SEDIMENT AND PHOSPHORUS INPUTS FROM PERENNIAL STREAMS TO LAKE WHATCOM, NORTHWESTERN WASHINGTON STATE

Nutrient enrichment presents a common problem in lakes and streams by promoting algae growth and the depletion of dissolved oxygen. For example, Lake Whatcom in northwestern Washington State is subject to a Total Maximum Daily Load (TMDL) to limit phosphorus input. The 20-km² lake is supported by runoff from numerous perennial streams in a steep, 125-km², moderately developed, forested watershed. Much of the phosphorus load occurs adsorbed to suspended sediment in streams and is transported to the lake during storm events. Understanding sediment and phosphorus transport to the lake is important for managing the TMDL and water quality in general because the lake serves as the drinking water source for about 100,000 people.

Our objectives are to calculate sediment and phosphorus fluxes to Lake Whatcom and examine relationships among precipitation, discharge, sediment, and phosphorus. A series of water samples were collected near the mouth of Smith Creek in the watershed during 22 storm events between February 2013 and January 2014 and analyzed for total suspended solids and total phosphorus. We used data from Smith Creek and four other streams to examine the effects of varying basin features on loading and to develop sediment-discharge and phosphorus-discharge models to estimate loading to the lake during the 2013 water year.

Relationships among sediment, phosphorus, and discharge vary temporally and spatially in the watershed. During most storm events, the sediment peak leads the discharge peak, indicating that transport is limited by sediment availability. In Smith Creek, the magnitude of hydrograph rise is the best predictor of the maximum sediment concentration during the event. Of the five streams studied, the steep, forested Smith Creek basin yielded the most sediment per area. The highest phosphorus yield was from a smaller, lower relief basin containing 29% residential development. Our sediment and phosphorous yields are comparable to estimates from similar streams in the Puget Sound region. Total suspended solids and total phosphorus are significantly correlated to discharge in most streams in the watershed, but variability within and among storm events results in uncertainty when calculating fluxes based on discharge. Continuous turbidity monitoring could allow for improved models and flux estimates.