---- 352.2 529.3 0.9500 537.0 536.7 ---- 441.0 611.3 0.9000 589.5 589.4 ---- 497.3 662.7 0.8000 661.2 661.2 ---- 574.3 734.7 0.6667 737.3 737.4 ---- 654.6 815.0 0.5000 828.1 828.3 ---- 746.1 918.4 0.4292 869.4 869.6 ---- 785.7 968.5 0.2000 1045. 1045. ---- 940.6 1202.0 0.1000 1184. 1183. ---- 1052.0 1406.0 0.0400 1355. 1354. ---- 1182.0 1674.0 0.0200 1480. 1479. ---- 1273.0 1881.0 0.0100 1604. 1603. ---- 1360.0 2092.0 0.0050 1728. 1726. ---- 1446.0 2310.0 0.0020 1893. 1889. ---- 1558.0 2609.0 1

Program PeakFq U. S. GEOLOGICAL SURVEY Seq.001.003 Version 7.1 Annual peak flow frequency analysis Run Date / Time 3/14/2014 11/26/2014 14:53

Station - 12369200 Swan River near Condon MT Page 2 12369200PEAK.PRT

I N P U T D A T A L I S T I N G

WATER PEAK PEAKFQ YEAR VALUE CODES REMARKS 1973 560.0 1974 1540.0 1975 1020.0 1976 994.0 1977 616.0 1978 821.0 1979 960.0 1980 1090.0 1981 876.0 1982 1060.0 1983 691.0 1984 846.0 1985 864.0 1986 1090.0 1987 665.0 1988 591.0 1989 942.0 1990 793.0 1991 720.0 1992 511.0

Explanation of peak discharge qualification codes

PeakFQ NWIS CODE CODE DEFINITION

- Minus-flagged discharge -- Not used in computation -8888.0 -- No discharge value given - Minus-flagged water year -- Historic peak used in computation

Program PeakFq U. S. GEOLOGICAL SURVEY Seq.001.004 Version 7.1 Annual peak flow frequency analysis Run Date / Time 3/14/2014 11/26/2014 14:53

Station - 12369200 Swan River near Condon MT

EMPIRICAL FREQUENCY CURVES -- WEIBULL PLOTTING POSITIONS WATER RANKED SYSTEMATIC B17B YEAR DISCHARGE RECORD ESTIMATE 1974 1540.0 0.0476 0.0476 1980 1090.0 0.0952 0.0952 Page 3 12369200PEAK.PRT 1986 1090.0 0.1429 0.1429 1982 1060.0 0.1905 0.1905 1975 1020.0 0.2381 0.2381 1976 994.0 0.2857 0.2857 1979 960.0 0.3333 0.3333 1989 942.0 0.3810 0.3810 1981 876.0 0.4286 0.4286 1985 864.0 0.4762 0.4762 1984 846.0 0.5238 0.5238 1978 821.0 0.5714 0.5714 1990 793.0 0.6190 0.6190 1991 720.0 0.6667 0.6667 1983 691.0 0.7143 0.7143 1987 665.0 0.7619 0.7619 1977 616.0 0.8095 0.8095 1988 591.0 0.8571 0.8571 1973 560.0 0.9048 0.9048 1992 511.0 0.9524 0.9524 1

End PeakFQ analysis. Stations processed : 1 Number of errors : 0 Stations skipped : 0 Station years : 20

Data records may have been ignored for the stations listed below. (Card type must be Y, Z, N, H, I, 2, 3, 4, or *.) (2, 4, and * records are ignored.)

For the station below, the following records were ignored:

FINISHED PROCESSING STATION: 12369200 USGS Swan River near Condon MT

For the station below, the following records were ignored:

FINISHED PROCESSING STATION:

Page 4 Montana Flood-Frequency and Basin-Characteristic Data http://wy-mt.water.usgs.gov/freq?page_type=gen_stats_4&estimate_nm...

