Bird Track Springs Basis of Design Report APPENDIX HYDROLOGIC ANALYSIS FOR BIRD TRACK RESTORATION PROJECT

Transcription

1 Bird Track Springs Basis of Design Report APPENDIX C HYDROLOGIC ANALYSIS FOR BIRD TRACK RESTORATION PROJECT

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3 Technical Memorandum Date March 1, 2016 To: From: RE: Mike Knutson, Bureau of Reclamation Nick Legg, P.G. and Michel Ybarrondo, P.E. Draft Hydrologic Analysis for the Bird Track Restoration Project (Grande Ronde River, Union County, OR). Table of Contents 1.0 Introduction Summary of Flow Estimates for Hydraulic Modeling General Setting Methods General Approach Drainage Area Rational Method for Ungaged Main-stem GRR Sites Flow Measurements and Calibration of Drainage Area Adjustments Tributary Flow Estimates Results Historical Flows Mainstem Peak Flows Tributary Peak Flows Flow Duration-Exceedance Discharges Historical Flow Trends Design Flows References Introduction This memorandum summarizes hydrologic analyses for the Bird Track Restoration Project Reach (Project) extending from approximately river miles (RMs) to of the Grande Ronde River (GRR) in Union County, Oregon. The overarching goals were to quantify stream flows in the project reach in both the mainstem GRR and tributaries entering the project reach. These flow estimates subsequently inform hydraulic modeling of existing conditions and proposed conditions in various design alternatives. This analysis augments Cardno s previous hydrologic analysis of the GRR from RMs to 165 (submitted to Bureau of Reclamation in September 2014; henceforth the 2014 hydrology study ), which encompassed the project reach. In general, an effort was made to avoid

4 repetition of information in the previous study, except where findings of the previous report were needed to maintain logical continuity. 2.0 Summary of Flow Estimates for Hydraulic Modeling Tables containing the primary inputs for hydraulic modeling efforts included the following: Table 5: Mainstem peak flows Table 6: Tributary peak flows Table 9: Mainstem design flows relating to targeted periods of salmonid life stages Table 10: Tributary design flows relating to targeted periods of salmonid life stages 3.0 General Setting The project reach sits at approximately 3,100 feet elevation and drains an approximately 475 mi 2 watershed extending to a maximum elevation of 7,923 feet. The mean annual precipitation is 26.2 in, most of which falls as snow during winter months. As a result, the annual hydrograph is dominated by snowmelt-derived high flows from April to May. Peak flows also occasionally occur from winter rains storms. The low flow season typically extends from August through December. Most of the basin is forested (over 73 percent) and has very little development (less than 0.1 percent estimated impervious area) (USGS 2014). Watershed characteristics of key points (Figure 1) along the main-stem GRR are shown in Table 1. Six tributary streams enter the project reach from adjacent valley walls. Figure 1 shows the project reach and the watersheds of Moss, Bear, and Jordan Creeks entering from the south (river right), and Spring Creek and two unnamed tributaries entering from the north (river left). All six tributary streams have no stream gaging records. Table 1 summarizes general attributes of the tributary basins. Despite their small drainage areas, the two unnamed tributaries (Unnamed Tributary 1 and 2) were included in the analysis to provide a full picture of possible flow inputs along the project reach. 2

