Comparison of estimated and measured sediment yield in the Gualala River

This study compares quantitative erosion rate estimates developed at different spatial and temporal scales. It is motivated by the need to assess potential water quality impacts of a proposed vineyard development project in the Gualala River watershed. Previous erosion rate estimates were developed using sediment source assessment techniques by the North Coast Regional Water Quality Control Board and by the California Geological Survey. Reported in this study is the estimated sediment yield under existing conditions in Soda Springs Creek (1.5 mi2) based on similar, but modified, techniques. The modified technique incorporated elements of both prior approaches, substituting detailed data for road erosion and adding site specific data for landslides not previously available. This study also reports sediment yields determined by a suspended sediment monitoring program utilizing Turbidity-Threshold Sampling techniques. The monitoring program produced data relating stream turbidity, suspended sediment concentration, and stream discharge during Hydrologic Years (HY) 2006 and 2007. Turbidity and streamflow data collected for HY 2008 to 2011 were used in conjunction with the relationship between turbidity and suspended sediment concentration from HY 2006 to 07 to estimate sediment yields over an additional 4 year period. Measured sediment yields are substantially lower than predicted by sediment source assessment techniques. Variation in geomorphic processes over time and space and methodological problems of sediment source assessments may be responsible for these apparent discrepancies.