Estimate Flood Discharges at Ungaged Sites in Montana -- (continued)

Summary of Estimation Parameters Selected:

Name for this estimation: Default Region: West Weighted estimate based on Basin and Climatic Estimation method: Characteristics, Active-channel width, and Bankfull width Drainage area in square miles: 69.1 Mean annual precipitation in inches: 54 Percent basin forested: 61 Width of active channel in feet: 72 Width of bank full channel in feet: 84

Flood Discharge Estimation:

(In the Flood Discharge table, RI is the Recurrence Interval, in years; STD ERR is the

Standard Error; and 90% PRED. INTERVAL is the 90% Prediction Interval, in cubic feet per second)

METHOD: Regression on basin characteristics Flood frequency estimates for Default West Region : A = 69.10: P = 54.00: F = 61.00 RI DISCHARGE STD ERR OF 90% PRED. INTERVAL (cfs) PREDICTION(%) 2 988. 60.3 392.1 2490. 5 1390. 55.2 591.5 3280. 10 1660. 54.1 714.7 3850. 25 2010. 54.3 864.0 4680. 50 2300. 55.7 969.3 5450. 100 2610. 58.2 1060.0 6390. 200 2950. 61.9 1150.0 7590. 500 3410. 67.5 1230.0 9430.

METHOD: Regression on active channel width

1 of 2 4/28/2015 11:00 AM Montana Flood-Frequency and Basin-Characteristic Data http://wy-mt.water.usgs.gov/freq?page_type=gen_stats_4&estimate_nm...

Flood frequency estimates for Default West Region: WAC = 72.00 RI DISCHARGE STD ERR OF 90% PRED. INTERVAL (cfs) PREDICTION(%) 2 1930. 63.3 734.1 5050. 5 2680. 60.9 1050.0 6800. 10 3160. 61.2 1240.0 8060. 25 3780. 63.1 1450.0 9890. 50 4290. 65.9 1580.0 11600. 100 4820. 69.3 1710.0 13600. 200 5390. 73.4 1810.0 16000. 500 6140. 79.4 1920.0 19600.

METHOD: Regression on bank full channel width Flood frequency estimates for Default West Region: WBF = 84.00 RI DISCHARGE STD ERR OF 90% PRED. INTERVAL (cfs) PREDICTION(%) 2 1820. 72.1 620.1 5320. 5 2550. 67.3 925.1 7030. 10 3030. 66.4 1110.0 8250. 25 3640. 67.2 1320.0 10000. 50 4130. 69.1 1460.0 11700. 100 4650. 71.9 1590.0 13600. 200 5210. 75.8 1700.0 16000. 500 5950. 81.3 1820.0 19400.

METHOD: Combined methods 1, 2 and 3 Flood frequency estimates for Default Region 1 RI DISCHARGE STD ERR OF 90% PRED. INTERVAL (cfs) PREDICTION(%) 2 1350. 49.8 617.5 2950. 5 1850. 46.8 881.7 3860. 10 2160. 46.9 1030.0 4520. 25 2550. 48.8 1180.0 5490. 50 2850. 50.8 1280.0 6300. 100 3170. 54.1 1370.0 7340. 200 3530. 58.7 1430.0 8710. 500 4010. 65.0 1500.0 10800.

Montana Flood-Frequency and Basin-Characteristic Data Retrieved on: 2015.04.28 12:59:10 Department of the Interior, U.S. Geological Survey Privacy Statement || Disclaimer || Accessibility || FOIA

0.044

2 of 2 4/28/2015 11:00 AM Site Number 12370000 12369200 Weighted Estimate Two Station MOVE.1 Weighted Basin Characteristics Basin Basin (equ. 22, p. 36) Comparison - Ext. Record Ext. on Estimation Estimate Regression + Channel Systematic Characteristics Systematic Characteristics Systematic + Basin on 12369200 12369200 Method (equ. 22, p. Characteristics Regression Regression Characteristics (correlation (correlation 36) Regression Regression coef=0.94) coef.=0.94) YOR 92 20 A 671 69.1 E 5020 5830 E6000 26 39 F 89.8 61 P2354 Wac 165 72