5 Figure 1. Map of key locations and tributary watersheds. 3

6 Table 1: Watershed characteristics of key GRR mainstem sites and tributaries contributing to the project reach. Watershed Outlet Description River Mile Drainage Area (mi 2 ) Outlet Elevation Maximum Elevation Mean Annual Precipitation (in) Mainstem Points Upper Project Reach Boundary Historic Stream Gage Location ( ) Lower Project Reach Boundary Tributary Outlets Unnamed Tributary 1 (enters left) Moss Creek (enters right) Bear Creek (enters right) Jordan Creek (enters right) Unnamed Tributary 2 (enters left) Spring Creek (enters left) Table 2: Stream gauges in the Grande Ronde River basin used in this hydrologic analysis. Station Number Name Agency River Mile Drainage Area (mi 2 ) Start Year End Year Grande Ronde R at La Grande, OR USGS Grande Ronde R Near Perry, OR OWRD Current Five Points Cr at Hilgard, OR OWRD Current Grande Ronde R at Hilgard, OR USGS Grande Ronde R Near Hilgard, OR* USGS * Historic gauge is located within the project reach. 4.0 Methods 4.1 General Approach The goals of this study were to estimate stream flows in the mainstem GRR and tributary streams (see locations noted in Table 1) along the project reach. Flows were estimated both in terms of peak flows and flow exceedance statistics. In terms of recurrence intervals, 1.05, 1.1, 1.25, 1.5, 2, 2.33, 5, 10, 25, 50, 100, 200, and 500-year peak flows were estimated. Flow duration estimates included 5%, 10%, 25%, 50%, and 95% annual flow exceedance values, and inform project design flows. With exception of a historic stream gage in the lower project reach, all other flow estimates are at ungauged sites and thus required various flow estimation techniques as described below. 4

7 The two primary flow estimation approaches used to estimate flows at ungauged sites included the drainage area-ratio method and regional regression equations. The drainage area-ratio method (Cooper, 2006) ties flow estimates at ungauged sites to gaging records up- or downstream, and thus was the preferred method of flow estimation. Since the drainage area method is only applicable at sites on the same stream and with drainage areas between 0.5 and 1.5 times that of the gaged site (Cooper, 2006), it could only be employed at mainstem locations where downstream GRR stream gages ( and ) were within the specified range. Given that tributary basin outflow points were outside the applicable range of the drainage area method, regional regression equations for peak discharges (Cooper, 2006) and annual flow duration (Risley et al., 2008) were needed to estimate flow. To corroborate regression equation estimates, Cardno used data from an active stream gauge on Five Points Creek (ID , see Table 2), a gage with a small drainage basin entering the GRR 4.2 miles below the lower project boundary. 4.2 Drainage Area Rational Method for Ungauged Main-stem GRR Sites Fundamentally, the drainage area ratio method adjusts known discharges (Qg) at a stream gage to estimate discharge at an ungauged site (Qu) using the ratio of drainage areas at the ungauged (Au) and gauged (Ag) locations (Au/Ag). That drainage area ratio is adjusted by an exponent (a) and then multiplied by the known streamflow at the stream gauge (Qg) to estimate discharge at the ungauged location. In equation form, the drainage area ratio approach is expressed as: QQ uu = QQ gg AA aa uu AA gg Exponent a in the above equation can be determined in two ways. The more general method involves simply using the exponent for drainage area in applicable regional regression equations. Cooper s (2006) regional regression equations for northeast Oregon for various flood magnitudes have relatively consistent drainage area exponents ranging from Thus, an average value of 0.74 is appropriate for ungauged sites in the GRR basin. Alternatively, the exponent can be solved for algebraically when two known stream gauge records overlapping in time are available. In this latter approach, Qg and Qu become Q1 and Q2, and, Ag and Au become A1 and A2, respectively, where subscripts denote stream gauges 1 and 2. The equation then can be rearranged to solve for a: log QQ 2 aa = QQ 1 log AA AA 2 1 In the above equation, the discharge ratio (Q2/Q1) is determined by a linear regressions of flows at two stations. To evaluate the value of the exponent a in the vicinity of the project reach, Cardno calculated exponent a using three gauge pairs with overlapping records: 5

