USGS Washington Water Science Center
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Under Washington regulations, bridges, culverts, and other stream-crossing structures need to be designed with fish passage in mind. For culverts, maximum flows cannot exceed a 10-percent exceedance probability flow (the flow that is equalled or exceeded 10 percent of the time) when fish are migrating upstream.
To help the Washington Department of Natural Resources manage its culverts at more than 20,000 crossing in eastern Washington, the USGS began a study of 10-percent exceedance probability flow at many of the sites. The study examines the flow at the sites during January, the month in which fish migration is at its peak, to determine if maximum flows were exceeded. The study also furthers the understanding of the characteristics of streamflow in eastern Washington, especially in small streams.