Basin Characteristics Wbf 185 84 10yr 6961 3224 6853 1180 1671 1216 2160 1374 1210 25yr 7858 3898 7703 1360 1972 1421 2550 1648 1373 50yr 8503 4514 8329 1480 2260 1567 2850 1856 1489 100yr 9132 5133 8946 1600 2580 1715 3170 2068 1603 500yr 10560 6693 10375 1890 3437 2075 4010 2584 1862

Ungaged basin Computed DA Average SEP, in coeff. for DA adj. coef. Based on Discharge A1 A2 P1 F1 SEP in log units percent EYR (table 13) two gages 10yr 3.63 0.86 1.25 -0.605 0.224 54.3 1.9 0.798 0.761 25yr 8.5 0.835 1.14 -0.639 0.222 54.6 2.7 0.776 0.743 50yr 13.2 0.823 1.09 -0.652 0.227 56 3.1 0.761 0.735 100yr 18.7 0.812 1.06 -0.664 0.235 58.5 3.4 0.747 0.727 500yr 35.4 0.792 1.02 -0.69 0.267 67.9 3.7 0.717 0.708

Gage Transfer - Interpolation Transfer based on Log Interpolation btwn Gages - Two Station Transfer based on Log Interpolation btwn Gages - MOVE.1 @ 12369200 Comparison @ 12369200 Cold Creek Glacier Creek Buck Creek Holland Upstream Cold Creek Glacier Creek Holland Creek Upstream Study Buck Creek Subbasin Subbasin Subbasin Subbasin Creek Study Subbasin Subbasin Subbasin Extents Subbasin 271.1 230.9 137.6 109.7 70.6 271.1 230.9 137.6 109.7 70.6 3645 3250 2246 1911 1395 3465 3062 2056 1727 1230 4215 3775 2645 2264 1672 3920 3465 2329 1958 1396 4635 4163 2943 2529 1883 4245 3754 2524 2122 1514 5052 4549 3244 2797 2097 4564 4036 2716 2283 1630 6024 5454 3958 3440 2618 5287 4677 3150 2650 1893 Long-Term Record Short-Term Record USGS Gage 12370000 USGS Gage 12369200

All Years: 1922-2013 All Years: 1973-1992 N = 92 N = 20

∑Xi = 340.3644322

x3̄ = 3.699613394 x̄ = 2.920370107 σX = 0.114983064 σX = 0.118174707 Sy3

Concurrent Years: 1973-1992 Concurrent Years: 1973-1992

N1 =20 N3 =20

∑Xi = 73.75982947 ∑Yi = 58.40740214 2 2 ∑Xi = 272.2427945 ∑Yi = 3411.424625

x̄1 = 3.687991473 ȳ1 = 2.920370107 σX = 0.106911763 Sx1 σy = 0.118174707 Sy1

∑X1Y1 = 215.6319562 Non-Concurrent Years: 1922-1972, 1993-2013

N2 =72 Equ. 7-1 b = 1.040441349

∑Xi = 266.6046027

x2̄ = 3.702841705 Equ. 7-2 r = 0.941279412 σX = 0.117634934 Sx2 Equ. 7-3 Var(ȳ) = 0.000217754 USGS Gage 12370000

Ranked Record Flows Equ. 7-4 is r > 0.23570226 ? Since r > rmean, improvement to the mean is possible 2 Water Year Date Gage Height (feet) Q (cfs) X=log10Q X Water Ranked Systematic B17B Year Discharge Record Estimate 1922 Jun. 08, 1922 5,500 3.740363 13.99031 1974 8890 0.0108 0.0108 Equ. 7-5a ȳ = 2.932462033 1923 Jun. 14, 1923 4,780 3.679428 13.53819 1997 8520 0.0215 0.0215 Equ. 7-5b ȳ = 2.932462033 1924 19-May-24 5,380 3.730782 13.91874 1948 8400 0.0323 0.0323