8 1. GRR USGS La Grande ( ) versus the GRR USGS Hilgard ( ): Discharge regression was performed in the 2014 GRR hydrology study (see Figure 1 in that report). Calculated adjustment exponent (a) = GRR USGS La Grande ( ) versus the GRR USGS Gauge near Hilgard ( ): Discharge regression was performed in the 2014 GRR hydrology study (see Figure 2 in that report). Calculated adjustment exponent (a) = GRR OWRD Gauge near Perry, OR ( ) versus the OWRD Gauge on Five Points Creek ( ): The discharge regression was performed in this study (see Figure 2) to estimate adjustment exponent a in a small watershed of greater similarity to tributary streams in the project reach. Calculated adjustment exponent (a) = 0.85 OWRD gage on Five Points Hillgard, OR ( , cfs) Monthly Maximum Daily Flows Annual Maximum Daily Flows Monthly Max y = x R² = Annual Max y = x R² = GRR OWRD gage near Perry, OR ( , cfs) Figure 2. Regressions of streamflow measurements ( ) measured at OWRD gages and Monthly and annual maximum daily flows (as opposed to daily flows) were used due to expected snow-melt timing differences in basins with such different size and elevation range. Two regressions are shown (blue and red), but they nearly overlap. The calculations above suggest an increasing trend in drainage area adjustment exponents (a) with drainage area (see Table 3). Given that these exponents can be replaced with the regional regression equation exponents for drainage area according to Cooper s (2006) methods, the decreasing trend suggests that the Cooper (2006) exponent of approximately 0.74 (see discussion above) is likely appropriate for estimating flows in the tributary streams entering the project site (which are smaller in drainage area than the Five Points Creek gage). This finding corroborates the use of the regional regression equation for estimates of tributary stream discharges, where the drainage area ratio method is out of applicable range. 6

9 Table 3. Calculated drainage area adjustment exponents at three gauges in the upper GRR basin. Station Number Name Drainage Area (mi 2 ) Drainage Area Adjustment Exponent (a) Grande Ronde R at Hilgard, OR Grande Ronde River Near Hilgard, OR* Five Points Creek at Hilgard, OR Peak flow hydrographs were estimated at the upstream and downstream project boundaries by first assembling a composite flow record of mean daily flows for the upstream site boundary. The composite flow record included drainage area adjusted flows measured at the historic USGS gauge at La Grande ( , ) and the active OWRD gauge at Perry ( , 1996-present). Drainage area adjustments utilized an adjustment exponent equal to the average of the two calculated for gauge pairs 1 and 2 (see above list). The composite record of peak flows was then input in PeakFQ (using the Bulletin 17B approach (Flynn et al 2006, IACWD 1982)). The peak flow hydrograph for the upstream boundary was then adjusted for the drainage area at the downstream boundary using the same adjustment exponent. 4.3 Flow Measurements and Calibration of Drainage Area Adjustments Field measurements of discharges were used to calibrate the adjustment exponents and factors developed using the above methods. The calibration efforts involved comparisons of the field measurements to instantaneous measurements (15-minute interval) at the active OWRD gauge at Perry. At times of rapidly changing flows (particularly during high flows), comparisons accounted for travel time between the measurement site and OWRD Gauge at Perry. Table 4 shows the measurements currently used to calibrate estimate high flows using drainage area ratio method. Table 4. High flow measurements and their comparison to estimated flows using an adjustment exponent of Measurement Location / Date Red Bridge / Lower Project Reach / RM Time of Day Measur ed Dischar ge (cfs) Average Flow Velocity (ft/s) Distance to Perry Gauge (mi.) Estimate Arrival Time at Perry Gauge Discharge at Perry Gauge at Arrival Time (cfs) Predicted Flow At Measurem ent Location (cfs) Predicted / Measured (%) : : % : : % Given the close correspondence (Table 4) between measured and predicted flows, it was deemed unnecessary to correct flow peak flow estimates for the upstream site boundary. However, future discharge measurement may necessitate corrections. Low flows were measured in the summer of 2015, permitting additional calibration of the drainage area ratio approach. The project team expected the drainage area adjustment exponents developed for the entire flow record would perform poorly during summer months 7

