Summary of Optical-Backscatter and Suspended-Sediment Data, Tomales Bay Watershed, California, Water Years 2004, 2005, and 2006

Abstract

The U.S. Geological Survey, in cooperation with Point Reyes National Seashore, is studying suspended-sediment transport dynamics in the two primary tributaries to Tomales Bay, Lagunitas Creek and Walker Creek. Suspended-sediment samples and continuous optical backscatter (turbidity) data were collected at three locations during water years 2004-06 (October 1, 2003-September 30, 2006: at two sites in the Lagunitas Creek watershed and at one site in the Walker Creek watershed. Sediment samples were analyzed for suspendedsediment concentration, grain size, and turbidity. Data were used to estimate mean daily and annual seasonal suspendedsediment discharge, which were published in U.S. Geological Survey Annual Water-Data Reports. Data were utilized further in this report to develop field-based optical-backscatter calibration equations, which then were used to derive a continuous time series (15-minute interval) of suspended-sediment concentrations. Sensor fouling and aggradation of the channel bed occurred periodically throughout the project period, resulting in data loss. Although periods of data loss occurred, collection of optical sensor data improved our understanding of suspended-sediment dynamics in the Lagunitas Creek and Walker Creek watersheds by providing continuous time-series storm event data that were analyzed to determine durations of elevated sediment concentrations (periods of time when suspended-sediment concentration was greater than 100 mg/L).Data derived from this project contributed baseline suspended-sediment transport information that will be used to develop and implement sediment total maximum daily loads for Tomales Bay and its tributary watersheds, and provides supporting information for additional total maximum daily loads (pathogens, nutrients, and mercury) and restoration efforts for four federally listed aquatic species that are affected directly by sediment loading in the Tomales Bay watershed. In addition, this project provided an opportunity to evaluate the suitability of using optical data as a surrogate for more traditional labor-intensive methods of measuring suspendedsediment transport in steep coastal watersheds.

Fluvial system dynamics derived from distributed sediment budgets: perspectives from an uncertainty‐bounded application

“…Suspended sediment rating curve for the Samuel P. Taylor State Park (SPT) gauging station. The rating curve is based on instantaneous discharge and corresponding instantaneous readings by a calibrated OBS sensor WY2004–2006 converted to sediment discharge values and scaled to tons/day by the USGS (for method see Curtis, ). Locally‐Weighted Scatterplot Smoothing (LOWESS, after Cleveland ) was employed with a bandwidth of 0.2 (percentage of points included in each of the local regression iterations) and 95% global prediction intervals applied following the method of Loader (, p.30).…”