Where Does California’s Water Come From?

Where Does California’s Water Come From?

California is a state of extremes in terms of its climate. Much of the state enjoys a Mediterranean-style climate, with wet winters followed by dry summers with little or no rain for months. Precipitation levels vary from one year to the next. Droughts come and go; significant, multi-year statewide droughts have occurred several times throughout the past century. Regional droughts have also plagued different areas of the state over the years.

In addition, most of California’s rain and snow fall in the northern half of the state: in the Sierra Nevada Mountains, along the North Coast, and in the Cascade Range. However, most of the state’s population lives in the considerably drier southern half.

Due to the variability in precipitation and the population imbalance, California depends on a network of reservoirs, dams, and aqueducts to store and transport surface water where and when people need it. Groundwater is also key to the state’s water supply.

Surface Water

One of the first big water infrastructure projects in the state was the Los Angeles Aqueduct, which was constructed in 1913 to transport water from the eastern Sierra Nevada Mountains to Los Angeles. The project provided a reliable source of surface water that the young city and its surrounding farms needed to grow and flourish. Other major water infrastructure projects followed in the mid-20th century—the federal Central Valley Project (CVP) and the California State Water Project (SWP).

From the Feather River watershed in the northern Sierras, SWP supplies water to 25 million Californians—about two-thirds of the state’s population—in the San Francisco Bay Area, San Joaquin Valley, and Southern California. SWP also provides irrigation for 750,000 acres of farmland.

Similarly, CVP provides irrigation for 3 million acres of farmland—about one-third of the state’s agricultural land—and drinking water to around 1 million households. The CVP system reaches from Shasta Lake near Redding in Northern California down to Bakersfield in the south.

In Southern California, the Colorado River constitutes another major source of water for both agriculture and urban areas. Over a period from 1933 to 1941, the Metropolitan Water District of Southern California built the Colorado River Aqueduct to tap into this vital resource. Today, the aqueduct stretches 242 miles from Lake Havasu near the California-Arizona border to a reservoir near Riverside, California, carrying more than a billion gallons of water each day. Los Angeles, San Diego, and other cities and farms in the southern half of the state depend on this imported water. Other canals, such as the All-American Canal, also import water from the Colorado River.

The river is truly a shared resource. California and six other Western states depend on the Colorado River and its tributaries for their water supply. The use of the river is governed by an agreement between these states that allots California 4.4 million acre-feet of water each year. (1 acre-foot of water is equivalent to 326,000 gallons, or the amount of water needed to flood 1 acre of land 1 foot deep.)


Besides surface water, groundwater is also important to the state’s water supply. Groundwater is the source of about 30% of the water used by California’s cities and farms. In dry years, that percentage can increase to 60%. Some communities in the state rely exclusively on groundwater for their supply, while others rely on a mixture of groundwater and surface water. Some have no access to groundwater at all.

Challenges and New Opportunities

California’s water supply faces many challenges, including a rising population, aging infrastructure, climate change, and other hurdles.

Population and Infrastructure

Ever since the SWP and CVP began in the mid-1900s, the state’s population has increased by some 18 million, straining these systems’ capacity. California’s population continues to increase and will surpass 40 million in 2018 and 45 million by 2035. The state’s water infrastructure is aging and in dire need of repair. The American Society of Civil Engineers estimated in 2013 that California needs $44.5 billion in repairs to its drinking water infrastructure. This past winter, the extensive damage to the spillway at the Oroville Dam—one of the linchpins in the SWP—demonstrated just how fragile much of that infrastructure is.

Climate Change

Scientists expect that climate change will lead to greater variability in weather patterns, potentially increasing the frequency and length of California’s droughts. The recent drought, which began in 2011 and ended this year, included the driest four-year period on record in California.

Warmer temperatures will cause more rain rather than snow in the mountains and will cause the snow to melt earlier and faster. The new pattern may challenge water managers’ ability to store water and to release it, as needed, during the dry months.


Besides the Oroville Dam crisis, the record wet winter of 2016-17 also revealed another challenge that plagues California’s water infrastructure: a relative lack of storage to capture water during wet years.

Heavy deluges of rain during the winter’s intense storms filled many reservoirs to capacity and prompted them to release water in order to avoid overflows. The water flows into rivers and streams and out to the Pacific Ocean, where it is essentially lost. Creating more storage could allow California to catch and save more of this runoff.

Traditional infrastructure such as dams and reservoirs can store large amounts of water, but underground water banks are also being explored as another solution to the storage crisis. Scientists estimate that up to 1,300 million acre-feet of water could be stored underground, as opposed to the 42 million acre-feet that the state’s current above-ground reservoir storage system can hold at capacity. Storing water underground also provides protection from evaporation, which may become a more significant problem as the climate becomes warmer.


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