About two-thirds of snow and rain in California falls north of San Francisco. However, about two-thirds of the state’s water demand is south of San Francisco.
To meet that demand, six cubic kilometers (1.6 trillion gallons) of water per year must be moved from Northern to Southern California. To give you an idea of how much that is, imagine the entire city of San Diego under 20 feet of water!
The water is moved from north to south this way: First it gathers in the Sacramento River and flows south until it reaches California’s delta, a complex system of farmlands and channels. Fresh water is winds its way through the delta to its southern end near the town of Tracy, Calif., located 60 miles east of San Francisco.
From there, the water flows through canals to farms in the southern Central Valley and to cities in Southern California. To get to Southern California, giant pumps lift the water some 2,000 feet over the Tehachapi Mountains. The water is then distributed to reservoirs and pipelines that carry it to most of the people and communities in Southern California.
The weak link in this system of rivers, canals, and pipelines is the delta, which is connected to seawater from the ocean through San Francisco Bay to the west and is bounded by aging levees. If parts of the levees collapse, in the case of an earthquake or flood, seawater would mix with freshwater, disrupting California’s water supplies. As a result, state officials are studying ways to protect the delta system.
-- Michael Dettinger, research hydrologist, Climate, Atmospheric Science, and Physical Oceanography Division