How California Reservoirs Will Change After Getting Week of Rain

Several reservoirs in Northern California could rise this weekend as an atmospheric river brings heavy rain to the region.

An atmospheric river is forecast to saturate much of the Pacific Northwest, including Oregon and Northern California, over the next week. An atmospheric river is 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 than a dozen similar storms battered California last winter, and although the deluge alleviated much of the state's drought, it also wreaked havoc through flooding and mudslides.

The incoming rain could bring up to 6 inches of rain to the Shasta Lake area and around 3 inches to Lake Oroville.

National Weather Service meteorologist Dakari Anderson told Newsweek that the NWS doesn't make predictions on lake level changes, but he confirmed that the region was expecting a "pretty active weather pattern" in the coming days.

Rain will begin to fall in California late Friday morning and move across the state in a west-to-east progression.

"From that point, we are seeing consistent rainfall at least through Monday...and lingering showers Tuesday to Wednesday," Anderson said.

Tim Daldrup, a meteorologist at the NWS office in Medford, Oregon, told Newsweek there could be a short break in the atmospheric river early next week before rainy conditions resume before the weekend.

Shasta Lake, which had concerningly low levels in 2022, has started to rise since the start of the year after falling slightly at the end of last year. More than a dozen atmospheric rivers last winter helped to bolster the lake's water levels and contributed to the reservoir's recovery. Lake Oroville's water levels tell a similar story.

Lake Oroville and Lake Shasta Lake are both at 70 percent capacity, according to a map by the California Department of Water Resources. However, despite further improvement before the lakes reach full pool, they are still 112 percent and 126 percent above the historical average, respectively.

As of Friday morning, Shasta Lake was at 1,015 feet, while Lake Oroville was at 825 feet.

Despite a series of atmospheric rivers that have hit the Golden State this winter, northern Sierra precipitation is only 67 percent of the average amount, the NWS office in Sacramento said on X (formerly Twitter) on Thursday.

"The upcoming wet pattern is expected to raise this amount and get us closer to average for this time of year," the office said.

The winter also hasn't brought as much snow as expected, which could pose a problem for the reservoirs during the spring months, when they rely on melting snow to supplement the water levels.