Where Delta Water Comes From and Goes

Water from the Delta provides some portion of water supply for more than 27 million Californians and 3 million acres of irrigated farmland

The 45,600-square-mile Delta watershed is California’s largest, draining 40% of California’s land mass and sending about half of the state’s runoff through the Sacramento and San Joaquin Rivers and into the Delta.  Because of its central location, Delta water provides some portion of water supply for more than 27 million Californians as well as 3 million acres of irrigated farmland, and plays a critical role in sustaining a portion of the state’s $1.9 trillion economy.

Most of the water enters the Delta from the north through the Sacramento River, providing about 86% of the Delta’s freshwater, while the San Joaquin River arrives from the south, providing the remaining 14%. 

If the water is not diverted or otherwise used, it flushes into the San Francisco Bay, under the Golden Gate Bridge, and out to the ocean.  However, an extensive network of dams, diversions and canals built on nearly every stream and river in the Delta watershed enables a significant portion of the water to be diverted in the upper watershed before it reaches the Delta.  Approximately 31% of the flow will be diverted before it ever reaches the Delta to support irrigated agriculture and municipal uses in the Sacramento Valley and the San Joaquin Valley. Of that 31%, 1.6% is diverted by the Mokelumne and Hetch Hetchy Aqueducts and is carried across the Delta to serve San Francisco and the East Bay. 

Delta communities and farmers use Delta water for agricultural and municipal use with in-Delta water remaining relative constant over the past 100 years at 4 percent of inflows.

In the south Delta lie the state’s two largest water projects, the Central Valley Project (CVP) and the State Water Project (SWP), rely on the Delta’s network of channels to convey water from upstream reservoirs to the export pumps which deliver Delta water to the Bay Area, the Central Coast, the San Joaquin Valley, and Southern California.

How much water the projects can export from the Delta varies from year to year.  In an average year, the two water projects combined export about 5.1 MAF or 24% of the Delta’s inflows; however, this amount can vary from as low as 3MAF in dry years to around 6.5 MAF in wet years. 

The location of the water project pumping facilities in the south Delta alter the flow in ways that are harmful to migrating fish. In the Delta, the natural direction of flow is from the Sacramento and San Joaquin Rivers westward to the San Francisco Bay.  However, when water is being pumped from the Delta, the currents tend to flow south towards the pumps, creating altered and even reverse flows.  This, in turn, affects fish movement and migration through the Delta, and can result in fish being drawn into the south Delta where they are subject to increased predation, or towards the pumps where they can become entrained.

Although water from the Delta only accounts for 8% of the state’s total water use, it is nonetheless a critical component of the State’s water supply, providing at least a portion of the water supply for two-thirds of the state’s population as well as irrigating 3 million acres of farmland.  However, reliance on Delta water varies from region to region as well as supplier to supplier. For some regions and suppliers, Delta is water is a small portion of their overall water supply portfolio; others with limited access to alternative sources can depend on water from the Delta for as much as 90% of their water supply, and there are some regions of the state that do not use any Delta water at all.

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