California doesn’t build more reservoirs because the best dam sites were taken decades ago, the ones left over cost billions and take 20 or more years to push through environmental review, water rights disputes, and lawsuits, and cheaper alternatives like groundwater banking and water recycling now deliver supply faster. The state already has roughly 1,500 reservoirs. Most were built during a mid-20th-century construction boom that claimed the deep canyons with strong bedrock first, leaving sites that are more expensive, more seismically risky, and more ecologically sensitive.
The Best Dam Sites Are Already Dammed
Two-thirds of California’s dams are at least 50 years old, and state and federal agencies manage 240 large reservoirs that hold about 60 percent of the state’s storage capacity. Builders in the 1930s through 1970s targeted deep, narrow canyons with strong bedrock foundations and large upstream catchment areas. What remains has weaker geology, shallower valleys, or sits near earthquake faults. California is one of the most seismically active regions in the country, so any new dam needs advanced engineering to survive major quakes, which pushes both construction and long-term maintenance costs higher.
Engineers are adapting by designing around the problem. The proposed Sites Reservoir in Colusa County would be off-stream: instead of damming a river, it would store up to 1.5 million acre-feet of water diverted from the Sacramento River during high flows. Off-stream designs avoid blocking fish migration and disrupting river ecosystems, but they require massive earthwork to create an artificial basin and pumps to lift water into it. That complexity costs money a traditional canyon dam wouldn’t need.
The Price Tags Run Into the Billions
Sites Reservoir’s estimated cost climbed from roughly $3.9 billion in 2021 to between $6.2 and $6.8 billion by 2024. The construction-and-roads package alone runs up to $3 billion. A project this size has to stitch together funding from multiple sources: California’s Proposition 1 water bond, the federal Bipartisan Infrastructure Law (which delivered more than $50 billion nationwide for water infrastructure), and the $10 billion Proposition 4 climate bond voters approved in 2024, which dedicates about $3.8 billion to drinking water, drought, flood, and water resilience programs.1US EPA. Water Infrastructure Investments2CA.gov. Climate Bond Prop 4
Temperance Flat on the San Joaquin River shows what happens when the numbers stop working. The project was estimated at $2.6 billion in 2014 and $3.2 billion by 2020. The project authority couldn’t attract enough public or private investors. The San Joaquin was already fully appropriated year-round, so no new water rights were available to fill the reservoir. Federal agencies concluded the dam would cause unavoidable harm to fisheries, wetlands, wildlife habitat, and a unique cave system. Temperance Flat quietly withdrew from state funding consideration in October 2020 and hasn’t returned.
Environmental Review Takes Years, and Lawsuits Add More
Every major reservoir proposal has to clear the California Environmental Quality Act and, if federal funding or permits are involved, the National Environmental Policy Act. Both laws require agencies to analyze and disclose potential environmental impacts, and CEQA requires them to minimize significant adverse effects where feasible.3Governor’s Office of Planning and Research. NEPA and CEQA: Integrating Federal and State Environmental Reviews For a reservoir, that means studying endangered fish, migratory birds, downstream water quality, and cultural resources. The federal Endangered Species Act adds another requirement: agencies must ensure projects they fund or authorize won’t jeopardize listed species or destroy critical habitat.4Environmental Protection Agency. Summary of the Endangered Species Act
The review is only the first delay. Environmental organizations routinely challenge approvals in court. Sites Reservoir faced a CEQA lawsuit filed in December 2023 and prevailed in mid-2024, but many reservoir proposals absorb multiple rounds of litigation that can add years even when the project ultimately wins.
No Unclaimed Water Left to Store
California’s water rights system combines two doctrines. Riparian rights belong to landowners whose property borders a natural water source. Appropriative rights follow a “first in time, first in right” rule: the earliest claimant to put water to beneficial use has priority over everyone who came later.5State Water Resources Control Board. Water Rights FAQs In dry years, junior appropriators can be shut off entirely so senior claims are satisfied.
A new reservoir doesn’t create new water. It only stores water that already belongs to someone, or captures surplus flows during wet periods. On rivers that are already fully appropriated, like the San Joaquin, the state has determined no additional rights are available. Existing rights holders and environmental advocates fight anything they see as threatening their allocations or the flows that sustain river ecosystems, and those fights move through administrative hearings and the courts.
