California Flood Fears as Reservoir Water Levels Exceed Seasonal Averages

Some of California's reservoirs have seen water levels spike above their historical averages in recent days as the state has faced a barrage of heavy rainfall storms that have led to flooding in some areas of the state and already claimed at least 12 lives.

While still in a drought, due to 2022's extremely dry conditions—the second most arid in 128 years, according to the U.S. government's National Integrated Drought Information System—California has seen up to 20 inches of precipitation between December 26 and January 6.

With the state forecast to be hit by what the National Weather Service described on Sunday as a "relentless parade of cyclones moving across the Pacific," there is confusion as to why a large swathe of the state still remains in extreme drought. Many of California's reservoirs remain below their usual historical levels for this point in the year, which is in large part due to depletion that occurred in the summer of 2022.

For instance, in response to a tweet by California's Department of Water Resources on January 7 stating that "major reservoir storage remains below average," @pl_www replied: "Such a weird thing to be saying when all our news stations are reporting rivers near the top."

"Our largest reservoirs, Shasta and Oroville, have plenty of room to take in more water," Jeanine Jones, interstate resources manager at the California Department of Water Resources, told Newsweek. "And there is still capacity in the major flood control reserves in the Sacramento flood system."

"The last three water years, from October 2019 to October 2022, have been the driest three-year period on record going back to 1896," she added. "This has created a huge water deficit that will take time—and much more rain and snow—to erase. While current storms have created a strong start to the wet season, we're still in early January with over half the wet season to come."

According to the California Department of Water Resources, Folsom Lake, near Sacramento, was at 462,132 acre feet (AF) of its 977,000 AF storage capacity on January 8, well above the 410,000 average for this time of year.

Despite the drought, federal managers of Folsom Dam let out a "substantial" amount of water into the American River on December 30, bringing its water level down from over 600,000 AF, the Central Valley Flood Protection Board said.

This was because the reservoirs also provide capacity for water to prevent flash flooding, and so, even when there is not enough water, managers of the water system want to ensure there is spare capacity for heavy rainfall.

With high-precipitation storms expected to batter California in the first weeks of January, the reservoirs will serve to contain flood water as more of a priority than filling to capacity ahead of the summer.

"Some of it can be captured for later, but the short answer is it falls so quickly that we lack the ability to take that water and set it aside quickly enough in a place where we can store it for later," Andrew Fisher, professor in hydrogeology at UC Santa Cruz, told NPR on Saturday. "Because the water falls at a very fast rate and it creates a hazard, we do tend to treat stormwater as a nuisance and try to get it off the landscape as quickly as possible."

"What if we decided not to dump that water, to just go ahead and keep more of the water?" Derek Morley, a senior principal and water management expert at Geosyntec, an engineering and consulting firm, explained in a LinkedIn post on January 4. "Well, then the reservoir can prevent flooding from smaller storms, but not bigger ones."

"So if a big one hits, the reservoir doesn't provide flood protection, and a bunch of people die," he added. "Seriously, for reservoirs like Folsom, it's hundreds or thousands of people dead, and billions of dollars."

Other reservoirs in California have yet to let off the large volumes of water they have taken on recently. Camanche Lake, to the southeast of Sacramento, has seen water levels rise from far below historical averages to well above them in the past two weeks. On January 8, its level stood at 301,710 AF, well above the circa 250,000 AF typical for the start of January.

Similarly, New Bullards Bar, north of Sacramento, was teetering along its historical average until a few days before the New Year, and has since shot up to 681,312 AF over a 610,000 AF historical average—putting it on course to reach capacity sooner than when it did in 1982-83.

If the intense, flood-inducing rain continues, more water may have to be dumped to allow for flood capacity to be maintained. When California reaches the latter part of its rainy season, according to Morley, water management will then shift towards holding more rainwater to avoid drought conditions come the summer.

"Major reservoir storage remains below average, and conditions could turn dry again this winter, offsetting recent rain and snow," said Jones. "Groundwater levels in much of the state have also been depleted by the numerous dry years in the last two decades and it would take more than a single wet year for groundwater levels to substantially improve at the statewide scale."

Update 01/10/23, 4:00 a.m. ET: This article was updated to include comment from Jeanine Jones, interstate resources manager at the California Department of Water Resources.