The government is releasing water from Folsom Lake during California’s drought. Here’s why
Despite recent storms, Folsom Lake isn’t even two thirds full, and California remains mired in a multi-year drought that’s left its biggest reservoirs even emptier.
Nonetheless, starting early Tuesday the federal managers of Folsom Dam began letting out a substantial amount of water into the American River through Sacramento, prompting warnings from local authorities to be mindful of rapidly rising, swift-moving water.
The dam’s operators say there’s a good reason they’re keeping space in the lake instead of letting it fill with water for the hot and dry months to come.
All that snow piling up in the Sierra could come rushing down in the weeks ahead and swamp Sacramento, one of America’s most flood-prone cities.
“We have a pretty relatively large American River watershed, and a correspondingly small reservoir,” said Drew Lessard, the area manager for the Central California office of the U.S. Bureau of Reclamation, which operates Folsom. “We have to … be very careful about incoming storms on the American River.”
The water releases from Folsom illustrate the delicate balancing act found at most of California’s major reservoirs: They’re there to store water for cities and farms during the state’s blistering-hot summers and falls, but they’re also designed to protect cities against flooding during the winter.
This year, Folsom Lake got so low boat ramps closed and people weren’t allowed to water ski due to underwater hazards getting exposed. It’s a long-standing gripe among those who boat or live along Folsom Lake’s 75 miles of shoreline that the lake is deliberately drained this time of year, when larger reservoirs such as Shasta and Oroville tend to be filling up.
Lake Shasta was at just 29% capacity as of Wednesday, and federal officials managing that reservoir said they’re “working to store as much water as possible from winter storms for releases later in the year when most needed.”
Lessard acknowledges that the “optics are difficult” to release water from Folsom during a drought, but he said the flood risks to Sacramento are too great.
Folsom Lake is one of the smallest of the state- and federal-managed reservoirs that ring California’s Central Valley, and it fills up quickly.
Folsom is capable of storing just 976,000 acre feet of water. An acre foot is the equivalent of flooding an acre of land with one foot of water, or 325,851 gallons. By comparison, Lake Shasta, the state’s largest reservoir, can store 4.6 million acre feet.
Because it’s so small, Folsom Lake isn’t nearly capable of holding all the water that can drain in from the American River watershed, a sprawling 1,900 square miles of the Tahoe and Eldorado national forests. Much of that is now covered with several feet of snow and ready to pour in behind Folsom Dam when it melts.
Plus, dam managers have to be mindful that a warmer storm could swamp the watershed with rain and snowmelt, sending a deluge into the lake and overwhelming Folsom Dam and the levees below it.
In 1986 and 1997, similar storms nearly brought a catastrophe to Sacramento on par with Hurricane Katrina, when the levees downstream of Folsom nearly failed.
In the years since, taxpayers have spent hundreds of millions of dollars upgrading the levees and building a new $900 million spillway at Folsom to help protect against flooding. Some of those upgrades remain underway. For instance, federal officials last year launched work on a five-year, $373 million project to raise the dam 3½ feet by adding rock and soil to its earthen dykes and seals on the gates at the top of the concrete portion of the dam.
Combined with the new spillway completed in 2017, federal dam officials say the flood-prone region is on its way to 300-year or more flood safety, meaning there will only be a one-in-300 chance in any given year that the combination dam and downstream levee system will fail.
Hope for the spring
Despite the upgrades, Lessard said there will always be a “balancing act and tug of war” between storing for future droughts and flood protection. But this time of year, flood control always wins out.
“And that’s really what it’s all about in the winter,” he said.
Releases at Folsom are so low they won’t trigger the use of the new spillway, and they’re not even close to what the downstream levees are rated to handle.
The flows went from 700 cubic feet of water per second to more than 2,200 on Tuesday, and Lessard said they’re going to gradually increase from 5,000 cubic feet per second on Thursday.
Lessard said the levees below Folsom are rated to withstand 115,000 cubic feet per second — a number that will climb to 160,000 in the coming years when levee upgrades are complete.
Meanwhile, the recent storms have helped raise reservoir levels statewide, but there’s still a long way to go to see them full again.
Recent storms added more than one million acre feet of water to the state’s 10 largest reservoirs, state data shows.
The 10 largest reservoirs in California held about eight million acre feet of water as of midnight on Tuesday, 65% of the historical average for this point in the year. A month earlier, those reservoirs held just under seven million acre feet, or 60% of the historical average for late November.
A year ago, in late December 2020, the 10 largest reservoirs held about 10.4 million acre feet, or about 2.4 million more acre feet than they hold today.
As of midnight on Wednesday, water levels in Folsom Lake were at 144% of the historic average for this point in the year.
Of the 10 largest reservoirs in California, only Folsom Lake had more water than normal for this point in the year.
Lessard said that hopefully will lead to a full reservoir in the spring.
“Hopefully, we find ourselves in a position in May where we can top off the reservoir,” he said. “But we’ll need to wait and see how the rest of the year continues.”
This story was originally published December 28, 2021 at 2:14 PM with the headline "The government is releasing water from Folsom Lake during California’s drought. Here’s why."