Culvert Design Flows for Fish Passage and Structural Safety in East Cascade and Blue Mountain Streams 2002The 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.
