Determination of upstream boundary points on southeastern Washington streams and rivers under the requirements of the Shoreline Management Act of 1971

Regulation of the shorelines of the State of Washington, as mandated by the Shoreline Management Act of 1971, requires knowledge of the locations on streams and river reaches where specific regulatory criteria are satisfied. The U.S. Geological Survey conducted a study in 1971 to determine the upstream boundary points of these reaches for many of the State's streams and rivers. Updated upstream boundary points were determined in the current study for all the streams and rivers in southeastern Washington that fall under the jurisdiction of the Shoreline Management Act of 1971. Upstream boundary point locations where the mean annual discharge equals 20 cubic feet per second were determined for 149 streams. In addition, upstream boundary point locations where the mean annual discharge equals 200 cubic feet per second or the drainage area equals 300 square miles were determined for 22 rivers.

Boundary point locations were determined by application of multiple-linear-regression equations that relate mean annual discharge to drainage area and mean annual precipitation. Southeastern Washington was divided into five hydrologically distinct regions, and a separate regression equation was developed for each region. The regression equations are based on data for gaging stations with at least 10 years of record. The number of stations in the regression analysis for each of the five regions ranged from 5 to 33. The coefficient of determination, R2, of the regression equations ranged from 0.953 to 0.997. The equation for the Upper Yakima region had the lowest standard error, ranging from -7 to +9 percent for a regression estimate of 20 cubic feet per second. The equation for the Columbia Basin to Palouse region had the highest standard error, ranging from -36 to +55 percent for a regression estimate of 20 cubic feet per second. The approximate error in the location of an upstream boundary point can be calculated using the variables mean annual precipitation of the basin upstream from a boundary point and average basin width in the vicinity of the boundary point. The calculation gives only a rough estimate of the error of the boundary point location, because of the uncertainty in estimating average basin width.