There’s a rule in California that may seem bizarre in a drought-stricken state: in the winter, reservoirs aren’t allowed to fill up completely.

In fact, even as this post goes up, a handful of reservoirs are releasing water to maintain empty space.

The practice, which has long inflamed combatants in California’s water wars, is due to a decades-old rule designed to protect public safety. If a major winter storm comes in, reservoirs need space to catch the runoff and prevent floods.

But with advances in weather forecasting, some say this preemptive strategy is outdated. A new, “smart” flood control system could save water in years when Californians need it most.

Hitting the ‘Magic’ Line

At one of the state’s major reservoirs — Folsom Lake, east of Sacramento — the volume of water spilling from the dam has swollen eight-fold in the past few weeks, sending billions of gallons downstream, much of it into San Francisco Bay.

Early in February the reservoir reached a key threshold: 60 percent full, which is the highest water level allowed during the winter months, according to rules from the U.S. Army Corps of Engineers and the Sacramento Area Flood Control Agency.

Last year, most of the reservoir was a dry, dusty lakebed.

“What reservoir was left was confined to the old river channels before we built the dam,” says Drew Lessard of the U.S. Bureau of Reclamation, the federal agency that manages the reservoir.

Just an average-size winter storm can send huge volumes of water down the American River into Folsom Reservoir, boosting the lake by 10 percent or more. A major storm can produce dramatically more than that.

“The watershed is pretty flashy and it responds pretty quickly to storm events,” says Lessard. “That’s why we need to reserve a space during those winter months in case that happens.”

Sitting 40 percent empty allows the reservoir to act as a buffer against floods, gulping the runoff without overflowing. In years where the upstream reservoirs are fuller, Folsom Reservoir is required to remain 60 percent empty.


Releasing water downstream does produce benefits, supporting wildlife and endangered fish. It also prevents salt water from San Francisco Bay from backing up into the Sacramento-San Joaquin Delta, which is a drinking water supply for many in the Bay Area.

But preemptively releasing water has its own risks, especially if it doesn’t rain again, as happened in 1997. The reservoir was lowered and, “it was dry the rest of the year and we never really rebounded,” recalls Lessard.

As California enters a fifth year of drought, seeing operators deliberately dumping water can be disconcerting.

A few weeks ago, Shauna Lorance of the San Juan Water District near Sacramento went to state regulators with a message: when water is disappearing downriver, it’s hard for water consumers to take the conservation message seriously.

“Anything they conserve right now is not held,” said Lorance. “For me to explain to customers, after everything they’ve done, that they need to continue to conserve so it can be spilled is going to be a nightmare.”

‘Smart’ Reservoirs

“The rules that govern water in the West were created in the 19th century,” says Marty Ralph, who directs the Center for Western Weather and Water Extremes at UC San Diego. “And yet here we live in the 21st century with its special and new needs: greater population and a changing climate.”

Ralph is piloting a more dynamic method of flood management at Lake Mendocino with the Sonoma County Water Agency.

Instead of a maintaining firm limits on reservoir levels dictated by the calendar, managers would use cutting-edge weather forecasts to gauge how much flood space they need.

“Weather predictions have been improving over the last decades, “ said Ralph. “Particularly on the West Coast, we’ve learned about the phenomenon that produces most flood-producing storms. We call them atmospheric rivers.”

Instead of emptying out a reservoir preemptively, managers would allow the reservoir to stay fuller, keeping an eye on the weather forecast.

If a big storm appears, “they’d have three, four, five days lead time, enough to release that extra water and get it out of the way safely,” said Ralph.

If storms don’t appear, the water would be saved for later in the year.

Ralph says “forecast-informed” operations are becoming possible thanks to major advances in weather research. New weather satellites and more precise forecast models are making predictions more accurate. Researchers are even flying planes into atmospheric rivers to gather information about how they behave.

Ralph says even with the improvements, there’s more fine tuning to be done.

