LOS ANGELES — As a series of atmospheric rivers pummeled California with heavy rain and residents braced for potential flooding, on Friday the state began releasing millions of gallons of water from a major reservoir — despite ongoing drought conditions.
The release at Lake Oroville, the state's second-largest reservoir and home to the nation's tallest dam, was the first since 2019 and came as a precaution against flooding to communities downstream in case of a possible spillover.
The shift from water conservation to flood prevention is just the latest in a winter weather whiplash in California, where 85% of the state was in severe drought three months ago, according to the U.S. Drought Monitor. Now just 19% of the state falls into that category.
“Water management in California is complicated, and it’s made even more complex during these challenging climate conditions where we see swings between very, very dry; very, very wet; back to dry,” Karla Nemeth, director of the Department of Water Resources, said at a Friday briefing. “We’re now back into wet.”
An atmospheric river known as the “Pineapple Express” because it carries warm, subtropical moisture across the Pacific from near Hawaii, is expected to melt snow at lower elevations. California's mountain ranges have built up significant snowpack this winter because of an onslaught of rain from nine atmospheric rivers and from storms fueled by blasts of arctic air.
The massive snowpack at high elevations is expected to absorb the rain, but snowmelt at elevations below 4,000 feet prompted the state Department of Water Resources to activate its flood operations center.
Water releases for flood control were underway or planned for some reservoirs that were depleted during three years of drought and have been filling with the winter’s extraordinary rains and snowfall.
The idea of releasing water that will eventually flow into the Pacific during a time of drought may seem counterintuitive, but state officials said they have to prepare for the possibility of floods.
"The primary management objective of flood operations is to reduce the risk of downstream flooding rather than conserve for the dry season to come," said Daniel Swain, a climate scientist at the University of California, Los Angeles, and the National Center for Atmospheric Research.
"It suggests that inflows are big enough and the reservoirs levels are high enough that officials don't want anymore water stored in them for safety reasons," he added.
California counts on a system of about 1,400 human-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 overall storage.
The reservoirs are designed not only to store water, but to manage streamflows during periods of heavy rainfall or snowmelt so downstream communities don’t flood.
Of the state’s 17 major reservoirs, seven are still below their historical averages this year. Water releases are also expected at Friant Dam, in central California, to free up space in Millerton Lake, according to the U.S. Bureau of Reclamation.
“After three years of drought and low lake elevations, it’s really good to see the lake rising from the standpoint of providing water to the local community and the statewide water users as well,” said Ted Craddock, deputy director of the State Water Project.
Because so much of California’s water reserves come from snowmelt at high elevations, water officials expect reservoirs to continue filling up through the spring.
As of this week, California’s Sierra Nevada snowpack, which provides about one-third of the state’s water supply, is more than 180% of the April 1 average, when it is historically at its peak.
“We know drought conditions will return to California, and it’s really these moments that we have to capture so we can be resilient in the event of future dry conditions,” Nemeth said.
Lake Oroville’s supply has risen 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 some 15,000 cubic feet per second. An acre-foot supplies enough water for two typical households for one year, water officials say.
The lake is crucial 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 “reconstructed to modern standards, and we’re very confident that it will be able to pass the flows that are coming into Lake Oroville.”