1925 23-May-25 6,760 3.829947 14.66849 1933 8280 0.0430 0.043 Equ. 7-6 Var(y3̄ ) = 0.000698263

1926 Apr. 21, 1926 3,860 3.586587 12.86361 1964 8100 0.0538 0.0538 is Var(y3̄ ) < ȳ ? if yes, then use Var(y3̄ ) Final estimate of mean = 2.932462 1927 Jun. 13, 1927 7,060 3.848805 14.8133 1928 7820 0.0645 0.0645 if no, then use ȳ 1928 28-May-28 7,820 3.893207 15.15706 1959 7520 0.0753 0.0753

1929 26-May-29 5,300 3.724276 13.87023 1970 7230 0.0860 0.086 Equ. 7-7 Ne = 64.13318478 (Equivalent years of record for the mean) 1930 Jun. 01, 1930 2,920 3.465383 12.00888 1927 7060 0.0968 0.0968

1931 18-May-31 5.57 5,140 3.710963 13.77125 1961 6810 0.1075 0.1075 Equ. 7-9 |r| = 0.941279412 > 0.469592 Since r > rStandardDeviation, improvement to the standard deviation is possible 1932 24-May-32 5.92 5,710 3.756636 14.11231 1956 6790 0.1183 0.1183 where, 1933 Jun. 18, 1933 7 8,280 3.91803 15.35096 1925 6760 0.1290 0.129 A = -11.70380623 1934 Dec. 25, 1933 5.42 4,750 3.676694 13.51808 2011 6710 0.1398 0.1398 B = 1.995847751 1935 25-May-35 5.33 4,650 3.667453 13.45021 1972 6540 0.1505 0.1505 C = 0.129011109 1936 17-May-36 5.97 6,180 3.790988 14.37159 1986 6340 0.1613 0.1613 2 1937 29-May-37 4.71 3,380 3.528917 12.45325 2013 6210 0.1720 0.172 Equ. 7-8 Var(Sy ) = 8.16729E-06 1938 30-May-38 5.26 4,350 3.638489 13.2386 1936 6180 0.1828 0.1828 2 1939 6-May-39 5.24 4,350 3.638489 13.2386 2006 6170 0.1935 0.1935 Equ. 7-10 Sy = 0.025233258

1940 27-May-40 4.82 3,650 3.562293 12.68993 1965 6010 0.2043 0.2043 Sy = 0.158849798 1941 Jun. 03, 1941 3.88 2,120 3.326336 11.06451 1947 5960 0.2151 0.2151 2 1942 28-May-42 5.06 3,980 3.599883 12.95916 2008 5930 0.2258 0.2258 Equ. 7-11 Var(Sy3 ) = 2.05293E-05 2 2 1943 Jun. 21, 1943 5.82 5,570 3.745855 14.03143 1958 5920 0.2366 0.2366 is Var(Sy3 ) < Var(Sy ) ? if yes, then use Var(Sy3)

