Nutrient and Sediment Production, Watershed Characteristics, and Land Use in the Tahoe Basin, California-Nevada1

:  In efforts to control the degradation of water quality in Lake Tahoe, public agencies have monitored surface water discharge and concentrations of nitrogen, phosphorus, and suspended sediment in two separate sampling programs. The first program focuses on 20 watersheds varying in size from 162 to 14,000 ha, with continuous stream gaging and periodic sampling; the second focuses on small urbanized catchments, with automated sampling during runoff events. Using data from both programs, we addressed the questions (1) what are the fluxes and concentrations of nitrogen and phosphorus entering the lake from surface runoff; (2) how do the fluxes and concentrations vary in space and time; and (3) how are they related to land use and watershed characteristics? To answer these questions, we calculated discharge-weighted average concentrations and annual fluxes and used multiple regression to relate those variable to a suite of GIS-derived explanatory variables. The final selected regression models explain 47-62% of the variance in constituent concentrations in the stormwater monitoring catchments, and 45-72% of the variance in mean annual yields in the larger watersheds. The results emphasize the importance of impervious surface and residential density as factors in water quality degradation, and well-developed soil as a factor in water quality maintenance.