How California's Major Reservoirs Changed After Atmospheric River

View of Lake Shasta at full capacity with mountains in the background. Water levels increased at several California reservoirs this week as an atmospheric river pummeled the region. Kirpal Kooner/Getty

Water levels at major reservoirs in northern California rose this week after a moisture-laden atmospheric river pummeled the region.

A bomb cyclone brought dangerous weather conditions across the Pacific Northwest earlier this week, causing power outages for more than a half-million people and killing at least two. It also ushered in an atmospheric river, which brought heavy rainfall across the northern half of California. Multiple weather warnings remained in place Friday morning, according to the National Weather Service (NWS), many of them flood related.

A bomb cyclone occurs when a storm's pressure drops quickly, which intensifies the storm and ramps up wind gusts. 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.

More heavy rain and mountain snow arrived on Thursday night and inclement weather is expected to continue through Saturday, the NWS Weather Prediction Center forecast said.

"Atmospheric river will produce one last round of heavy rain/mountain snow and strong wind gusts across the Pacific Northwest today," the center posted on X, formerly Twitter, on Friday.

The deluge contributed to a rise in several major reservoirs in the state.

Lake Shasta

California's largest reservoir has risen more than 2 feet since Monday. As of Friday morning, its water levels were measured at 987.72 feet.

Despite the increase, Lake Shasta is at only 56 percent of capacity, though that level is 105 percent of average for this time of year, according to data from the California Department of Water Resources (DWR).

Lake Shasta exhibited an impressive recovery in the winter of 2023 and even neared capacity the past two summers. The recent rise in water levels comes after a months-long decline.

Lake Oroville

Water levels at California's second-largest reservoir also rose this week.

On Monday, water levels were just over 755 feet, nearly 4 feet below their Friday level. Lake Oroville also has been experiencing a months-long decline, which has put the lake at only 49 percent capacity, slightly below historical average, according to DWR data.

A DWR spokesperson told Newsweek that the timing of when Lake Oroville begins to rise each year is weather dependent.

"For example, for the 2023 Water Year, Lake Oroville saw minor elevation increases in December 2022, but January 2023 marked a significant increase in elevation due to numerous atmospheric rivers arriving in California," the spokesperson said. "For the 2024 Water Year, Lake Oroville saw steady increases starting mid-December 2023 with higher elevation gains in late January 2024."

Lake Oroville hit capacity the past two summers after battling drought for years.

Lake Sonoma

The greatest increase in water levels occurred at Lake Sonoma, in Sonoma County, where the lake jumped by nearly 9 feet since Monday. The increase puts the lake at 62 percent of capacity, 121 percent above its historical average.

Rainfall broke records at Sonoma County Airport earlier this week.

Slight increases also occurred at Trinity Lake in northwest California and New Bullards Bar Reservoir in northeast California.

The atmospheric river also will be "extremely beneficial" for California's snowpack levels, even as much of the precipitation is falling as rain, the DWR spokesperson said.

"For California to benefit from snowmelt in the spring and summer, we need good rainstorms in the fall and early winter," the spokesperson said. "These storms wet the soils in the mountains so that more of the snowmelt in the spring or summer results in runoff and is not lost to dry soils."

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