1944 20-May-44 4.53 3,120 3.494155 12.20912 1950 5900 0.2473 0.2473 in no, then use Var(Sy) 1945 Jun. 04, 1945 5.01 3,940 3.595496 12.92759 1979 5840 0.2581 0.2581 1946 30-May-46 5.34 4,610 3.663701 13.4227 1980 5810 0.2688 0.2688 Final estimate of standard deviation = 0.1588498 1947 11-May-47 6.11 5,960 3.775246 14.25248 1996 5810 0.2796 0.2796 1948 24-May-48 7.12 8,400 3.924279 15.39997 1932 5710 0.2903 0.2903 1949 17-May-49 5.64 5,130 3.710117 13.76497 1982 5710 0.3011 0.3011 1950 Jun. 23, 1950 6.01 5,900 3.770852 14.21932 2012 5710 0.3118 0.3118 Equ. 4a G = 0.11007295 1951 14-May-51 5.45 4,750 3.676694 13.51808 1954 5630 0.3226 0.3226 Equ. 4b G = 0.110072949 1952 Apr. 30, 1952 5.37 4,590 3.661813 13.40887 1953 5610 0.3333 0.3333 1953 Jun. 05, 1953 5.87 5,610 3.748963 14.05472 1943 5570 0.3441 0.3441 1954 22-May-54 5.88 5,630 3.750508 14.06631 1967 5540 0.3548 0.3548 1955 Jun. 16, 1955 5.6 5,060 3.704151 13.72073 1981 5520 0.3656 0.3656 1956 Jun. 04, 1956 6.42 6,790 3.83187 14.68323 1922 5500 0.3763 0.3763 1957 8-May-57 5.55 4,960 3.695482 13.65658 1960 5500 0.3871 0.3871 1958 27-May-58 6.05 5,920 3.772322 14.23041 1971 5440 0.3978 0.3978 1959 Jun. 22, 1959 6.72 7,520 3.876218 15.02506 1975 5410 0.4086 0.4086 1960 Jun. 06, 1960 5.75 5,500 3.740363 13.99031 1924 5380 0.4194 0.4194 1961 29-May-61 6.38 6,810 3.833147 14.69302 1978 5320 0.4301 0.4301 1962 31-May-62 5.26 4,530 3.656098 13.36705 1929 5300 0.4409 0.4409 1963 Jun. 02, 1963 4.84 3,740 3.572872 12.76541 2003 5290 0.4516 0.4516 1964 Jun. 10, 1964 6.98 8,100 3.908485 15.27626 1985 5160 0.4624 0.4624 1965 Jun. 20, 1965 6 6,010 3.778874 14.27989 1931 5140 0.4731 0.4731 1966 Jun. 02, 1966 5.25 4,520 3.655138 13.36004 1949 5130 0.4839 0.4839 1967 25-May-67 5.77 5,540 3.74351 14.01387 1955 5060 0.4946 0.4946 1968 Jun. 25, 1968 5.23 4,480 3.651278 13.33183 1993 5050 0.5054 0.5054 1969 Jun. 01, 1969 5.18 4,380 3.641474 13.26033 2009 4990 0.5161 0.5161 1970 Jun. 08, 1970 6.58 7,230 3.859138 14.89295 1957 4960 0.5269 0.5269 USGS Gage 12369200 Water Gage Height B17B Date Q (cfs) Y2 X*Y (Y-x̄)^3 Y3 1971 31-May-71 5.72 5,440 3.735599 13.9547 1989 4940 0.5376 0.5376 Year (feet) Y=log10Q Estimate 1972 Jun. 12, 1972 6.25 6,540 3.815578 14.55863 1976 4860 0.5484 0.5484 1973 20-May-73 3.45 560 2.748188 