The Napa River Watershed is almost entirely within Napa County (a small portion is also in Solano County). It is defined by Mt. St. Helena to the north, the Mayacamas Mountains to the west, Howell Mountain, Atlas Peak, and Mt. George to the east, and the Napa-Sonoma Marsh to the south.
The Putah Creek Watershed lies in four counties: Napa County, Lake County, Solano County and Yolo County. Putah Creek begins in Lake County, and empties into the Sacramento River. The watershed is bounded by Howell Mountain and Atlas Peak to the west and the Blue Ridge and Vaca Mountains to the east.
The Suisun Creek watershed falls within Solano and Napa Counties. Separated from the Napa Valley by Mt. George in the west and bounded by the Vaca Mountains in the east, the watershed contains the upper reaches of Suisun Creek and several of its tributaries including Wooden Valley and Gordon Valley Creeks.