El Niño could bring California a water windfall. Can it keep it?
With some reservoirs nearly full and a potentially wet winter ahead, California faces a tricky challenge: preventing floods while saving as much water as possible for the next drought.
Oceanfront homes along Long Beach and Newport Beach are in danger of crumbling into the water as Tropical Storm Rachel brought high surf crashing into the Southern California coast on Oct. 11.
California could face a remarkable dilemma this winter, releasing water from reservoirs to prevent flooding while desperately trying to save as much as possible for the next drought.
A potentially historic El Niño could bring a parade of powerful storms to parts of the state, AccuWeather Lead Long-Range Meteorologist Paul Pastelok said. But California is entering the rainy season with some reservoirs already nearly full, raising a complicated question: where will all that water go?
Lake Casitas in Ventura County was 94% full on Oct. 21 while California's reservoirs statewide held approximately 104% of their historical average storage for this time of year, according to California Water Watch. That doesn't mean every reservoir is full, but it illustrates the balancing act facing water managers as they prepare for potentially heavy winter rainfall.
Why California can't simply keep all the rain
Reservoirs do more than store water for drinking, farming and the dry summer months. Many also serve as flood-control systems, requiring operators to maintain empty space where incoming stormwater can collect without threatening downstream communities.
Water flows through the Oroville Spillway at Lake Oroville on Saturday, March 25, 2023, in Butte County, Calif. The California Department of Water Resources was releasing water to create room at the reservoir for anticipated snowpack melt. (AP Photo/Noah Berger)
That means water sometimes has to be released before a major storm, even when California knows another drought could be around the corner. Release too little, and a reservoir could be overwhelmed. Release too much, and there is no guarantee the next storm will replenish what was lost.
Marty Ralph, director of the Center for Western Weather and Water Extremes at Scripps Institution of Oceanography, described exactly that predicament at Lake Mendocino in Northern California. Following a major atmospheric river during the 2013 water year, operators released water to prepare for another storm, but the anticipated rain never arrived.
As Ralph explained in a 2024 interview with Maven's Notebook, “no more storms came for the rest of the winter, and that unnecessary water release left the whole watershed in a terrible situation that summer.”
An aerial view of the Rio Hondo Spreading Grounds in Montebello, California, on Sept. 23, 2026. The facility captures stormwater runoff, allowing it to soak underground and replenish local groundwater supplies. (Image: Allen J. Schaben/Los Angeles Times via Getty Images)
How California stores floodwater underground
One solution lies beneath California's cities. Los Angeles County operates 27 spreading grounds, large basins where stormwater diverted from rivers and flood-control channels can soak into the earth instead of rushing directly toward the ocean. The basins are built over porous soil, allowing water to replenish natural underground storage areas called aquifers.
“When we bring in the water it'll slowly percolate, soak deep into the ground and recharge these groundwater aquifers that we have in LA County,” Sterling Klippel of Los Angeles County Public Works told ABC7 Los Angeles.
The water fills tiny spaces between layers of sand, gravel and rock, where it can remain long after the storms have passed. During drier months, water utilities pump groundwater back to the surface through wells, treat it as needed and deliver it to homes and businesses.
The system is already making a difference. Los Angeles County captured approximately 120 billion gallons of stormwater during the 2025–26 storm season, roughly ten times the 11.9 billion gallons captured the previous year. That's enough to meet the water needs of approximately 3 million people for a year, according to an April report from Los Angeles County.
Similar efforts are underway in California's Central Valley, where farmers and conservation groups are allowing floodwater to spread across suitable fields and restored floodplains. At Dos Rios State Park near Modesto, 2,100 acres of former farmland have been restored to absorb seasonal flooding and replenish groundwater while also reducing downstream flood risks. When rivers overflow onto these floodplains, the water spreads across a larger area, slowing its movement and allowing more time for it to soak into the ground.
A canal and pumping infrastructure at Terranova Ranch in Helm, California, designed to divert Kings River floodwater onto agricultural land to replenish underground aquifers, Sept. 20, 2021. (Image: Brian van der Brug/Los Angeles Times via Getty Images)
But even underground storage has limits. Soil can absorb water only so quickly, and when powerful storms deliver more runoff than spreading grounds and floodplains can handle, the excess continues downstream. Capturing more requires suitable land, diversion infrastructure and available groundwater capacity, along with the legal authority to divert the water.
California is preparing to capture more of that water this winter. The State Water Resources Control Board is helping water districts and farmers secure the permits needed to divert excess runoff underground while local operators prepare the equipment and land needed to capture it when storms arrive.
How better forecasts could save more water
Another solution doesn't require building new reservoirs. It involves using increasingly accurate weather forecasts to make smarter decisions about the water California already has.
A technique called Forecast-Informed Reservoir Operations, or FIRO, uses improved high-resolution weather forecasts to help operators decide when to hold water and when to release it. If an atmospheric river is approaching, they can create room for the expected runoff. If flood risk appears low, they may be able to safely retain water that traditional operating rules would otherwise require them to release.
A 2026 study from Scripps Institution of Oceanography estimated that expanding this approach across 20 major California reservoirs could allow the state to retain an additional 100 billion to 160 billion gallons of water annually without building new dams.
Even with better forecasts and expanded storage, California can't capture every drop. Reservoirs still need room to absorb floodwaters, and rivers must continue flowing to support wildlife, ecosystems and communities downstream.
The challenge ahead is finding the right balance—keeping enough room for the next storm without letting too much of the next drought's water supply slip away.
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