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July 25, 2025
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Securing Our Imported Supplies
East Branch of the California Aqueduct, photo courtesy of California Department of Water Resources
Our Foundation: Securing Our Imported Supplies
It is a simple reality: Southern California doesn’t produce enough water locally to meet the demands of our population and economy. The region imports more than half of the water it uses, most of it through Metropolitan. We bring this water to the region from two sources: the Sacramento and San Joaquin rivers, via the State Water Project, and the Colorado River, via the Colorado River Aqueduct. Together these imported supplies provide a foundation of water reliability in Southern California. Knowing that these critical imported resources cannot easily be replaced, Metropolitan works hard to ensure their continued reliability in the face of climate change.
The Colorado River Aqueduct: Where It All Begins
The Colorado River has been the backbone of Southern California’s imported water supply for 80 years. Built and operated by Metropolitan, the Colorado River Aqueduct carries water from the river 242 miles across the desert to Southern California. It provides about 20 percent of the water used in Metropolitan’s 5,200-square-mile service area.
Originally conceived by William Mulholland and designed by Metropolitan’s first Chief Engineer Frank Weymouth, the Colorado River Aqueduct was the largest public works project built in Southern California during the Great Depression. Over seven years, 35,000 people toiled across the desert to construct the engineering feat.
Lifeblood of the Southwest
Since Metropolitan’s first CRA deliveries in 1941, water from the Colorado River has helped transform Southern California into the thriving region it is today. The river also sustains people, farms, businesses, tribal nations, and wildlife across six other states and Mexico. But while demands for Colorado River water have grown, supplies from the river have not. The resulting imbalance will likely grow as the climate warms, reducing run-off in the river’s watershed. This is the latest challenge in the enduring struggle to share the Colorado River among the 40 million people and 5 million acres of farmland that rely on it.
Hoover Dam, photo courtesy of United States Bureau of Reclamation
A New Era of Sustainability
For the past two decades, Metropolitan has been committed to increasing the sustainability of the Colorado River by building partnerships inside and outside California to develop creative conservation and storage programs based on collaboration, not conflict. The agreements and trust forged provide a critical foundation as water agencies across the west negotiate the next steps needed to address the river’s imbalance.
State Water Project: Delivering Supplies from the Northern Sierra
Lake Oroville, photo courtesy of DWR
Along with the Colorado River, Southern California’s other main source of imported water — typically about 30% of the region’s annual supply — comes from the Northern Sierra. It is delivered here through the system of reservoirs and aqueducts known as the State Water Project, the largest state-built water and power system in the nation. The water collects in the Feather River, passes through Lake Oroville and weaves its way through the Sacramento-San Joaquin Delta before entering the 444-mile California Aqueduct to Southern California. Along the way, the State Water Project provides a critical water supply to more than 27 million Californians from the Bay Area to San Diego and sustains farmland that produces nearly half of the nation’s fruits and vegetables.
For several decades, mounting pressures on the Delta — ranging from environmental protections and water quality regulations to a changing climate marked by increasingly extreme weather patterns — have reduced the reliability of SWP deliveries to Southern California. Recognizing the importance of the Delta’s health to our water future, Metropolitan is committed to science-based watershed management, multi-benefit environmental initiatives and community-based partnerships with local stakeholders in the Delta. As we evaluate long-term investments related to water supply, we are ensuring they are aligned with our Climate Adaptation Master Plan for Water planning process.
The Sacramento-San Joaquin Delta:
Hub of the State Water Project
The Delta is critical to the state’s economy, water supply and quality of life for millions of Californians. It is home to an important ecosystem and diverse historical communities. Learn more about this unique place, its challenges and Metropolitan’s role in helping to develop cooperative regional and state solutions.
Saving for a Not-So-Rainy Day: Exchanges, Transfers & Water Banking
California has always had highly variable weather – swinging from very dry years to very wet ones. As the climate changes, those swings are expected to become even more extreme, which means greater fluctuation in the water resources Metropolitan imports. To ensure Southern California has reliable water supplies, regardless of these swings, Metropolitan has made significant investments in storage. We’ve increased our storage capacity by 13 times since 1990, so when there is a very wet year in the Sierra or Colorado River Basin, we can store water for use in dry years. Some of that storage increase has come through investments in surface reservoirs, like Diamond Valley Lake and Lake Mead, through the Intentionally Created Surplus program. But equally important is the storage capacity Metropolitan has developed through partnerships with water agencies across California for groundwater banking and exchanges.
Antelope Valley-East Kern High Desert Water Bank
Antelope Valley-East Kern High Desert Water Bank
Banking Across the State
Through its groundwater banking agreements, Metropolitan stores water with partner agencies along the State Water Project and the Colorado River Aqueduct. They either put the water into their groundwater basins using spreading grounds or exchange it for water that they would have pumped out of the ground for use. In dry years, when Metropolitan’s imported supplies are limited, these partners either pump up some of the stored water for Metropolitan’s use or provide their other supplies in exchange.
Metropolitan’s Partners:
- Semitropic Water Banking and Exchange Program, Wasco
- Arvin-Edison Water Management Program, Arvin
- Kern Delta Water District Water Management Program, Taft
- Mojave Storage Program, Mojave
- Antelope Valley-East Kern Water Agency (AVEK) High Desert Water Bank
- Desert Water Agency/Coachella Valley Water District Advanced Delivery Program, Coachella Valley
Water Transfers & Other Agreements
Metropolitan has developed other partnerships and programs to improve the reliability of imported supplies. This includes direct water transfers and exchanges, in which water is directly purchased in dry years, or exchanged for more water in wet years. Metropolitan’s portfolio includes such agreements with state and federal water agencies, water districts and individuals. On the Colorado River, Metropolitan also has developed specialized conservation programs in which Metropolitan invests in agricultural conservation and receives the conserved water.
Palo Verde Valley