Upper Klamath Lake Basin nutrient-loading study-assessment of historic flows in the Williamson and Sprague Rivers

Abstract The Williamson River Basin, located in southcentral Oregon, has a drainage area of approximately 3,000 square miles. The Sprague River, which flows into the Williamson River Basin, has a drainage area of 1,580 square miles. Together, the Williamson and Sprague Rivers supply about one-half of the inflow to Upper Klamath Lake. Various statistical techniques, which included trend tests, double-mass curves, and two-sample tests, were used to detect significant changes in the precipitation-runoff relation for the Williamson and Sprague River Basins. Flows from these two rivers were compared with the precipitation and air temperature records collected at Klamath Falls to assess the effect of climate on flow variations. Most of the double-mass curves showed a major break in the slope of the curve occurring around 1950 and a smaller one near 1990. For the years 1930-50 and 1990-96, February through May flows were relatively lower in the Williamson River than in rivers in nearby basins