LOS ANGELES — As a series of atmospheric rivers battered California with heavy rains and residents braced for possible flooding, the state began releasing millions of gallons of water from a large reservoir on Friday — despite ongoing drought conditions.
The release into Lake Oroville, the state’s second-largest reservoir and home to the nation’s tallest dam, was the first since 2019 and was done as a precaution against flooding downstream communities in the event of a potential spill.
The shift from water conservation to flood prevention is just the latest in a winter crisis in California, where 85 percent of the state was in severe drought three months ago, according to the US Drought Monitor. Now only 19% of the state belongs to this category.
“Water management in California is complex and has become even more complex during these difficult climates where we see swings between very, very dry. very, very wet; back to dry out,” Carla Nemeth, director of the Department of Water Resources, said in a Friday briefing. “Now we’ve got rain again.”
An atmospheric river known as the “Pineapple Express” because it carries warm, subtropical moisture across the Pacific near Hawaii is expected to melt snow at lower elevations. California’s mountain ranges have produced significant snow this winter due to the influx of rain from nine atmospheric rivers and from storms fueled by bursts of arctic air.
Massive snow at high elevations is expected to soak up the rain, but melting snow at elevations below 4,000 feet prompted the state Department of Water Resources to activate the flood center.
Flood-control water releases were underway or planned for some reservoirs that have been depleted during three years of drought and have filled with extraordinary winter rains and snowfalls.
The idea of releasing water that will eventually flow into the Pacific during a drought may seem counterintuitive, but state officials said they need to prepare for the possibility of flooding.
“The primary goal of flood management operations is to reduce the risk of downstream flooding rather than sustain the coming dry season,” said Daniel Swain, a climate scientist at the University of California, Los Angeles, and the National Center for Atmospheric Research. .
“It indicates that inflows are large enough and reservoir levels are high enough that officials no longer want water stored in them for safety reasons,” he added.
California relies on a system of approximately 1,400 man-made surface reservoirs and thousands upon thousands of miles of levees to manage surface water. About two dozen large reservoirs are responsible for more than half of the total storage.
Reservoirs are designed not only to store water, but also to manage stream flows during periods of heavy rainfall or snowmelt so that downstream communities are not flooded.
Of the state’s 17 major reservoirs, seven are still below their historic averages this year. Water releases are also expected at Fried Dam in central California to make room for Lake Millerton, according to the US Bureau of Reclamation.
“After three years of drought and low lake levels, it’s great to see the lake rising from the perspective of providing water to the local community and water users across the state,” said Ted Craddock, deputy director of the State Water Project .
Because much of California’s water supply comes from high-elevation snowmelt, water officials expect reservoirs to continue filling into spring.
As of this week, snowpack in California’s Sierra Nevada, which provides about a third of the state’s water supply, is more than 180 percent of the April 1 average, when it is historically at its peak.
“We know that drought conditions will return to California, and it’s really these moments that we need to capture so we can be resilient in the event of future dry conditions,” Nemeth said.
Lake Oroville Supply has increased by 180 feet since Dec. 1 and was just 60 feet below capacity before Friday’s storms. Capable of storing more than 3.5 million acre feet, officials planned to release about 15,000 cubic feet per second. An acre-foot provides enough water for two typical households for a year, water officials say.
The lake is vital to the State Water Project, which provides water to about 27 million residents and flood protection to downstream communities.
The reservoir was repaired in 2018 after a massive flood collapsed the main spillway and forced more than 180,000 people to evacuate.
Craddock expressed confidence in the 1960s-era Oroville Dam and said upgrades to the spillway have been “rebuilt to modern standards and we’re very confident it will be able to handle the flows coming into Lake Oroville.”