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Scientific Investigations Report 2007–5224

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

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Introduction

The Federal Clean Water Act requires states to identify impaired water bodies and to define total maximum daily loads (TMDLs) that a water body can receive and still meet water-quality criteria and attain beneficial uses. Tomales Bay and its two primary tributary watersheds, Lagunitas Creek and Walker Creek, are listed as “impaired by sediment” by the San Francisco Bay Regional Water Quality Control Board (SFRWQCB). Sediment TMDLs are scheduled for development by 2007 ( http://www.waterboards.ca.gov/tmdl/docs/303dlists2006/final/r2_final303dlist.pdf ). Data derived from this project provide baseline information that will be used to develop and implement sediment TMDLs for Tomales Bay and its watersheds, and to provide supporting information for additional TMDLs (pathogens, nutrient, and mercury) and restoration efforts for four federally listed aquatic species that are affected directly by sediment loading in the Tomales Bay watershed. The federally listed species include coho salmon, steelhead trout, California freshwater shrimp, and California red-legged frogs.

Data collection included daily and seasonal (October to May) suspended-sediment monitoring and more frequent (15-minute) optical-backscatter (turbidity) monitoring at two U.S. Geological Survey (USGS) streamflow-gaging stations: Lagunitas Creek at Samuel P. Taylor State Park, California (Lagunitas Creek SPT) and Walker Creek near Marshall, California, (Walker Creek). Available funding was limited for data collection at the Lagunitas Creek near Point Reyes gaging station (Lagunitas Creek PRS). Data collection at this site included periodic seasonal suspended-sediment sampling, and more frequent (15-minute) optical-backscatter monitoring. Lack of sufficient sediment sampling (n=14 over the 3-year project period) at Lagunitas Creek PRS resulted in insufficient data to develop a reasonable field-based calibration equation; thus, only the uncalibrated raw sensor-output voltages are available for this station. Locations of the stream-discharge gaging stations are shown on figure 1 .

Although sediment discharge in Lagunitas Creek and Walker Creek can vary significantly due to changes in concentration and streamflow, collection and calibration of optical-backscatter data provided a continuous time-series of concentrations under changing streamflow- and sediment-supply conditions. These continuous time-series sediment data are valuable to the TMDL process and habitat restoration efforts because they capture temporal variability and provide detailed information about the timing of sediment peaks and duration of elevated sediment concentrations.

Purpose and Scope

This report summarizes suspended-sediment and optical-backscatter data collected by the USGS at three gaging stations in Lagunitas Creek and Walker Creek during water years 2004–06. These data were used to determine mean daily, annual, and event-based suspended-sediment transport within the two primary tributaries to Tomales Bay. Traditional suspended-sediment sampling was supplemented with more frequent (15-minute) optical-backscatter monitoring at all three gages to capture the rapid temporal variations in sediment concentration and to test the use of optical backscatter as a sediment surrogate in this environment.

Daily mean and total seasonal suspended-sediment discharge for stations Lagunitas Creek SPT and Walker Creek are published in the USGS Annual Water-Data Reports (Webster and others, 2005; U.S. Geological Survey, 2006; 2007). This report describes data-collection methods and summarizes suspended-sediment concentration, grain size, turbidity, and optical-backscatter data. Optical-backscatter calibration curves, plots of edited and calibrated optical-backscatter data, and a table of storm-event data also are presented.

Study Area

Lagunitas Creek and Walker Creek are the two primary tributary watersheds to Tomales Bay. Land use in the 103-mi 2 Lagunitas Creek watershed is mixed, with about half of the watershed used for public purposes such as water supply and for a State Park and the other half used for limited agriculture, grazing, and rural residential development. Land use in the 76-mi 2 Walker Creek watershed consists primarily of privately owned beef and dairy operations.

Pervasive cutbank and hillslope erosion throughout the study area delivers sediment to mainstem channels, resulting in aggradation of lower tributary channels. Analyses of digitized bathymetric maps from 1861, 1931, 1957, and 1994 indicate that the highest sedimentation rates in Tomales Bay occurred from 1931 to 1957 (Rooney and Smith, 1999). Haible (1980) documented chronic erosion (channel incision) of 3–15 ft in the upper reaches of Walker Creek and sedimentation (aggradation) of 3–6 ft in the lower reaches of Walker Creek over the period 1915–75. Comparison of topographic maps published between 1862 and 1976 indicates that accumulation of sediment at the Lagunitas delta was greatest from 1861 to 1931 (Neimi and Hall, 1996). Although rates have slowed, erosion and sedimentation in both watersheds continues, Anima and others (1988) reports the accumulation of 10 million ft 3 of sediment [roughly more than 37,000 dump truck loads (at 10 yd 3 each)] on the Lagunitas and Walker deltas after the 1981–82 flood.

Acknowledgments

This project was funded by the joint National Park Service—USGS Water Quality Assessment and Monitoring partnership program and was developed through coordination with Point Reyes National Seashore. Brannon Ketcham (Point Reyes National Seashore) and Scott Wright (USGS–Sacramento, California) were responsible for project development. Neil Ganju (USGS–Sacramento, California) provided insightful advice throughout the project. Reviews by Scott Wright and Mark Uhrich (USGS–Portland, Oregon) improved the original manuscript.

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