<P> The Central Valley watershed, which incorporates the Sacramento River, San Joaquin River and Tulare Lake regions, is the largest in California, draining over a third of the state--60,000 square miles (160,000 km)--and producing nearly half the total runoff . The Sierra Nevada snowpack feeds Central Valley river systems and is a critical source of water in the state's long dry season when little if any precipitation falls . Up to 30 percent of California's water supply is from snowpack . Much of California's extensive reservoir and aqueduct system is designed to store and capture runoff from the Central Valley watershed . The Sacramento and San Joaquin Rivers converge at the Sacramento--San Joaquin River Delta, a large fresh - water estuary from where much of the state's water supply is withdrawn . The Central Valley watershed provides most of the water for Northern and Central California, as well as a significant chunk of Southern California's usage . </P> <P> The North Coast watershed receives the highest annual precipitation of any California watershed . It incorporates many large river systems such as the Klamath, Smith and Eel, and produces over a third of the runoff in the state . With a few exceptions, the North Coast watersheds are relatively undeveloped and provides water only to local communities . The vast majority of water flows uncontrolled into the Pacific Ocean . There have been many proposals to divert water from North Coast rivers to increase water supplies in the rest of California, but these projects were never realized, both due to cost and potential environmental harm . </P> <P> The Colorado River originates more than 1,000 miles (1,600 km) from California in the Rocky Mountains of Colorado and Wyoming and forms the state's southeastern border in the Mojave Desert . Unlike the other California watersheds, essentially all of the water flowing in the Colorado originates outside the state . The Colorado is a critical source of irrigation and urban water for southern California, providing between 55 and 65 percent of the total supply . </P> <P> The Central and South Coast watersheds include the most populous regions of California--the San Francisco Bay Area, Los Angeles and San Diego--but have relatively little natural runoff, requiring the importation of water from other parts of the state . </P>

Where does southern california get most of its water