7.552537 9.828458 -0.005104625 20.75579 0.9048 1973 21-May-73 4.86 3,770 3.576341 12.79022 2005 4860 0.5591 0.5591 1974 Jun. 18, 1974 4.88 1,540 3.187521 10.16029 12.58721 0.019066393 32.38613 0.0476 1974 Jun. 20, 1974 7.34 8,890 3.948902 15.59383 1984 4820 0.5699 0.5699 1975 Jun. 16, 1975 4.22 1,020 3.0086 9.051675 11.2317 0.000686831 27.23287 0.2381 1975 Jun. 17, 1975 5.74 5,410 3.733197 13.93676 1983 4790 0.5806 0.5806 1976 15-May-76 4.19 994 2.997386 8.984325 11.05027 0.000456823 26.92949 0.2857 1976 16-May-76 5.45 4,860 3.686636 13.59129 1923 4780 0.5914 0.5914 1977 3-May-77 3.5 616 2.789581 7.781761 9.861988 -0.002237266 21.70785 0.8095 1977 4-May-77 4.69 3,430 3.535294 12.4983 2010 4770 0.6022 0.6022 1978 Jun. 09, 1978 3.87 821 2.914343 8.493396 10.85859 -2.18924E-07 24.75267 0.5714 1978 Jun. 10, 1978 5.69 5,320 3.725912 13.88242 1934 4750 0.6129 0.6129 1979 27-May-79 4.1 960 2.982271 8.893942 11.23246 0.00023719 26.52415 0.3333 1979 28-May-79 5.95 5,840 3.766413 14.18587 1951 4750 0.6237 0.6237 1980 26-May-80 4.29 1,090 3.037426 9.22596 11.43341 0.00160393 28.02317 0.0952 1980 27-May-80 5.88 5,810 3.764176 14.16902 1990 4670 0.6344 0.6344 1981 Jun. 07, 1981 3.85 876 2.942504 8.65833 11.01067 1.08438E-05 25.47717 0.4286 1981 27-May-81 5.75 5,520 3.741939 14.00211 1935 4650 0.6452 0.6452 1982 Jun. 17, 1982 4.25 1,060 3.025306 9.152476 11.36497 0.001155502 27.68904 0.1905 1982 Jun. 19, 1982 5.83 5,710 3.756636 14.11231 1946 4610 0.6559 0.6559 1983 30-May-83 3.63 691 2.839478 8.062636 10.45023 -0.000529319 22.89368 0.7143 1983 Jun. 01, 1983 5.45 4,790 3.680336 13.54487 1952 4590 0.6667 0.6667 1984 Jun. 22, 1984 3.91 846 2.92737 8.569497 10.78164 3.43038E-07 25.08609 0.5238 1984 Jun. 26, 1984 5.43 4,820 3.683047 13.56484 1991 4570 0.6774 0.6774 1985 Jun. 08, 1985 3.98 864 2.936514 8.623113 10.90225 4.20731E-06 25.32189 0.4762 1985 Jun. 10, 1985 5.59 5,160 3.71265 13.78377 1962 4530 0.6882 0.6882 1986 31-May-86 4.37 1,090 3.037426 9.22596 11.54857 0.00160393 28.02317 0.1429 1986 Jun. 01, 1986 6.15 6,340 3.802089 14.45588 1966 4520 0.6989 0.6989 1987 1-May-87 3.61 665 2.822822 7.968322 10.23361 -0.000928242 22.49315 0.7619 1987 3-May-87 5.14 4,220 3.625312 13.14289 1968 4480 0.7097 0.7097 1988 Jun. 08, 1988 3.41 591 2.771587 7.681697 9.703492 -0.003293492 21.2905 0.8571 1988 Jun. 09, 1988 4.57 3,170 3.501059 12.25742 2002 4480 0.7204 0.7204 1989 11-May-89 4.09 942 2.974051 8.844979 10.98533 0.000154688 26.30542 0.381 1989 12-May-89 5.49 4,940 3.693727 13.64362 1969 4380 0.7312 0.7312 1990 Jun. 26, 1990 3.82 793 2.899273 8.405785 10.63835 -9.38982E-06 24.37067 0.619 1990 Jun. 28, 1990 5.36 4,670 3.669317 13.46389 1938 4350 0.7419 0.7419 1991 19-May-91 3.74 720 2.857332 8.164349 10.4576 -0.000250495 23.32826 0.6667 1991 21-May-91 5.31 4,570 3.659916 13.39499 1939 4350 0.7527 0.7527 1992 9-May-92 3.29 511 2.708421 7.335544 9.471157 -0.009521281 19.86774 0.9524 1992 10-May-92 4.55 3,140 3.49693 12.22852 1987 4220 0.7634 0.7634 1993 18-May-93 5.47 5,050 3.703291 13.71437 1942 3980 0.7742 0.7742 1994 14-May-94 4.9 3,760 3.575188 12.78197 1945 3940 0.7849 0.7849 1995 Jun. 07, 1995 4.98 3,910 3.592177 12.90373 1995 3910 0.7957 0.7957 1996 Jun. 11, 1996 5.88 5,810 3.764176 14.16902 2001 3910 0.8065 0.8065 1997 18-May-97 7.27 8,520 3.93044 15.44836 1926 3860 0.8172 0.8172 1998 29-May-98 4.99 3,860 3.586587 12.86361 1998 3860 0.8280 0.828 1999 29-May-99 5.06 3,820 3.582063 12.83118 1999 3820 0.8387 0.8387 2000 Jun. 17, 2000 4.82 3,580 3.553883 12.63008 1973 3770 0.8495 0.8495 2001 30-May-01 5.08 3,910 3.592177 12.90373 1994 3760 0.8602 0.8602 2002 Jun. 03, 2002 5.27 4,480 3.651278 13.33183 1963 3740 0.8710 0.871 2003 Jun. 02, 2003 5.65 5,290 3.723456 13.86412 1940 3650 0.8817 0.8817 2004 Jun. 08, 2004 4.58 3,150 3.498311 12.23818 2000 3580 0.8925 0.8925 2005 Jun. 05, 2005 5.45 4,860 3.686636 13.59129 2007 3510 0.9032 0.9032 2006 Jun. 18, 2006 6.06 6,170 3.790285 14.36626 1977 3430 0.9140 0.914 2007 Jun. 07, 2007 4.76 3,510 3.545307 12.5692 1937 3380 0.9247 0.9247 2008 22-May-08 5.94 5,930 3.773055 14.23594 1988 3170 0.9355 0.9355 2009 Jun. 02, 2009 5.51 4,990 3.698101 13.67595 2004 3150 0.9462 0.9462 2010 Jun. 06, 2010 5.41 4,770 3.678518 13.5315 1992 3140 0.9570 0.957 2011 Jul. 02, 2011 6.2 6,710 3.826723 14.64381 1944 3120 0.9677 0.9677 2012 Apr. 29, 2012 5.73 5,710 3.756636 14.11231 1930 2920 0.9785 0.9785 2013 15-May-13 6.08 6,210 3.793092 14.38754 1941 2120 0.9892 0.9892 Two Station Comparison per Bulletin 17B - Appendix 7 Guidelines Calculation of weighted skew for short term record, all years

