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Aerial view of the Sacramento-San Joaquin Bay Delta. Two-thirds of Californians get some water from here. Mark Andrew Boyer/KQED
Aerial view of the Sacramento-San Joaquin Bay Delta. Two-thirds of Californians get some water from here. (Mark Andrew Boyer/KQED)

Bay Area: Do You Know Where Your Water Comes From?

Bay Area: Do You Know Where Your Water Comes From?

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Listen to all six episodes of our State of Drought series on Spotify. Episode transcript.

The Bay Area water system is a byzantine patchwork of agencies — more than 50 in all — that provides water to customers. Some are the ones you see on your water bill. Others are middlemen that provide water to local agencies at the wholesale level.

And some of that water makes a long journey. Southern California has the reputation for tapping far-flung sources for its water needs, but the Bay Area is in the same boat.

More than two-thirds of the Bay Area’s water supply comes from outside the region, which means in extreme drought years, local water districts are competing with many others around the state for limited supplies.


As we suffer under another dry period, some parts of the Bay Area are experiencing the drought more acutely because of where they get their water. We’re going to break it down for you.

Bay Area Water Districts By Major Source of Supply

Graphic showing where bay area water districts get their water.
A breakdown of where different regions of the Bay Area get their water. (Source: ABAG Infrastructure Vulnerability & Interdependencies Study (2014))

Hetch Hetchy Water System

The system originates more than 100 miles from its primary customers, in Yosemite National Park. O’Shaughnessy Dam was built on the Tuolumne River in 1923 to create Hetch Hetchy Reservoir. The water travels through a series of pipelines before it reaches the Bay Area and blends with five local reservoirs. The Tuolumne River joins the San Joaquin River and flows into the Delta.

San Francisco depends on Hetch Hetchy for its water. The city’s direct access to a large amount of stored water means that even when precipitation levels are lower than average, residents don’t start seeing mandatory water restrictions right away. The San Francisco Public Utilities Commission is also a water wholesaler, selling water to places on the peninsula like Burlingame, Palo Alto, Hillsborough and Redwood City.

The city has long resisted proposals to drain Hetch Hetchy. In 2012, advocates of restoring Hetch Hetchy Valley put a measure on the San Francisco ballot that would have required the San Francisco Public Utilities Commission to study draining the reservoir and shifting the water to other storage facilities. The measure was defeated. In 2018, then Interior Secretary Ryan Zinke met with proponents of restoration, but the discussions didn’t lead anywhere.

Click to enlarge map. (Credit: By Shannon1 [GFDL or CC-BY-SA-3.0-2.5-2.0-1.0], via Wikimedia Commons)
Click to enlarge map. (Credit: By Shannon1 via Wikimedia Commons)

The Sacramento-San Joaquin Delta

California’s two major rivers, the Sacramento and San Joaquin, fed by half a dozen others, come together in this inland delta just east of San Francisco Bay. The Delta’s watershed makes up about 45 percent of the state in all. Two-thirds of Californians use Delta water, delivered mainly through two major canal systems, the State Water Project and the Central Valley Project.

Map of the Sacramento-San Joaquin Bay Delta
The Sacramento-San Joaquin Bay Delta is a massive system of waterways that provides water to fish, wildlife and people throughout California.

When we have prolonged drought, water deliveries from both these projects diminish, except for some drinking water supplies. Napa, Santa Clara, and Contra Costa counties all get some water out of the Delta. The Sacramento-San Joaquin Delta has seen dramatic ecological decline due to habitat loss, invasive species and highly altered water flow.

Santa Clara County — and San Jose, its largest city — are in a particularly difficult water situation. In 2021, Santa Clara Valley Water, the water agency that serves San Jose, only received 5% of the water it contracts from the state, a quarter of what it sources from the feds, and has seen little local rainfall.
On top of all that, the largest reservoir in Valley Water’s system is virtually empty at 3% full, after it was emptied so that the Anderson Dam near Morgan Hill could undergo seismic retrofitting. Valley Water is asking Santa Clara residents to cut water use by 15% to help stretch their water supplies in case we have another dry winter.

Russian River Water System

New vegetation grows on what was once the lake bed of Lake Mendocino on June 11, 2021.
New vegetation grows on what was once the lake bed of Lake Mendocino on June 11, 2021.

The 110-mile Russian River begins north of the Bay Area in Mendocino County and flows south until it reaches the Pacific Ocean west of Santa Rosa. The water system consists of reservoirs at Lake Sonoma, Lake Mendocino, and water diverted from the Eel River into the Russian River.

This region is unique in that no water comes from the Sierra Nevada snowmelt. The Russian River watershed sits isolated from the rest of the state, and in dry times, communities in the region are on their own. In normal years, Sonoma also sells water to Marin County, so in dry years both counties feel the pinch.

Marin Water officials are concerned enough about their situation to be considering two pricey options. One is to lease a desalination plant for $37 million. That would provide a third of the county’s drinking water needs. The other option is to build a water pipeline over the Richmond-San Rafael Bridge to transport water purchased from an entity with access to Delta water. That pipeline would cost between $66 – $88 million. Marin built a similar pipeline back in the late 1970s when it was especially dry.

Mokelumne River Water System

This river originates in the Central Sierra Nevada and flows west until it reaches the Sacramento-San Joaquin Delta and San Francisco Bay. The East Bay Municipal Utility District built Pardee Dam on the river near Stockton in 1929. Water is delivered to the Bay Area through the 85-mile Mokelumne Aqueduct, which diverts the river’s water before it reaches the Delta.

Map of the EBMUD aqueducts from the Mokelumne River Watershed.
Map of the EBMUD aqueducts from the Mokelumne River Watershed. (Courtesy EBMUD)

Lake Berryessa

The 23 mile-long reservoir was created in Napa County in the 1950s, when the U.S. Bureau of Reclamation built Monticello Dam on Putah Creek. Lake Berryessa water feeds several big cities in Solano County.

Local Water Supplies

Many water districts use water from the surrounding watershed. It comes from local streams and rivers, fed by rainfall or is pumped from underground aquifers. Some districts also recycle water, which is primarily used for landscape irrigation.

Additional research by Shara Tonn.

A version of this article originally published in 2014.


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