What Sites Reservoir Could Mean for California’s Water Future

Joe Grindstaff

Oct 2

7 min read

Golden grassy hills with scattered trees overlook a deep blue lake under a clear sky.

California has a water storage problem that cannot be measured only by the amount of water currently sitting behind its dams.

The larger challenge is timing.

California can experience powerful storms and periods of heavy runoff, followed by long stretches of dry weather. If water cannot be captured when it is available, some of that opportunity is lost. At the same time, storing more water involves difficult questions about cost, environmental protection, water rights, local impacts, and who will receive the benefits.

Sites Reservoir is one of California’s most significant proposed water infrastructure projects. The project is planned for Northern California, but its participants and potential benefits extend across the state, including to Southern California water agencies.

As Sites Reservoir approaches major water rights, funding, and participation decisions, it is worth understanding how the project would work and what it could mean for California’s long-term water supply.

What Is Sites Reservoir?

Sites Reservoir would be a new off-stream reservoir constructed in rural Colusa and Glenn counties, west of the Sacramento River.

Unlike a traditional reservoir created by building a dam directly across a major river, Sites Reservoir would not block the Sacramento River. Water would be diverted from the river through existing and new facilities when flows are available and regulatory requirements are met. That water would then be stored in the reservoir for later use.

The proposed reservoir would have a storage capacity of approximately 1.5 million acre-feet.

According to the California Department of Water Resources’ most recent Sites Reservoir project report, 30 local agencies are conditionally participating in the project, with 22 expecting to receive a direct water supply benefit.

Participants include urban and agricultural water agencies from the Sacramento Valley, Bay Area, San Joaquin Valley, and Southern California. The state and federal governments are also involved because the project is expected to provide public benefits beyond the water stored for participating agencies.

How Would an Off-Stream Reservoir Work?

Sites Reservoir is designed to capture water during periods when Sacramento River flows are higher and water can be diverted under the project’s permit conditions.

That distinction is important. The reservoir would not continuously remove water from the Sacramento River. Its operations would be governed by water rights, environmental requirements, minimum flow protections, and conditions intended to protect fish and wildlife.

Once stored, the water could be released when participating agencies need it or when it is required to provide an identified public benefit.

This creates a different kind of flexibility than a reservoir located directly on a major river. Water managers could use the reservoir to store water during wetter conditions and reserve it for dry years, emergencies, environmental needs, or periods of greater demand.

However, the reservoir could only store water when sufficient water is available and diversions are allowed. Its existence would not guarantee that it fills every year.

Why Is California Considering Additional Water Storage?

California’s existing water system was largely designed around historical precipitation and snowpack patterns.

Much of the state’s water supply has traditionally been stored as mountain snow during winter and released gradually as the snow melts during spring and summer. Rising temperatures are changing that pattern. More precipitation may fall as rain rather than snow, and snow may melt earlier in the year.

That can produce more runoff during periods when demand is lower and less natural storage later in the season.

Additional California water storage could help agencies capture water when it is available and hold it for times when it is more valuable. Storage can also create operational flexibility when drought, wildfire, infrastructure repairs, or environmental requirements affect other supplies.

Sites Reservoir would not replace conservation, groundwater storage, recycled water, desalination, or repairs to California’s existing water infrastructure. It would be another component of a diversified statewide water supply strategy.

What Could Sites Reservoir Provide?

The most obvious benefit is water storage for the agencies investing in the project. Participating agencies would receive a share of storage capacity and potential water deliveries based on their agreements and level of investment.

The state is also considering Sites Reservoir through the Water Storage Investment Program, which provides Proposition 1 funding for the public benefits created by qualifying water storage projects.

The public benefits identified for Sites Reservoir include:

Water for wildlife refuges and wetlands
Drought emergency supplies
Flood-risk reduction in parts of Colusa County
Recreation, including two recreation areas and a day-use boat ramp
Additional operational flexibility for California’s interconnected water system

These public benefits matter because not every advantage of a reservoir can be assigned to the agencies receiving water.

For example, water released for an ecosystem benefit may support wetlands or wildlife habitat without becoming part of an agency’s regular water supply. Flood protection and recreation can also benefit the public beyond the project’s direct participants.

The agreements that define how these public benefits will be administered and measured are among the items that must be completed before the project receives its final state funding award.

Why Would Southern California Agencies Participate?

A reservoir in Northern California may seem far removed from communities in Riverside County. However, California’s major water systems are connected through reservoirs, rivers, canals, aqueducts, pumping facilities, treatment plants, and water exchanges.

Water stored in one part of the system can create benefits somewhere else, even when the exact same water does not travel directly to the participating agency.