10 when the mainstem GRR likely carries a disproportionate flow relative to the tributaries entering between project reach and the gauges downstream. Eleven flow measurements from June to September of 2015 confirmed this hypothesis, revealing that flow predictions tied to the Perry gauge (using adjustment exponent of 1.19) under-estimated flows in the lower project reach by an average of 17%. The differences warranted an estimation of an adjustment exponent specific to low flows. Flow measurements collected at two locations bracketing the project reach were utilized for this analysis; the first is located just upstream of the project reach (drainage area = 453 mi 2 ), and the second at the staff gauge in the lower project reach (drainage area = 496 mi 2 ). The analysis produced nearly identical estimates of the adjustment exponent (~0.49) at the two measurement locations. The adjustment exponent of 0.49 results in an adjustment factor of 0.83 from the Perry Gauge to the upstream project boundary, and an adjustment factor of 0.96 from the staff gauge in the lower project reach to the upstream project boundary. Table 5. Flow measurements made in the summer of Measures Discharge at Measurement Date Forest Service Corner downstream of Beaver Creek (cfs) Measured Discharge in Lower Project Reach (cfs) Mean Daily Discharge at OWRD Gauge at Perry (cfs) 6/16/ /23/ /29/ /7/ /14/ /20/ /29/ /10/ /19/ /24/ /8/ Estimated Adjustment Exponent The field measurements indicate a correspondence between adjustment exponents and discharge (i.e. exponents increase with discharge); however, the nature of the transition in adjustment exponents from low to high flow remains an unknown. The team elected not to pursue additional analysis to constrain this transition given that future flow measurements at moderate flows should facilitate further refinement. Unknowns related to this transition most likely affect estimates of annual and monthly flow exceedance statistics (as opposed to peak flows). As a temporary solution until addition flow measurements, exceedance statistics were determined by averaging the following two approaches: 8

11 Statistics for the OWRD gauge at Perry, OR ( , 1996-Present) adjusted to the upper site boundary (RM 146.1) using an adjustment exponent of 0.84 (average of low and high flow adjustment exponents). Statistics for the historic USGS gauge within the project reach (USGS gauge , , in the lower project reach) adjusted to the upper project boundary using an adjustment exponent of 0.84 (average of low and high flow adjustment exponents). The rationale for averaging the two approaches relates to differing sources of uncertainty with each approach. The first estimate provides increased certainty due to its more recent record and likelihood of representing current conditions, but also has elevated uncertainty relating to the flow measurements at a distance downstream of the project reach. The latter estimate uses measurements collected within the project reach (thus involving smaller errors associated with drainage area adjustment), but over a historic period when flow conditions may have differed from present. 4.4 Tributary Flow Estimates Regional regression equations provided tributary streamflow estimates in the project reach. Cooper (2006) developed regression equations (requiring only drainage area) for 2, 5, 10, 25, 50, 100, and 500-year recurrence interval peak flows at ungauged sites. This study also sought to estimate 1.05, 1.1, 1.25, 1.5, 2.33, and 200-year recurrence interval floods. Cardno estimated these flows (without a corresponding regression) by interpolating and extrapolating fit curves to the flow estimates using the Cooper equations. Risely et al. (2008) regression equations for flow exceedance probabilities require drainage area, mean annual precipitation, and maximum basin elevation. All of these input variables were measured for individual watersheds using the USGS Streamstats Program. 5.0 Results 5.1 Historical Flows Figure 3 shows the historic flows for the historic USGS gauge location ( ) in the lower project reach. Figure 3 also serves as reference for the multiple streamflow records used to reconstruct flows in the project reach. The top ten peak flows for the project reach are shown in Table 4. The peak flow of record occurred on January 30 th, 1965 in response to an intense rain storm with exceptionally high freezing levels. The below excerpt from Waananan et al. (1971, pg. A20) describes the conditions in the storm. During January a series of Pacific storms dropped the freezing level to about 4,000 feet and caused almost daily rain in the valleys and snow at the higher altitudes. After January 26 the freezing level rose to about 8,000 feet and precipitation intensities increased; the heaviest rain occurred January Precipitation during the January storm was notably heavy in northeastern Oregon, southeastern Washington, and north-central Idaho. As much as 7.38 inches was recorded at Meacham, 20 miles northwest of La Grande, OR; 8.79 inches, 13 miles east-southeast of Walla Walla, Wash.; and 7.71 inches, at Elk River, 40 miles east of Moscow, Idaho. Precipitation exceeding 5 inches occurred also during this period in a small area north of Boise, Idaho; 5.76 inches was recorded at Idaho City. These heavy rains produced record breaking 9

