Tri-Valley Water Supply

Where Does Tri-Valley Water Come From?

California relies on three main, interconnected water sources: mountain snowpack, reservoirs, and aquifers:
• The Sierra Nevada snowpack, which melts in the spring and summer, provides runoff to rivers and reservoirs. In normal years, melted snowpack typically supplies about 30 percent of the state’s water supply.
• The state’s reservoirs store water from precipitation events and receive the runoff from melting snowpack. Less precipitation and snowpack results in decreased reservoir storage.
• When the supply of surface water is unable to meet demand, groundwater is pumped from aquifers, accounting for nearly 60 percent of the state’s water supply in a dry year.

Diagram of the water cycle, courtesy of NASA. Describes how water evaporates from the surface of the earth, rises into the atmosphere, cools and condenses into rain or snow in clouds, and falls again to the surface as precipitation. The water falling on land collects in rivers and lakes, soil, and porous layers of rock, and much of it flows back into the oceans, where it will once more evaporate.


Currently 80 percent of our water starts as snow melt from the Feather River watershed on the western slopes of the Sierra Nevada.  Another ten percent of the water provided to the Tri-Valley area is from the local watershed, stored in Lake Del Valle.  The other ten percent comes from groundwater.

Included below is a video which illustrates the natural water cycle (Courtesy of NASA), which produces groundwater flowing through our creeks and streams to the San Francisco Bay.


Aquifers are bodies of rock and sediment that are saturated with water.  The aquifer can be made of sand and gravel, sandstone and gravel, and other rocks.  Each is made up of permeable material.

Aquifers are important because…

  • We can only use about 1% of earth’s surface water.
  • 99 percent of all usable water is actually groundwater.
  • Aquifers provide 99 percent of groundwater.
  • 50 percent of all potable water (suitable for drinking) comes from aquifers.

Livermore Valley Groundwater Basin

Unlike most other Bay Area communities, the Livermore Valley benefits from local water-storage capacity in an underground basin that provides an increased water-supply reliability. The Valley’s main groundwater basin has an estimated storage capacity of 250,000 acre-feet (an acre-foot is about 326,000 gallons, enough to supply two households per year). To prevent overpumping, the basin is cooperatively managed by Zone 7 and its retailers so that, even during multiyear droughts, groundwater levels do not drop below historic low levels of 130,000 acre-feet. The groundwater basin extends from the Pleasanton Ridge east to the Altamont Hills (about 14 miles) and from the Livermore Upland north to the Orinda Upland (about 3 miles). Surface drainage features include Arroyo Valle, Arroyo Mocho, and Arroyo las Positas as principal streams, with Alamo Creek, South San Ramon Creek, and Tassajara Creek as minor streams. All streams converge on the west side of the basin to form Arroyo de la Laguna, which flows south and joins Alameda Creek in Sunol Valley.

Principal Streams
Arroyo Valle and Arroyo Mocho are local streams that originate in Santa Clara County and flow through the Livermore-Amador Valley into Lake Del Valle as well as to Arroyo de la Laguna, ultimately flowing to the San Francisco Bay. Arroyo de la Laguna is the main tributary to Alameda Creek, the second largest drainage to the San Francisco Bay.














A watershed is the land over which water flows into a common water body. Bethany Reservoir and Lake Del Valle collect the water that flows across the land surrounding them. Limiting the impact on the watersheds will help to protect your drinking-water supplies.

The Alameda Creek Watershed consists of many streams and groundwater channels that converge and drain the region’s stormwater runoff into Alameda Creek, which flows to the Bay. Covering about 700 square miles from Mt. Diablo in the north to Mt. Hamilton in the south, it is one of the Bay Area’s largest watersheds.

Alameda Creek is the third largest tributary to San Francisco Bay (after the Sacramento and San Joaquin rivers). The Alameda Creek watershed is the geographic area drained by Alameda Creek and its tributaries, encompassing more than 680 square miles of the East Bay. The watershed ranges from streams draining the south slopes of Mount Diablo in the north, most creeks from Mount Hamilton in the south, and arroyos as far east as Altamont Pass. Along its course, Alameda Creek provides wildlife habitat, water supply, a conduit for flood waters, opportunities for recreation, and a host of aesthetic and environmental values. The creek and three major reservoirs in the watershed are used as water supply by the San Francisco Public Utilities Commission, Alameda County Water District and Zone 7 Water Agency

Who Provides The Water?

The State Water Project‘s (SWP) main purpose is to store water and distribute it to 29 urban and agricultural water suppliers in Northern California, the San Francisco Bay Area, the San Joaquin Valley, the Central Coast, and Southern California. Of the contracted water supply, 70 percent goes to urban users and 30 percent goes to agricultural users.

To reach many of us, water must travel long distances through complex delivery systems such as the California Aqueduct Project. The SWP is the nation’s largest state-built water and power development and conveyance system.The Project is also operated to improve water quality in the Delta, control Feather River flood waters, provide recreation, and enhance fish and wildlife.

Planned, designed, constructed and now operated and maintained by the California Department of Water Resources, this unique facility provides water supplies for 25 million Californians and 750,000 acres of irrigated farmland.

Tri-Valley water wholesaler, the Zone 7 Water Agency, has a 2 percent share of the water provided by the State Water Project.  Zone 7 treats the water so it’s drinkable and sells it to four water retailers and a few ranchers and farmers.

Map of Water Suppliers

Identify who provides your water by referencing the map below.