Record heat, melting snow: What does it mean for California’s reservoirs?

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A record-baking heat wave is scalding California, with major consequences for the state’s most important reservoir: its snowpack. 

Providing about a third of the state’s water supply, the Sierra Nevada snowpack is a vital source of spring and summer runoff that refills reservoirs when the state needs the water most. 

But a warm wet storm followed February’s snow, and now, March temperatures are shattering records — prompting warnings of rapid snowmelt and swift rivers. 

Historically, the snowpack is at its deepest in April. But climate change is shifting runoff earlier, leaving less water trickling down the mountains in warmer months for homes, farms, fish, hydropower and forests. 

“In an ideal world, you’d have your reservoir full right now, and this additional huge snowpack reservoir that we know will help replenish and provide more water supply,” said Levi Johnson, operations manager for the Central Valley Project, the massive federal water system that funnels northern California river water to the Central Valley and parts of the Bay Area. 

This year, he said, “we’re not going to have that.” 

California’s reservoirs are in good shape, brimming above historic averages with many nearing capacity. But that summertime snow bank on the slopes of the Sierra Nevada is disappearing early, and fast — dropping to 38% of average for mid-March statewide.

It’s not yet the worst snowpack on record: that distinction belongs to 2015, when then-Gov. Jerry Brown stood on brown, barren slopes of the Sierra Nevada to watch scientists measure the most meager snowpack in history.  

But this year’s snowpack is rapidly approaching the worst five on record for April 1, state climatologist Michael Anderson said — and it’s likely to worsen still as temperatures climb. From early to mid-March, the snowpack has been disappearing at a rate of roughly 1% per day. 

It’s a sharp departure from the near-average conditions of last year, and presents both a challenge and a glimpse of the future for reservoir operators in the state. 

Conflicting roles for reservoirs

Many of California’s reservoirs serve a dual role: stoppering flood flows and storing water for drier times ahead. 

Those roles sometimes conflict — as they did at Lake Mendocino, which dried to a mud puddle during the 2012–16 drought. Rigid federal operating rules forced the U.S. Army Corps of Engineers to release vital water supplies from the dam to make room for winter floods that didn’t come. 

The dire water shortages that followed spurred an experimental partnership called Forecast Informed Reservoir Operations, between the Scripps Institution of Oceanography at UC San Diego’s Center for Western Weather and Water Extremes and state, federal and local agencies. 

The program incorporates advanced forecasting and weather observations into reservoir release decisions at Lake Mendocino. It prevented the reservoir from going dry during the most recent drought, according to Don Seymour, deputy director of engineering at Sonoma Water, which co-manages the reservoir. 

Now, 165 miles away in the Sierra Foothills, Yuba Water Agency is eyeing adopting the same program for New Bullards Bar, a reservoir roughly eight times bigger than Lake Mendocino that’sfed by Sierra snowmelt on the North Yuba River. 

The reservoir supplies water to more than 60,000 acres of farmland in Yuba County as well as users south of the Delta. But early snowmelt is complicating efforts to store that water. 

“We’re seeing snowmelt conditions in mid-March that we normally don’t see until at least mid-May,” said general manager Willie Whittlesey. “It’s pretty obvious that this is the runoff — this is the snowmelt — and it’s just happening about two months early.” 

The reservoir is nearly full at 114% of average for this date and 84% of total capacity. 

But when snowmelt arrives early, the agency can’t catch it once the reservoir reaches a certain level — even when no storms are in the immediate forecast. Federal rules require Yuba Water to maintain a certain amount of empty space until June to absorb potential floodwaters, according to Whittlesey. 

Yuba Water is working with the U.S. Army Corps of Engineers to update this decades-old rulebook, Whittlesey said, but until then it must request special permission to store the extra water. 

Though the agency has received permission in the past, this year it’s also contending with a rupture in a major pipe to one of its hydropower facilities, which is forcing the agency to hold back more water behind the dam. 

Whittlesey said he suspects that the combination of flood-control requirements and damage control after the pipe failure is likely costing them tens of thousands of acre-feet of snowmelt. 

The California Department of Water Resources, which manages Lake Oroville — the state’s second-largest reservoir — told CalMatters that it’s storing water beyond its normal flood control limits, with permission from the U.S. Army Corps of Engineers.

In the Bay Area, the East Bay Municipal Utility District, California’s second-largest urban water supplier, owns and operates the Camanche and Pardee reservoirs in the Central Sierra foothills. 

“We’re working to save every drop in light of the warm temperatures that we are experiencing now, and in light of all the zeros that we are seeing in terms of a rain or snow forecast,” said spokesperson Andrea Pook. “The last time that we had run off this early was in 2015.”

Pook said the district is releasing less water from its reservoirs now, in order to preserve more for the fall when salmon migrate upriver to spawn. 

“We’re tracking to not necessarily be in a drought situation. But I am not convinced that we’re going to fill our reservoirs by July 1st, which is our usual goal,” Pook said. 

Improved forecasts after a major miss

Even as California suffers record heat and early snowmelt, the state is better prepared than in the past. 

Five years ago, state forecasters badly missed their runoff predictions — overestimating the snowmelt expected to refill reservoirs by up to 68%. Dry soils and a parched atmosphere drank up the runoff before it could flow into storage. Farms and cities scrambled in the middle of a drought as supplies fell far short of expectations.

This year is different. Major reservoirs are already above historic averages, and early season storms soaked the soil beneath the snowpack, making it less likely to swallow the runoff. 

The state has also been working on better forecasts. 

“Things have substantially improved,” said Andrew Schwartz, Director of UC Berkeley’s Central Sierra Snow Laboratory, in an email to CalMatters. 

Johnson, at the federal Central Valley Project, said that the state and federal water delivery systems are in a better spot than five years ago, and that forecasts haven’t made a major miss since. 

But the season’s early melt may still leave a gap.

“It’s going to get us through this year just fine,” Johnson said. “But it’s not as ideal as having that additional snow reservoir ready to run off through summer, and replenish what we’re going to be releasing.” 

Improved snowpack modeling and soil moisture estimates, experimental temperature measurements at different snow depths, university collaborations and incorporating weather outlooks are helping, according to the Department of Water Resources. 

Still, between state budget shortfalls and federal cuts, challenges remain, Anderson said.

Efforts to install more soil moisture sensors in national forests have run into permitting slowdowns at the U.S. Forest Service, which has shed thousands of employees under President Donald Trump. 

“You wait in line a lot longer,” Anderson said. “That’s been the biggest limitation of late. There just isn’t anybody there.”

This article was originally published on CalMatters and was republished under the Creative Commons Attribution-NonCommercial-NoDerivatives license.