Sediment and phosphorus inputs from perennial streams to Lake Whatcom, northwest Washington State

Abstract

Relationships among suspended sediment, phosphorus, and discharge vary temporally and spatially in the Lake Whatcom watershed, a 125-km2, high-relief, moderately developed, forested basin in northwestern Washington State. The lake is subject to a Total Maximum Daily Load (TMDL) to limit phosphorus inputs. Phosphorus, which largely enters the lake adsorbed to suspended sediment in streams, has led to increased algae growth and depletion of dissolved oxygen. We used the results of high-resolution storm event sediment and phosphorous sampling in five streams to examine the effects of varying watershed features on loading and to develop sediment-discharge and phosphorus-discharge models to estimate phosphorus loading to the lake during the 2013 water year. During most storm events, the sediment peak preceded the discharge peak. The magnitude of hydrograph rise was the best predictor of the maximum sediment concentration during the event. Of the five basins studied, a large, forested watershed that contains steep slopes susceptible to mass wasting yielded the most sediment per area. The highest phosphorus yield was from a smaller, lower-relief watershed containing 29% residential development. Our sediment and phosphorous yields were comparable to estimates from similar streams in the Puget Sound region in northwest Washington State. Total suspended solids and total phosphorus were significantly correlated to discharge in most streams in the watershed, but variability within and among storm events resulted in uncertainty when calculating fluxes based on discharge.