“We get down to three days’ lead time and we’re down to plus or minus 300 kilometers (186 mi.) in terms of the location of the storm’s landfall,” he explains. “That’s the difference between San Francisco and the Oregon border. That’s a big difference if you’re operating a reservoir on a much smaller scale.”

But he says potential is clear in a drought-prone state like California.

“Imagine we have the option to keep, say, ten percent extra water behind each dam in the winter,” he muses. At Folsom Reservoir alone, that amount would supply 200,000 households for a year.

Changing the Rules

“We see potential,” agrees Greg Kukas of the Army Corps of Engineers, the federal agency that sets flood rules for reservoirs. “What we’re not clear on is what the risks associated with that potential might be. What are the trade offs?”

The Corps is considering using the new weather-based operations at Folsom as part of a major update to the reservoir operations plan.

The question is whether weather forecasts are precise enough — not just as to when a storm is coming, but how big it is.

“When it comes to forecasting the size of events that we’re most concerned about, they are about 20 percent off,” says Kukas.

Getting it wrong is not an option. If the dam overflows, it could flood Sacramento and hundreds of thousands of people downstream, and potentially put the dam itself at risk of failing.

“The consequences, you don’t even want to imagine them.”

Working with reservoir operators around the state, the Corps says it currently takes weather forecasts into consideration in making water release decisions, though in a minimal way.

“We don’t make our reservoir releases in a vacuum,” says Christy Jones, also with the Corps of Engineers. “We always are looking at the balance. We know water supply is incredibly important but we also know public safety is incredibly important.”

The Corps plans to decide whether to manage Folsom Reservoir using weather forecasts next year. Changing current flood rules for how much the reservoir can hold in the winter would require an act of Congress. But if it works at Folsom, Jones agrees that it would be a model for reservoirs across the Western U.S.

“This could have a huge impact on the West.”

California Reservoirs Are Dumping Water in a Drought, But Science Could Change That 29 February,2016Lauren Sommer

  • Mike

    “When it comes to forecasting the size of events that we’re most concerned about, they are about 20 percent off,” says Kukas.

    What kind of number is that a percent of? And, how does that relate to reservoir capacity?

    • Ted Swift

      From context, I think the “number” is forecast total precipitation from a given storm event: They can forecast precipitation to within +/-20%. But that’s not the same as reservoir inflow; other variables add to the uncertainty. Rain-on-snow events are especially scary for flood managers. The number relates to reservoir capacity via the (unnecessarily and confusingly 3D) graph. You can see that starting in March the flood storage “headroom” tapers to zero by June (“Reservoir is allowed to fill completely in late spring”). Compare that to how fast the reservoir filled in January.

  • Uwe

    I don’t know where the author thinks San Francisco is in relation to the Oregon border, but it’s a lot more than 186mi

    • FauxFlat

      I suspect both the author and the weather expert, Ralph, know that 186 miles is the midpoint between San Francisco and the Oregon border.

  • Bob Johnson

    And now an atmospheric river is headed our way until at least March 16th, so it’s good that Folsom wasn’t full.

  • solodoctor

    I did not know about the requirement to save space in order to prevent flooding. Seems to me that with today’s more accurate forecasting of rain, etc it would be possible to adjust some of these limits. A trial run or two with carefully calibrated changes is worth a try.

    Also, why not build some other systems to capture/store more of this water? Eg, channels which could allow for it to replenish badly depleted underground aquifers might be helpful. Or how about building secondary reservoirs or cisterns in which ‘excess’ water could be stored and used later? Or, not so simply, raising the walls of the existing reservoirs so more water could be stored in these? Admittedly, these kinds of things would be expensive in today’s world. But is it more costly to simply send this water out into the SF Bay when it could be put to other uses?

  • Rg

    They release water to make space. Isnt there already plenty of space? How big of storm are they expecting?

    • Jony

      It is bullshit excuse. Only agenda was to “save” the bait fish which are not endangered in the first place.

