The pervasive problem of restoring fish runs to their natal streams is characterized in many regions of Washington by improperly placed culverts. The replacement of these fish migration barriers requires knowledge of design flows: floods for structural safety and migration season high and low flows. High flows block fish passage with velocities that exceed their swimming capabilities. During low flows, the migration barrier is caused by a lack of enough water depth to support the bodies of the fish. The estimation of these fish passage and safety flows in ungaged streams is impeded in eastern Washington due to: the wide range of climatic conditions (5 to 110 in. per year of precipitation); diverse geology and soils; a lack of stream-gaging stations with long-term records; changes in land use; and the seasonal impacts of irrigation diversions and well pumpage on the remnant instream flows. Past efforts to estimate these flows have not been successful.
Therefore, the Washington State University (WSU) project was undertaken to model the high migration season flow in all of eastern Washington. In addition, WSU established 20 stream-gaging stations on salmon streams along the east side of the Cascade Mountains plus the Blue Mountains. This complementary project was developed to estimate other design flows in the Water Resource Inventory Areas (WRIA) 29, 30, 32, 35, 38, 40, 45, 48 and 49, in which the WSU gage sites are located. United States Geological Survey (USGS) gage records in those WRIAs (and in WRIAs 39, 46 and 47) were used to develop models that estimate the following statistical flows: 100-year, 25-year and 2-year daily and peak floods; the average annual flow and its variability; ranges of mean monthly flows (maximum, average and low); the 7-day average, 2-year, 10-year, 20-year low flow; and the 30- and 60-day average low flows. The model results are, for the most part, very good. The standard error of estimate ranges are: for floods, 2-37%; average annual flow, 10-37%; maximum annual flow, 6-14%; minimum annual flow, 13-23%; and low flows, 3-22%. Monthly average flows reflected the strong influences of seasonal variability and irrigation withdrawals: 3-242%, due to winter frozen low flows (3%) in the Entiat-Wenatchee Region, and August low flows (242%) in the Blue Mountains due to irrigation. All flow estimation equations were based on USGS stream-gaging data from continuous gages located in the WRIAs