The Tomales Bay Watershed is a 223 square mile coastal watershed in western Marin County, with three major creek systems and a number of smaller drainages flowing into it. The Bay itself is approximately 11 square miles in area. The major land uses in the watershed are livestock grazing, dairy farming, low-density residential, and parklands. There are a total of 483 acres of aquaculture leases within the bay, primarily for oyster cultivation. Samples taken during a comprehensive study by the Department of Health Services in the winter of 1995-96 showed that there are high loadings of total and fecal coliform in most of the eastshore creeks and tributaries discharging into Tomales Bay during rainfall events. These coliform levels threaten the existence of the commercial shellfish industry and also pose a threat to water contact recreation in the Bay. Highest fecal coliform loadings occur in the Walker Creek and Lagunitas Creek watersheds, as well as along a number of small east shore tributary streams. Walker Creek is a particular source of interest to the shellfish industry because of its location in the north central part of the Bay where the majority of the shellfish leases are located. These results were consistent with a number of earlier studies by the Health Department and Regional Water Quality Control Board. The studies have concluded that nonpoint source runoff from dairies and grazing lands are the likely source of the coliform loadings. A recent illness outbreak attributable to a human virus has also highlighted the need to address potential human sewage inputs to Tomales Bay, including residential on-site sewage systems and dumping from recreational boaters. The Regional Board and Department of Health Services are currently doing an onsite sewage system survey along the northeast side of Tomales Bay; this is expected to be expanded to other areas of the Bay in the future. The Regional Board also oversees eight sewage treatment facilities in the watershed, all of which operate under Waste Discharge Requirements. Board staff are also working with local, state, and federal park agencies to improve sewage facilities for boaters and to increase boater education regarding sewage discharges.