 The United States Government has designated 13,000 acres (53 km2) in the Napa Sonoma Marsh as the San Pablo Bay National Wildlife Refuge.The marsh is fed by Sonoma Creek (which drains the Sonoma Valley), Tolay Creek (originating in the Tolay Lake basin), and the Napa River (which drains the Napa Valley).Although the marsh extends north as far as State Route 12, as a practical matter, most of the marsh is only accessible by boat.