California’s second largest reservoir is shrinking

A new study from California’s Department of Water Resources (DWR) has found that Lake Oroville is shrinking.

Water levels at the state’s second largest reservoir are in a much better place than they were two years ago, when severe drought gripped much of California. Two back-to-back wet winters, accompanied by atmospheric rivers, have supplemented the water levels at many California reservoirs and contributed greatly to their recovery, although the atmospheric rivers also caused flooding and mudslides.

Atmospheric rivers are a “long, narrow region in the atmosphere—like rivers in the sky—that transport most of the water vapor outside of the tropics,” according to the National Oceanic and Atmospheric Administration.

Despite Lake Oroville’s recovery, water officials recently discovered that its capacity was shrinking and that the lake had lost 3 percent of capacity since it was created in the 1960s.

“DWR utilized the latest terrain-mapping technology to determine if there have been any changes in the lake’s volume to optimize how the reservoir is operated and ensure accuracy in estimating California’s water supply availability,” a DWR webpage said.

“What resulted were highly detailed 3D topographic terrain models of the bottom of the lake, which DWR engineers used to calculate a new storage capacity of 3,424,753 acre-feet, approximately 3 percent less than previously estimated,” the webpage added.

The DWR attributed the loss to “weather swings and almost six decades of service.” Newsweek reached out to the DWR by email for comment.

Despite the loss, DWR officials said Lake Oroville remains the state’s second largest reservoir, behind only Lake Shasta.

“Having updated storage capacity data allows us to operate Lake Oroville in a more efficient manner,” said John Yarbrough, the DWR’s deputy director of the State Water Project.

“It ensures we are providing adequate flood storage protection during winter months and accurately accounts for the state’s water supply, which is especially important as we experience climate change-driven weather extremes,” he said.

During the winter months, water officials occasionally release water from the reservoir to provide flood mitigation for downstream communities, such as in February when atmospheric rivers brought a deluge of rain to the area. Once California enters its dry season, officials transition to retaining as much water as possible in the reservoir.

Lake Oroville’s water levels began rising last December and reached full capacity in May. The levels have been steadily declining over the past few weeks as California enters its dry season.

However, the lake is in a much better state than it was in 2022. As of Tuesday, Lake Oroville’s water levels were at 887 feet, only 12 feet below full pool of 900 feet. During the summer of 2022, the water levels were at only 750 feet.