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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5224

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Summary

Suspended-sediment transport in the Tomales Bay watershed during water years 2004–06 is characterized in this report. The U.S. Geological Survey collected suspended-sediment concentration, grain size, turbidity, and optical backscatter data at three gaging stations: Lagunitas Creek Samuel P. Taylor (SPT) (station 11460400), Lagunitas Creek Point Reyes Station (PRS) (station 11460600), and Walker Creek (station 11460750). Sediment samples analyzed for concentrations and grain size were used to calibrate optical sensor output to suspended-sediment concentrations at Lagunitas Creek SPT and Walker Creek. Insufficient sediment data were collected at Lagunitas Creek PRS to develop a calibration equation. These data will be valuable during the development and implementation of total maximum daily loads (TMDLs) for the Tomales Bay watershed, which is scheduled for completion in 2007, and for restoration efforts for four federally listed aquatic species that are affected directly by sediment loading in the Tomales Bay watershed. This project also provided an opportunity to evaluate the suitability of optical sensors in steep coastal watersheds, and a chance to compare optical sensors with daily sediment stations.

Significant optical-data loss occurred due to measurements that exceeded the sensor’s upper measurement limit, malfunctioning of the sensor and (or) recording equipment, aggradation of the channel bed, and fouling of the sensors. Percent valid data ranged from 20 to 87 percent during the 3-year project period (water years 2004–06). 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 where SSC was greater than 100 mg/L).

Data collection is challenging in dynamic systems with high sediment yields, unstable channel bed conditions, and wide variations in the concentration and particle size of the suspended load. Poorly fit calibration equations decreased the reliability of using optical data to determine actual sediment discharge in this study. Improving the reliability of optical sensors as surrogates for more traditional labor-intensive methods of measuring suspended-sediment transport would require relocation of the optical sensors to more stable stream reaches to avoid changing channel bed elevations. Use of multiple optical sensors at a single site, with the gain of two sensors adjusted to measure average conditions (lower sensor gain) and peak conditions (higher sensor gain), also may significantly improve data collection. Limited funding was available for sediment data collection at Lagunitas Creek PRS, which has a relatively stable channel bed and deep gaging pool. The Lagunitas Creek PRS site shows the greatest potential for using an optical backscatter sensor (OBS) as a surrogate for more traditional sampling. Interruptions in the data record at Lagunitas Creek PRS primarily were due to biofouling; therefore, data loss could be improved easily at this site by regular cleaning and maintenance of the OBS.

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