Ḡ= 0.125 Generalized skew coefficient (from Peak FQ) G = 0.110 Station skew (matches PeakFQ? = yes) N = 20 Number of years of record

MSEḠ= 0.410 (from PeakFQ)

Equ. 6 A = -0.321 Equ. 6 B = 0.911

Equ. 6 MSEG = 0.254

pg. 12 Gw = 0.116 Weighted skew (matches PeakFQ? = yes)

orig M = 2.9204 (from PeakFQ) adj M = 2.932462 orig S = 0.1182 (from PeakFQ) adj S = 0.15885 Short-term Concurrent Data Weibull Rank Year Q abscissa Probability 1 0.047619048 1974 1540 1.6683912 Short Term Frequency Curves 2 0.095238095 1975 1090 1.3091717 Original Adjusted 3 0.142857143 1986 1090 1.0675705 T P K X Q K - B17B X Q abscissa 4 0.19047619 1982 1060 0.8761428 1.005025 0.995 -2.46702 2.628799 425 -2.4659 2.540754 347 -2.57583 5 0.238095238 1975 1020 0.712443 1.010101 0.99 -2.24087 2.655529 452 -2.23997 2.576643 377 -2.32635 6 0.285714286 1976 994 0.5659488 1.020408 0.98 -1.99112 2.685049 484 -1.99042 2.616284 413 -2.05375 7 0.333333333 1979 960 0.4307273 1.052632 0.95 -1.61128 2.729947 537 -1.61086 2.676577 475 -1.64485 8 0.380952381 1989 942 0.3029804 1.111111 0.9 -1.26851 2.770462 589 -1.26831 2.730991 538 -1.28155 9 0.428571429 1981 876 0.1800124 1.25 0.8 -0.84676 2.820313 661 -0.84675 2.797956 628 -0.84162 10 0.476190476 1985 864 0.0597171 1.428571 0.7 -0.53806 2.856801 719 -0.53817 2.846974 703 -0.5244 11 0.523809524 1984 846 -0.0597171 1.666667 0.6 -0.27124 2.888339 773 -0.27141 2.889349 775 -0.25335 12 0.571428571 1978 821 -0.1800124 2 0.5 -0.0193 2.918119 828 -0.01945 2.929372 850 0 13 0.619047619 1990 793 -0.3029804 5 0.2 0.8355 3.019156 1045 0.835379 3.065162 1162 0.841621 14 0.666666667 1991 720 -0.4307273 10 0.1 1.29331 3.073269 1184 1.293356 3.137911 1374 1.281552 15 0.714285714 1983 691 -0.5659488 25 0.04 1.789894 3.131965 1355 1.79022 3.216838 1648 1.750686 16 0.761904762 1987 665 -0.712443 50 0.02 2.115313 3.17043 1481 2.115875 3.268568 1856 2.053749 17 0.80952381 1977 616 -0.8761428 100 0.01 2.411137 3.205396 1605 2.411961 3.315602 2068 2.326348 18 0.857142857 1988 591 -1.0675705 200 0.005 2.68447 3.237704 1729 2.685555 3.359062 2286 2.575829 19 0.904761905 1973 560 -1.3091717 500 0.002 3.019042 3.277251 1893 3.020505 3.412269 2584 2.878162 20 0.952380952 1992 511 -1.6683912

10,000 Annual Original Adjusted Recurrence Exceedance Discharge Discharge Original Interval Probability Estimate (cfs) Estimate (cfs) Adjusted 2-year 0.5 828 850 peak flows 5-year 0.2 1045 1162 10-year 0.1 1184 1374 25-year 0.04 1355 1648 50-year 0.02 1481 1856 second

100-year 0.01 1605 2068 200-year 0.005 1729 2286 per 500-year 0.002 1893 2584 feet

1,000 cubic

Flows,

Peak

100 ‐3 ‐2 ‐10123 Exceedance Probability

PROGRAM OPTIONS USED IN THIS ANALYSIS:

imode = 1 [1=linear regression, 2=move.1, 3=mo.2, 4=rpn] method = 1 [1=cyclic, 2=noncyclic, 3=variable] numb = 0 [0=use all sites as independent variables, 1-12=indicated site] ibeg = 1 iend = 2 [index of first and last dependent variable] iout = 0 [1=show table of independent variables, 0=no table] istrt = 1922 [first year of extended record] anogo = 10. [minimum number of concurrent values needed]

Variable Station number number 1 12369200 2 12370000

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Analysis for variable 1 station 12369200

Sample size (years of record) = 20. Sample mean of the logs = 2.92 Sample std dev of the logs = 0.118

Variable Concurrent samples Potential predictors Predictors Correlation coefficient 2 20. 72 72 0.94

resultant sample size = 92. Predicted mean of the logs = 2.93 Predicted std dev of the logs = 0.121 Average standard error of prediction = 9.977 Number of variables with no predictors = 1012.

********************************************** **********************************************

Analysis for variable 2 station 12370000

Sample size (years of record) = 92. Sample mean of the logs = 3.70 Sample std dev of the logs = 0.115

Variable Concurrent samples Potential predictors Predictors Correlation coefficient

resultant sample size = 92. Predicted mean of the logs = 3.70 Predicted std dev of the logs = 0.115 Average standard error of prediction = 0.000 Number of variables with no predictors = 1012.

1 The total predicted error was 718.35 for 72. predictions