Most of the San Francisco Bay Area’s drinking water starts as snow in the Sierra Nevada mountains, traveling hundreds of miles through aqueducts and pipelines before reaching your tap. But the specific source depends heavily on where in the Bay Area you live. The region has no single water system. Instead, a patchwork of agencies each pulls from different rivers, reservoirs, and underground basins to serve roughly 8 million people.
San Francisco and the Peninsula: Hetch Hetchy
If you live in San Francisco, on the Peninsula, or in parts of the South Bay and southern Alameda County, 85% of your water comes from a single source: snowmelt stored in Hetch Hetchy Reservoir, which sits on the Tuolumne River inside Yosemite National Park. The water travels about 167 miles by gravity through a system of tunnels and pipelines to reach the Bay Area, losing elevation the entire way.
Hetch Hetchy water is famously clean. The reservoir is one of only a handful of surface water sources in the country that is exempt from federal and state filtration requirements. It still gets treated with ultraviolet light and chlorine disinfection before delivery, along with fluoride and pH adjustments, but it skips the heavy filtration process most city water goes through. The San Francisco Public Utilities Commission (SFPUC) serves about 2.7 million people across four counties with this system, supplementing the Sierra supply with rainfall collected in local reservoirs on the Peninsula and in the East Bay, plus some groundwater.
East Bay: The Mokelumne River
The East Bay Municipal Utility District (EBMUD) serves 1.4 million customers in Alameda and Contra Costa counties, and its water comes from a different Sierra Nevada river entirely. EBMUD captures snowmelt across 575 square miles of mostly undeveloped watershed along the Mokelumne River, storing it at Pardee Reservoir about 90 miles east of the Bay Area. Pardee holds roughly 204,000 acre-feet of water, enough to supply EBMUD’s customers for about 10 months.
If you live in Oakland, Berkeley, Richmond, Walnut Creek, or surrounding cities, this is your primary water source. EBMUD also maintains local reservoirs like Briones, San Pablo, and Upper San Leandro for supplemental storage, but the Mokelumne system is the backbone.
South Bay: A 50/50 Split
Santa Clara County has a more complicated water picture. About half of the county’s supply comes from local sources, mainly groundwater pumped from underground basins and runoff captured in local reservoirs. The other half is imported from hundreds of miles away, arriving as Sierra Nevada snowmelt and rainfall that flows into the Sacramento-San Joaquin River Delta before being pumped south through massive state and federal water conveyance systems.
Santa Clara Valley Water District manages this balancing act, blending local groundwater with imported supplies that travel through the State Water Project and the federal Central Valley Project. Some areas in the northern part of the county also receive Hetch Hetchy water through the SFPUC’s wholesale system. This reliance on multiple sources gives the South Bay some flexibility, but it also means the region is sensitive to disruptions in the Delta, which serves as a critical chokepoint for water moving from Northern to Southern California.
Tri-Valley: State Water Project and Groundwater
Livermore, Pleasanton, and Dublin get their water through Zone 7 Water Agency, which has been importing water from the California State Water Project via the South Bay Aqueduct since 1962. Zone 7 blends this imported supply with local groundwater from the Livermore Valley basin and surface water captured in Lake Del Valle. The agency also banks water in Kern County during wet years, storing it underground for retrieval during droughts.
Zone 7 actively recharges its local groundwater basin with surface water, essentially refilling the underground aquifer so it can be tapped during peak demand, dry years, or emergencies. This strategy makes the Tri-Valley less vulnerable to year-to-year swings in state water deliveries, which fluctuate dramatically. In Water Year 2025, the State Water Project delivered about 2.1 million acre-feet statewide, roughly half of what contractors are theoretically entitled to under full allocations.
North Bay: Local Rain and the Russian River
Marin County’s water supply is unusually local compared to the rest of the Bay Area. About 75% of the district’s supply comes from rainfall on the Mt. Tamalpais watershed and the hills of west Marin, flowing into seven reservoirs. The remaining supply comes from Sonoma County Water Agency, which draws from the Russian River system.
In practice, the local reservoirs do most of the heavy lifting. Recent production data showed Marin Water pulling about 14 million gallons per day from its reservoirs compared to under 1 million gallons per day from the Russian River. Sonoma County itself relies heavily on the Russian River for its own residents, making that system a shared resource across much of the North Bay. Other North Bay communities, like those in Napa County, draw from local sources like Lake Hennessey and other creek-fed reservoirs.
How Full Are Bay Area Reservoirs?
As of late February 2026, most Bay Area reservoirs were at or above their 30-year historical averages for that time of year. North Bay reservoirs were particularly healthy: Nicasio Reservoir sat at 108% of its historical average, Alpine Lake at 105%, and Kent Lake at 131%. East Bay reservoirs were also in solid shape, with San Pablo Reservoir at 102% of average and Briones at 101%.
Lake Del Valle in the Tri-Valley stood out at 192% of its historical February average, reflecting aggressive storage during recent wet conditions. Los Vaqueros Reservoir in eastern Contra Costa County, which serves as a regional backup supply, was at 172% of average. These strong storage levels provide a buffer heading into the dry season, though Bay Area water managers plan on multi-year timelines since a single dry winter can shift the picture quickly.
Recycled Water as a Growing Source
The Bay Area is increasingly turning to recycled water to stretch its supply. San Francisco’s largest current project, the Westside Enhanced Water Recycling Project, is a $213 million system designed to produce up to 2 million gallons per day of recycled water for irrigation in Golden Gate Park, Lincoln Park Golf Course, and other landscaped areas. Construction began in 2017, and deliveries are expected to begin in late 2026.
Two million gallons per day is modest compared to the city’s total consumption, but every gallon of recycled water used for irrigation is a gallon of Hetch Hetchy drinking water that stays available for taps. Similar projects are underway across the region as agencies look for ways to reduce dependence on distant Sierra snowmelt, which climate models suggest will become less reliable as more precipitation falls as rain rather than snow in warming winters.