The Central Valley used to be an arm of the sea, but deposits from the Coast Range and Sierras gradually filled it in. The westernmost part of the lower bioregion, nearest the Coast Range, is somewhat rolling, but the area flattens until it is nearly plane at the Yolo Bypass. Water Though water is relatively abundant in the upper bioregion, the Central Valley section uses more water than it normally receives. Irrigation water comes largely from Putah and Cache Creeks, with the sloughs carrying off the tailwater from irrigated fields. Cache Creek Diversion Dam supplies irrigation water for Yolo County, while the Putah Creek diversion supplies water mainly to Solano County, outside the historic watershed.
Most drinking water comes from groundwater.
Some irrigation water is also pumped from groundwater. One lake in the bioregion, Clear Lake, is natural, the result of a landslide that dammed its original outlet. The other large lake in the bioregion, Lake Berryessa, is a reservoir created when Putah Creek was dammed in 1957. Virtually all other lakes in the region are also reservoirs. Water quality and abundance are issues in the region.