12 floods having recurrence intervals greater than 50 years in some basins, notably the Grande Ronde River basin in Oregon. The conditions in the 1965 storm show that peak flows can be generated by winter rain-on-snow events in addition to the more common source of spring snowmelt. Figure 3. Reconstructed flow record for water years for the historic gauge location in the lower project reach. The reconstructed record includes measured flows from , and drainage area adjusted flows from the USGS gauges at La Grande ( ) and Perry ( ). Years with missing data include 1910, , , and Table 4. The top 10 historical peak flows measured and reconstructed for the upstream project boundary from water years Reconstructed flows are adjusted from measurements at gauges and Rank Date Peak Flow (cfs) 1 1/30/ /1/ /18/ /31/ /23/ /16/ /8/ /10/ /13/

13 5.2 Mainstem Peak Flows 10 2/20/ To evaluate the peak flow hydrograph in the mainstem, annual peak flow records from multiple gauges were compiled, adjusted to the upstream site boundary, and then input into PeakFQ. The resulting hydrograph is presented graphically in Figure 4 and in Table 5. Figure 4. PeakFQ output showing the discharges of various annual exceedance probabilities for the upstream project boundary (RM 146.1). Table 5. PeakFQ results for the upstream site boundary. 95% Confidence Intervals Annual Probability Return Interval Bull 17b Low High

14 Tributary Peak Flows The Cooper (2006) regression equations were the basis for estimating peak flows in tributaries entering the project reach. However, raw estimates using the Cooper equations require a correction prior to being input into hydraulic models. Specifically, a correction is needed to ensure that the contributed flow from tributaries aligns with the expected mainstem flow gain along the project reach (as estimated using the drainage area ratio method). A correction factor was developed by estimating the sum of tributary stream flows at each of the peak flow recurrence intervals, and comparing these sums to estimated flow gain along the project reach. On average, the raw aggregate flow contribution from tributaries is 2.13 times that of the flow gain along the mainstem estimated using the drainage area ratio method. While some of this mismatch may relate to estimation error, the larger tributary flow contribution also probably relates to differences in peak flow timing between the small tributary basins and the mainstem GRR. In general, peak flows are expected to occur earlier in tributaries than in the mainstem. By this idea, tributary streams experience the falling limb of their flood hydrograph when the mainstem GRR reaches its peak flow for a given event (rain or snowmelt event). To account for these differences (and to avoid larger than reality flows in the lower project reach), hydraulic modeling efforts should use the flows shown in Table 6 as inputs. 12

15 Table 6. Corrected peak flow estimates for the tributary streams entering the project reach (for input into hydraulic model). Tributary Unnamed Trib. #1 (L) Moss Cr. (R) Bear Cr. (R) Jordon Cr. (R) Unnamed Trib. #2 (L) Spring Cr. (L) Outlet GRR Mile Basin Size (sq. mi) Return Interval Peak Flow (cfs) 1.05** ** ** ** * ** * * * * * ** * * Estimated using Cooper (2006) Region 3 Regression Equations. ** Estimated by interpolation or extrapolation of curve formed by regional regression equation estimates. 5.4 Flow Duration-Exceedance Discharges Flow duration-exceedance statistics (Table 7) are derived from a combination of streamflow records collected at the historic USGS gauge within the lower project ( ) and the active OWRD gauge at Perry ( ) in line with the rationale in Section 4.3. Annual exceedance statistics for tributaries (estimated using the Risely et al. (2008) regression equations) are listed in Table 8. Table 7. Exceedance statistics for the upper project boundary (GRR RM 146.1). Month 5 percent exceedance discharge (cfs) 50 percent exceedance discharge (cfs) 95 percent exceedance discharge (cfs) October November December January