    • McVader

      These lakes also take runn off in spring after the snow melts. Shasta and Folsom will be at 100% in May/June and they will start to lose water in August/September

  • Patrick Ridosh

    The Truth is that flood insurance companies own our politicians on this issue. Use stilts if you build a house in a flood zone. Stop making others suffer for the sake of poorly built river shacks and their Fat Cat flood insurance company.

  • Brutus

    So they are stopping flooding by building reservoirs, implying that without them, the state would flood. Sounds suspect, but the logical conclusion is that we need more reservoirs, which also happens to be the solution to lack of sufficient water storage for use.

    There are so many more people here than 100 years ago, how could we possibly expect to have no increase in capacity.

    The suggestion that we shouldn’t let water out of reservoirs before they’re full shouldn’t be novel, and the fact that it is shows how inept those making the decisions about our water are.

    • PDuncan12345

      They are not completely inept, nor do they let water out AFTER winter. In the spring reservoirs are allowed to fill to 100% so we can make it through summer. But you’re right, WE NEED MORE RESERVOIRS. And the biggest problem is that the people we have elected aren’t doing their jobs. They need to set aside funds and invest in desalination projects but no one seems to want to do that either. Instead, they shake their fingers at the average citizen and tell us to simply “Stop using so much water.” RIDICULOUS! The earth is called the WATER Planet. WE have PLENTY of water. Scolding citizens even to the point of having neighbors spy on each other and local authorities handing out citations for water usage is NOT a long term solution! It’s infuriating that this entire issue is mired in politics as usual.

    • Jonny

      California has not built a reservoir in years! That is an outright lie!

  • PDuncan12345

    It seems besides more accurate weather forecasting, that we could create more reservoirs to store water. Why isn’t anyone talking about infrastructure?

  • Jony

    They ARE NOT ENDANGERED bait fish! Obama’s agenda was for the saving of bait fish. Build more reservoirs & save the water RETARDED FUCKS! Save it for us & for farmers, fucking retards!

    • lmanningok

      “Screaming” in print with capital letters and using foul, insulting language does not imply an intelligent person whose post should be respected. Calm down and try again.

      • Jonny

        Shut the fuck up liberal fucktard! Pretending to be intelligent does not make my comment into fiction. OK fucktard?

      • Jonny

        Fucking pointless retard!

      • Jonny

        So a fucking retard like you live to appear intelligent? Appearing intelligent is the reason you live fucking blowfuck? Post needs to be respected? So if someone like you types out a calm bullsgit comment, it should be respected even if it is fucking bullshit? Huh faggot????? Retarded fuck!!!!!!!

  • McVader

    Why did we go 5 years in drought without building additional reservoirs? The state government is useless. It is an obvious need for the millions of people in CA. We should be building massive reservoirs for Southern California, so much water goes to the south, let’s build a couple just to satisfy their needs and keep the current water supply in the north. Who’s responsibility in the state government is this?

  • Walter Crunch

    So, basically, they are running their water storage based on 1950s ideas and rules. Makes complete and utter sense

  • bfraz

    Climate change! Climate Change! What BS!
    Fast forward a year from this article and SEE how stupid it is to base the single most important task a dam does – FLOOD CONTROL, on a “climate change” fallacy. California has a history of drought and flood leading back to beyond the ice age.

    “We don’t need no stinking 20% flood capacity? Tell it to Oroville.

  • Jonny

    Solution is to keep building reservoirs. They lie to keep their retard liberal agendas hidden. Whole retarded shit is done over saving of bait fish. It has nothing to do with weather or safety. If that is the case, build a reservoir every couple of years. DUH!!!!!!!!!!!!


Lauren Sommer

Lauren is a radio reporter covering environment, water, and energy for KQED Science. As part of her day job, she has scaled Sierra Nevada peaks, run from charging elephant seals, and desperately tried to get her sea legs – all in pursuit of good radio. Her work has appeared on Marketplace, Living on Earth, Science Friday and NPR’s Morning Edition and All Things Considered. You can find her on Twitter at @lesommer.

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