Land Acquisition and Local Opposition
A large reservoir can flood thousands of acres. When that land is privately owned, the project authority has to negotiate purchases or use eminent domain, which allows the government to take private property for public use but requires just compensation. Disputes over fair value routinely end up in court. The process is expensive, slow, and politically damaging.
Local resistance adds to the delay. Temperance Flat would have flooded up to 5,000 acres of public land, including established recreation sites and irreplaceable natural features. Even projects that don’t displace residents draw opposition over construction disruption, changes to local waterways, and the loss of land that could serve other purposes. That resistance turns into organized political pressure, public comments, and more legal challenges.
Climate Change Is Making Surface Storage Less Reliable
California’s reservoir system was designed around a predictable pattern: Sierra Nevada snow accumulates in winter, melts gradually through spring and summer, and feeds reservoirs when demand peaks. That pattern is breaking down. As of early 2026, the western United States is experiencing record-breaking snow drought, and California is seeing abnormally early snowmelt.6Drought.gov. Snow Drought Current Conditions and Impacts in the West When runoff arrives weeks or months early, rivers can surge before reservoirs have been drawn down enough to capture it, and managers have to release water to prevent overtopping.
Warmer temperatures also increase evaporation from reservoir surfaces. The hot Central Valley, where most major storage sits, loses substantial water to evaporation every year. Underground aquifers lose almost none, which is a large part of why groundwater banking has gained ground.
Reservoirs themselves produce greenhouse gases. Lakes and reservoirs worldwide account for roughly 10 percent of global methane emissions, generated when organic material decomposes in the oxygen-poor sediment at the bottom. Methane has about 81 times the warming potential of carbon dioxide over 20 years, and these emissions are projected to grow as temperatures rise.7Nature.com. Future Methane Emissions from Lakes and Reservoirs
What California Is Building Instead
With new reservoirs this slow and expensive, the state has shifted investment into a portfolio of water strategies that move faster and cost less per acre-foot.
- Groundwater banking stores surplus water underground during wet periods and recovers it during droughts. Recharge costs run roughly $90 to $1,100 per acre-foot, with a median around $390, often well below the cost of new surface storage, and aquifers don’t lose water to evaporation.
- Direct potable reuse became legal in California under regulations effective October 2024, allowing highly treated municipal wastewater to be introduced directly into drinking water systems after public notification and meetings. The rules effectively turn a supply once discharged to the ocean into a drought-proof source.8State Water Resources Control Board. 22 California Code of Regulations – Direct Potable Reuse
- Stormwater capture channels urban rainfall into spreading grounds and infiltration basins that replenish local groundwater instead of letting it run to the ocean through storm drains.
- Conservation standards for indoor fixtures, restrictions on ornamental grass, and incentives for drought-tolerant landscaping have reduced per-capita urban water use significantly over the past two decades.
- Desalination of brackish and seawater provides drought-proof supply but remains energy-intensive and expensive, which makes it more useful for coastal communities with few alternatives than as a statewide answer.
The Sustainable Groundwater Management Act, passed in 2014, pushes communities toward underground storage by requiring local agencies in high- and medium-priority basins to develop plans that eliminate overdraft within 20 years.9California Department of Water Resources. Sustainable Groundwater Management Act (SGMA)
The One Big Reservoir That Is Advancing
California hasn’t given up on reservoirs entirely. Sites would be the first major new one in roughly 50 years. The project authority was soliciting construction proposals in early 2025 and expects a water rights permit decision in 2025. With 1.5 million acre-feet of storage capacity, and up to 370,000 acre-feet reserved for environmental flows, it would be a meaningful addition to the state’s water grid.10Sites Project Authority. Sites Reservoir Progressing Towards Construction
Sites has been in some stage of planning and review for decades. Its costs have doubled since 2021. It survived a CEQA lawsuit, secured multiple funding commitments, and adopted an off-stream design specifically to reduce environmental harm. Even with all of that, it still isn’t built. That is the clearest answer to why California doesn’t simply build more reservoirs. A single project absorbs billions of dollars and decades of political capital before a shovel breaks ground, and the state has concluded, through hard experience, that spreading the same investment across many smaller, faster alternatives tends to deliver more water per dollar than betting on the next big dam.