16 February March April May June July August September Annual Table 8. Annual exceedance statistics for the tributary streams entering the project reach. Estimates are based on Risely et al. (2008) regression equations for region 6 requiring input variables of watershed area, mean annual precipitation, and maximum basin elevation. Tributary Unnamed Trib. #1 (L) Moss Cr. (R) Bear Cr. (R) Unnamed Trib. #2 (L) Jordon Cr. (R) Outlet GRR Mile Basin Size (sq. mi) Mean Annual Precipitation (in.) Max Elevation in Basin (ft) Annual Exceedance Flow Discharge (cfs) 5 % % % % % Spring Cr. (L) 5.5 Historical Flow Trends A simple analysis of flow trends identified potential effects of climate change and other factors on flows in the GRR. A flow record was compiled for the gauge location at Perry by assembling flow records measured at the OWRD Gauge (1996-Present) and historical flow records measured at the historic gauge at La Grande ( , adjusted for drainage area difference). Then flow statistics and peak flow averages were calculated for each 30-year period within the compiled record. The 30-year period intentionally extends beyond the typical decadal Pacific Decadal Oscillation cycle of years (Mantua and Hare, 2002). Trends in annual flow exceedance statistics and peak flows are shown in Figures 5 and 6, respectively. The most noticeable trends in annual flow are in the 95% and 50% exceedance statistics, which appear to have increased over the period of record. These increases likely relate, at least in part, to increases in annual precipitation amounts as measured at local, long-term precipitation gauge 14

17 data compiled by the Office of Washington State Climatologist ( 1. Peak flows also show an apparent increasing trend with time. The exact cause of this trend is not certain, but it may relate to a combination of increasing precipitation (and potential increases in extreme precipitation) and temperature 2. These increases have the potential to increase the frequency and/or magnitude of rain-on-snow events like those responsible for the peak flow of record in the Grande Ronde Basin. It should be noted that the discussed trends were identified visually, but were not tested for statistical significance. FLOW (CFS, STATISTIC FOR PREVIOUS 30- YR PERRY, OR) % Exceedance 10% Exceedance 25% Exceedance 50% Exceedance 1 95% Exceedance YEAR Figure 5: Trends in annual exceedance statistics for a flow record compiled for the gauge location at Perry, OR. Exceedance statistics are calculated for the 30-year period preceding a given year. Notice the logarithmic vertical axis. 1 Local precipitation gauges with observed increases in annual precipitation as measured from 1895-present include UNION EXP STN (average of 0.14 per decade), WALLOWA (0.02 per decade), and PILOT ROCK 1SE (0.14 per decade). Note that the BAKER FAA AP gauge (near Baker City) had an observed decrease in annual precipitation of 0.14 per decade for the same period of record. 2 Local temperature stations with observed mean annual temperature increases (1895-present) include UNION EXP STN (average of 0.11oF per decade), WALLOWA (0.25oF per decade), PILOT ROCK 1SE (0.25oF per decade) and BAKER FAA AP (average of 0.22oF per decade). 15

18 yr Running Average Peak Flow FLOW (CFS) YEAR Figure 6: Peak flow trends for a flow record compiled for the gauge location at Perry, OR. 6.0 Design Flows In addition to an evaluation of the flow hydrograph in the project reach, a key outcome was a determination of design flows relating to key periods of salmonid use in the project reach. Winter and summer rearing were identified as the target life stages (Figure 7). The proposed design flows are listed in Table 9, and the subsequent text provides the supporting rationale. Table 9: Design flows for the upstream project boundary (RM 146.1). Design Flow Description Flow (cfs) Exceedance statistic Low Flow (Winter and Summer) 18 95% exceedance for critical winter rearing period (October-March). 50% exceedance flow for August Winter median flow 82 50% exceedance for critical winter rearing period (Oct.-Mar.). Median March flow 400 Winter high flow 900 Approximately the 50% exceedance flow for March. 5% exceedance for critical winter rearing period (Oct.-Mar.) Winter Rearing Flows The critical winter rearing period is October to March (see Figure 7). Habitat suitability criteria (Maret, 2005) indicate the following for juvenile chinook and steelhead: 16

