California Reservoirs Emit Millions of Tons of Methane, State Finally Agrees to Track It

Methane has been bubbling up from California's dams and reservoirs for decades without anyone in state government counting it — but that's about to change. The California Air Resources Board has adopted a new policy to start tracking estimates of climate-warming methane and carbon dioxide released from the state's artificial reservoirs, responding to a petition filed by a coalition of environmental groups.

The decision follows years of scientific concern that reservoir emissions represent a significant blind spot in climate accounting. As reported by the Los Angeles Times, plants and algae break down underwater in reservoirs and form methane, which then bubbles to the surface — a process invisible to the smokestack-focused monitoring that has historically shaped greenhouse gas policy. Researchers have estimated that waters held behind the world's dams emit between 10 million and 22 million metric tons of methane per year, according to the same Los Angeles Times report, amounting to roughly 3% to 7% of methane emissions from human activities.

A Petition Years in the Making

The push originated with a legal petition submitted to CARB in March, in which the advocacy group Tell The Dam Truth and a coalition of environmental and Indigenous conservation organizations estimated that California's 368 largest reservoirs generate at least 5.3 million metric tons of carbon dioxide equivalent annually — comparable to the yearly emissions of roughly 1.2 million gas-powered vehicles. Patagonia joined that petition, and the outdoor apparel company's chief executive, Ryan Gellert, said California's plan to assess emissions from dams and reservoirs is a critical step toward cleaner energy and water infrastructure, per the Los Angeles Times.

Friends of the River, another petitioning group, said reservoir methane data has been a blind spot in California's climate work, the Los Angeles Times reported. Gary Wockner, an environmental advocate quoted in the same report, called the state's new tracking policy “a really big step forward,” adding that California is now the first U.S. state to start tracking reservoir greenhouse gases based on the latest science.

Why Reservoirs Slipped Through the Cracks

Under California's Mandatory Greenhouse Gas Reporting Regulation, established under Assembly Bill 32, commercial and industrial facilities that generate 10,000 metric tons or more of carbon dioxide equivalent per year must submit verified annual reports — but artificial reservoirs were historically omitted from that requirement, according to Net Zero Compare. That program currently covers roughly 600 facilities across power generation, industrial manufacturing and fuel supply, but dams and reservoirs were never folded in.

CARB did not go as far as environmental petitioners wanted. The agency turned down a request to require emissions reports directly from dam and reservoir managers, opting instead to calculate estimates internally using methods recommended by the U.N. Intergovernmental Panel on Climate Change, per the Los Angeles Times. CARB Executive Officer Steven Cliff said the agency is committed to further study of emissions from dams and reservoirs, according to the same report, and the board plans to consider how best to incorporate the new reservoir emissions information into its ongoing work.

The Sites Reservoir Flashpoint

The debate over reservoir methane has already shaped one of Northern California's biggest water fights. An independent analysis of the proposed $4.5 billion Sites Reservoir project estimated the 1.5 million acre-foot facility would produce approximately 362,000 metric tons of carbon dioxide equivalent annually — more than 14 times CARB's mandatory facility reporting threshold, according to a 2023 Los Angeles Times report. Project proponents dispute that analysis, arguing that off-stream reservoirs like Sites generate lower emissions than reservoirs built on dammed river systems.

Dam removal has cut the other way in California's methane accounting, too. Environmental petitioners noted that before four major hydroelectric dams along the Klamath River were removed, they produced combined annual greenhouse gas emissions estimated at 25 times CARB's minimum mandatory reporting threshold, according to Tell The Dam Truth. The Klamath project stands as the largest dam removal and river restoration effort in U.S. history. Reservoirs also release a burst of carbon dioxide when they are first constructed and filled, the Los Angeles Times noted, adding another wrinkle to how emissions are measured over a reservoir's lifespan.

California Steps In Where Federal Reporting Stopped

The state's move also fills a gap left by Washington. The Trump administration stopped reporting data on various sources of climate pollution to the United Nations in 2025, and California is now stepping in to take up some of that slack, according to the Los Angeles Times. The U.S. Environmental Protection Agency had only begun including estimated methane emissions from reservoirs in its national Greenhouse Gas Inventory in 2022, following updated IPCC guidance, before that federal U.N. reporting was later halted.

The scientific foundation for treating reservoirs as a serious emissions source dates back further. A landmark global study published in BioScience in 2016 estimated that reservoirs behind dams worldwide emit roughly 1 gigaton of carbon dioxide equivalent annually — about 1.3% of global human-caused greenhouse gases — with methane responsible for 79% to 80% of that warming impact. Researchers behind that study found that biological productivity and nutrient levels were stronger predictors of a reservoir's methane output than its age or geographic latitude.

Fitting Into California's Broader Climate Math

California's total statewide anthropogenic greenhouse gas emissions reached 360.4 million metric tons of carbon dioxide equivalent in 2023, a 17.6% reduction from 1990 levels, according to official CARB inventory data published in 2025 and cited by Next10. Transportation remains the state's largest single emissions source, followed by industrial processes and electricity generation. The new reservoir tracking effort feeds into California's broader methane target: reducing methane emissions to 40% below 2013 levels by 2030, a goal backed by Senate Bill 1383, a 2016 law that mandates state air regulators curb short-lived climate pollutants. SB 1383 originally targeted landfill organic waste and dairy manure emissions before regulators turned their attention to aquatic impoundments like reservoirs.

The tracking initiative also arrives alongside other state efforts to close gaps in methane detection. California launched the $100 million California Satellite Methane Project in March 2025, deploying satellite-mounted sensors to track and map major point-source methane leaks in real time, according to StateScoop. That project focuses on super-emitter events from landfills, energy infrastructure and agricultural operations, though it remains an open question how CARB will reconcile model-based reservoir estimates with physical satellite data going forward.

The physical scale of California's water storage infrastructure underscores why the new tracking effort matters. The California Department of Water Resources' Division of Safety of Dams inspects and regulates more than 1,200 non-federal jurisdictional dams statewide, a footprint that gives a sense of just how many reservoirs could eventually be folded into the state's climate accounting. How water district managers will respond to newly published, state-estimated carbon footprints for their own facilities remains to be seen.