According to the U.S. Census Bureau, the county has a total area of , of which is land and (5.5%) is water. Two main watercourses drain the county: Cache Creek, which is the outlet of Clear Lake; and Putah Creek. Both of these flow to the Sacramento River. The main streams which flow into Clear Lake are Forbes Creek, Scotts Creek, Middle Creek and Kelsey Creek. At the extreme north of the county Lake Pillsbury and the Van Arsdale Reservoir dam the Eel River, providing water and power to Ukiah in Mendocino County.