Land-Use and Water Quality in Tributary Streams of Lake Tahoe, California-Nevada

We examined land-use and water quality monitoring data for 10 watersheds of tributary streams to Lake Tahoe, California-Nevada, to describe relationships between watershed disturbance and water quality degradation from nonpoint sources. Discharge-weighted annual average concentrations of nitrate (as NO3-N), soluble P, total P and suspended sediment were plotted against the proportion of each watershed represented as disturbed and imperviously covered land of various land-use classes. Comparisons between land-use and runoff water quality demonstrated significant relationships between increased coverage and disturbance in the watersheds and decreased water quality. The concentrations of NO3-N, total P, and suspended sediment in the streams increased significantly with the disturbance of high hazard lands (erodible soils, steep slopes, stream environment zones). Increased disturbance of lower hazard lands (less erodible soils) resulted in increases in the concentrations of soluble and total P. The patterns of significance and slopes of the relationships demonstrated increased nonpoint source water quality degradation with increased land disturbance in these Sierra Nevada watersheds.