HEADWATER STREAM SEDIMENT LOADS AND ASSOCIATED CARBON AND NITROGEN, SIERRA NEVADA, CALIFORNIA

Knowing the spatial and temporal range of variability of soil erosion and stream sediment loads is important for understanding geomorphology processes and land management effects in forested mountains with changing climate. The Kings River Experimental Watersheds provide seven years of data for eight headwater watersheds with granite-derived soils in the southern Sierra Nevada, California. The average annual sediment loads range from 2 to 17 kg/ha with one watershed having a much higher load of 61 kg/ha. Annual sediment mass, carbon, and nitrogen fluxes were strongly correlated with streamflow and precipitation and have high interannual variation. Somewhat surprising is that snow-dominated streams transport similar loads as streams at the rain-snow transition zone. Also, the stream in the undisturbed watershed (no roads or timber harvest) carries as much sediment as the other streams. Current erosion rates are low compared to long-term erosion rates determined by cosmogenic nuclide analyses. Coarse and fine particulate organic matter makes up 30-50% of the material trapped in the sediment basins; this is expected for heterotrophic, low-order forest streams. Our results suggest that carbon and nitrogen are relatively less protected in material exported from watersheds with coarse-textured soils than in watersheds with finer textured soils, but more carbon and nitrogen overall are leaving the basins with finer textured soils.