For example, a Southern California agency could receive water through the State Water Project or an exchange agreement while Sites Reservoir water is used elsewhere to meet another obligation. This creates operational flexibility across the larger system.

Participation in Sites Reservoir could give an agency access to:

Additional water storage
Greater flexibility during drought
A supply that can complement existing imported and local water sources
Emergency water reserves
More options when other parts of the system face restrictions or infrastructure problems

The value of the project is therefore not just the amount of water it stores. Its value also depends on when water can be captured, when it can be released, and how it can be coordinated with the rest of California’s water system.

How Much Would Sites Reservoir Cost?

The current estimated project cost is between $6.2 billion and $6.8 billion in 2025 dollars. That estimate is expected to be refined before participating agencies make their final long-term commitments.

The project would be financed through a combination of local participant funding, state funding, and federal participation.

Local agencies that expect to receive water supply benefits would pay for their shares of the project. The State of California would fund eligible public benefits through Proposition 1’s Water Storage Investment Program. The federal government is also negotiating its role through the Bureau of Reclamation.

Large infrastructure projects must be evaluated over many decades. Sites Reservoir would require significant upfront construction spending, along with ongoing costs for energy, operations, maintenance, environmental compliance, and eventual repairs.

Participating agencies must determine whether the expected water supply and storage benefits justify their share of those costs.

That calculation is not as simple as dividing the project cost by the reservoir’s total capacity. Water managers must consider how often the reservoir is expected to fill, how much water can be delivered, the cost of moving that water, the value of having it during drought, and how those benefits compare with other supply options.

Important Environmental and Community Considerations

Every major water project creates tradeoffs, and Sites Reservoir is no exception.

Even though it would be off-stream, filling and operating the reservoir would still involve diverting water from the Sacramento River. The timing and volume of those diversions could affect river flows, fish, wildlife, and water quality.

Construction would also affect land, local roads, biological resources, cultural resources, and communities in the project area.

The project has gone through state and federal environmental review. The U.S. Bureau of Reclamation issued its federal Record of Decision in January 2026. Additional permits, agreements, biological reviews, and tribal consultation activities are still underway.

Project planning has included consultation with tribes traditionally and culturally affiliated with the area. Cultural resource surveys, monitoring, cemetery planning, and procedures for addressing historic properties are part of the ongoing work.

These requirements are not side issues. They are part of determining whether the project can operate in a way that provides reliable water storage while meeting environmental and cultural responsibilities.

The Water Right Is a Critical Part of the Project

A reservoir cannot simply take water whenever its operators want to fill it. Sites Reservoir needs a water right that defines when water may be diverted, how much may be taken, and what conditions must be met.

The Sites Project Authority applied for water rights in 2022. The State Water Resources Control Board received protests and conducted a formal hearing process.

A draft water right decision and permit was released in March 2026. A revised draft decision was this month (September 2026), to be followed by an additional public comment period.

The final water right will help determine how the reservoir can operate and how much benefit participants can reasonably expect.

This is why the water rights process is closely connected to project costs and final investment decisions. Agencies need to understand the operating conditions before committing billions of dollars to construction.

What Decisions Are Expected Next?

Sites Reservoir is moving from planning and environmental review toward final agreements, permits, and participant commitments.

Based on the state’s July 2026 project report, several important steps are expected during the remainder of the year:

A revised draft water right decision is anticipated in September
Final public benefit agreements are expected during the fall
The State Water Board is expected to consider the project’s water right later in 2026
Participating agencies will review long-term benefit and obligation contracts
Federal and state funding arrangements must be finalized
Additional environmental permits and biological reviews must be completed
The project is expected to seek a final Water Storage Investment Program award

These dates may change as permits, agreements, and public review move forward. The larger point is that Sites Reservoir is entering a period when agencies must decide whether to convert conditional participation into long-term financial commitments.

Is Sites Reservoir the Answer to California’s Water Problems?

No single reservoir can solve California’s water challenges.

Sites Reservoir would not create rainfall, eliminate drought, repair the state’s canals, resolve Colorado River negotiations, or remove the need for conservation and local water development.

It could, however, give California another place to store water when conditions allow. That storage could provide value during drought, support environmental needs, and increase flexibility across the state’s interconnected water system.

The final question is whether those benefits are large and reliable enough to justify the financial, environmental, and community costs.

That is the decision participating agencies and state and federal regulators must make based on the final water right, operating rules, project cost, and long-term agreements.

Sites Reservoir represents both the potential and the complexity of planning California’s water future. Building additional storage may be important, but the value of that storage depends on how carefully it is designed, funded, governed, and operated.