19 Habitat suitability (Maret, 2005) for juvenile chinook and steelhead increases from 0 to 2 feet depth, and remains at a maximum at depths greater than 2 feet. Thus, a critical design consideration is depth (likely in pools) during the lowest winter flows, when depths are also at a minimum. Habitat suitability is greatest at velocities below 2 feet per second (ft/s), and is least above the 2 ft/s threshold. A second critical consideration is therefore velocity during the highest winter flows when average velocities are high and velocity habitat suitability is minimized. The project design would ideally provide slow-water refuge that meets velocity suitability criteria during these periods. Within winter months (October-March), the highest flows are most likely in March (Figure 7). Median flows inform normal habitat suitability during winter conditions. Figure 7: Annual hydrograph at the upstream end of the project reach (RM 146.1). Fish periodicity data generated in the Atlas Process are shown. Darker portions of fish periodicity bands show the critical period and lighter bands show secondary periods of a given life stage. The design flows shown in Table 9 are similar to the annual 95%, 50%, 25%, and 10% exceedance flow statistics. For simplicity, the same annual flow statistics were used for tributary streams entering the project reach (see Table 10). 17

20 Table 10: Design flows for tributaries entering the project reach (estimated using Risely et al. (2008) regression equations). Tributary Unnamed Trib. #1 (L) Moss Cr. (R) Bear Cr. (R) Unnamed Trib. #2 (L) Jordon Cr. (R) Spring Cr. (L) Design Flow Discharge (cfs) Low Flow (Annual 95% exceedance) Winter Median Flow (Annual 50% exceedance) March Median Flow (Annual 25% exceedance) Winter High Flow (Annual 10% exceedance) Summer Base Flows Summer base flow was estimated using a combination of flows measured (summer of 2015) and the historic USGS gauge location in the lower project reach (USGS gauge , ). The 2015 flow measurements and the historic gauge location are within close proximity to one another, so are complimentary and easily compared datasets. Grande Ronde River (GRR) flows are the lowest seasonally in August, and represent base flow conditions. Measured low flows in August 2015 were approximately 13 cfs (Bureau of Reclamation, 2015). The drought in 2015 led to some of the lowest flows historically in the GRR. The measured August 2015 flow of 13 cfs approximates the 80% exceedance flow statistic for August. A single design flow of 18 cfs addresses both winter low flows and summer base flows. The proposed design low flow is the 95% exceedance statistic for the critical winter rearing period (October-March), and approximately the median flow for August. 7.0 References Cooper, R. M. (2006). Estimation of peak discharges for rural, unregulated streams in eastern Oregon: Oregon Water Resources Department Open-File Report SW Salem, Oregon. Flynn, K.M., Kirby, W.H. and Hummel, P.R., User's manual for program PeakFQ, annual flood-frequency analysis using Bulletin 17B guidelines (No. 4-B4). Mantua, N.J. and Hare, S.R., The Pacific decadal oscillation. Journal of oceanography, 58(1), pp Maret, T.R., Hortness, J.E., and Ott, D.S., Instream flow characterization of upper Salmon River basin streams, central Idaho, 2005: U.S. Geological Survey Scientific Investigations Report pp. Office of the Washington State Climatologist, Northwest Temperature, Precipitation, and SWE Trend Analysis. Accessed February Risley, J. C., Stonewall, A., & Haluska, T. L. (2008). Estimating flow-duration and low-flow frequency statistics for unregulated streams in Oregon (No. FHWA-OR-RD-09-03). US Department of the Interior, US Geological Survey Scientific Investigations Report Waananen, A. O., Harris, D. D., & Williams, R. C. (1971). Floods of December 1964 and January 1965 in the Far Western States; Part 1 Description (No A). US Govt. Print. Office 18