Thanks to storm runoff, there's a lot of water over California's second-largest reservoir

At 8 a.m. Saturday, a sheet of water began spilling from the brim of California’s second-largest reservoir and washing down a partly cleared hill to the Feather River.

It was the first time in the 48-year history of Lake Oroville that the reservoir was so full that it triggered uncontrolled releases down an emergency spillway.

Bloated with storm runoff, the reservoir had gone from 80% full to overflowing in less than a week when managers were forced to reduce releases on Oroville’s heavily damaged concrete spillway.

The emergency spillway was doing what it was supposed to do: letting water out of the huge lake so it wouldn’t top the dam.

State water officials called the spill a small one and said they expected it to end Sunday or Monday as lake levels dropped below the emergency threshold.

"The flow rates that we see now pose no threat to the dam and no flood threat to downstream waters,” said Bill Croyle,  acting director of the state Department of Water Resources.

But he made it clear that while this is Oroville’s first emergency spill, it may not be the last.

With northern Sierra precipitation levels tracking ahead of the wettest year on record so far this winter, Oroville managers have a nerve-racking several months ahead of them. “There’s a lot of snow up there,” Croyle observed.

The snowpack in the northern Sierra, which includes the Feather River watershed, is 150% of normal for the date. Statewide it is 180% of average.

Reservoirs around the state are making flood-control releases.

In Mariposa County, the small reservoir behind Mariposa Dam was overflowing, sending water down its spillway for the first time since the 1950s, said Merced County spokesman Mike North. About 25 homes were flooded Saturday by the swollen Mariposa Creek.

This winter’s turnaround from five parched years has been dramatic. A year ago at this time, Oroville was slightly less than half full. In 2014, it was a little more than a third full and its receding shoreline served as a vivid symbol of the drought’s punishing toll on the state.

But Californians shouldn’t be surprised.

“If you look at a map of the U.S. and weather variability, California is the most variable place in the country,” said Jeff Kightlinger, general manager of the Metropolitan Water District of Southern California. This is an extreme. But it’s not outside of what we’ve experienced in the past.”

Completed during the administration of Gov. Ronald Reagan, Oroville is the keystone of the State Water Project that sends Northern California water hundreds of miles to the Southland.

As the head of the project’s biggest customer, Kightlinger has been anxiously watching since Tuesday, when a hole big enough to swallow a small house opened in the long concrete chute that is the lake’s normal spillway.

Dam managers briefly shut down releases and then restarted them at a reduced rate as they struggled to keep the lake from rising to a point that would trip emergency spills.

Friday they thought they had managed to do that. But it rained Friday night. By Saturday morning the lake level had risen to an elevation of 901 feet —  the point at which water started washing over the 1,700-foot-long top of the emergency spillway.

It snaked down the hill, a shallow rippling stream glinting in the sunlight.

Dam managers wanted to avoid that sight for two big reasons. They don’t control the emergency releases. Nature does. The spill won’t stop until inflow volumes drop below the discharge.

And as the water streams down the hillside, it carries sediment and debris into the Feather River — material that can clog the river channel, the dam’s powerhouse and downstream diversion facilities.

Crews worked frantically at the end of the week clearing trees and brush from the water’s emergency path. Utility crews used helicopters to remove power lines.

Booms and boats were brought in to collect debris from the pool at the dam’s base. More than 10 million salmon were evacuated from the Feather River hatchery downstream because the river water had grown too muddy for them.

Engineers are limiting flows down the damaged spillway so as not to further erode it. But that is also cutting releases, which means that after this emergency spill is over, more may be necessary to counter runoff in future storms.

Croyle said the state is already studying repair options, including building a new spillway to replace the broken one. But that work can’t begin until the runoff season ends. And it will be expensive.

Croyle estimated that it will cost $100 million to $200 million to fix the spillway — a bill that will largely go to State Water Project contractors, including Metropolitan.

 “Obviously down the road, we’ll be talking with FEMA and the federal government and the state government about disaster funds,” Kightlinger said. “But … there will eventually be a sizable bill for the state water contractors — and ultimately to Metropolitan.”

Kightlinger said his agency is volunteering technical and engineering assistance to the state to develop repair plans.

He agreed that Oroville’s earthen dam “should be in fine shape.” But he said the unprecedented use of the emergency spillway comes with risks of hillside erosion and sending damaging sediment downstream.

“We just hope they make it through this winter without catastrophic damage and go from there,” Kightlinger added.

Greg and Doreen Schmidt live in a low-lying area of downtown Oroville, not far from the dam.

"Once it spills over" the emergency spillway, "who knows what's going to happen,” Greg Schmidt said. “But it seems like they have it under control."

Nonetheless, Doreen said, "I have my bags packed and I'm on alert."

Ralph Thomas was more sanguine as he watched the roiling dam releases flow under the Table Mountain Boulevard Bridge in Oroville.

"This ain't nothing compared to ’97," he said. "Back then the water was almost up to the bridge."

To read the article in Spanish, click here

bettina.boxall@latimes.com

patrick.mcgreevy@latimes.com

Times staff writer Angel Jennings contributed to this report. McGreevy reported from Oroville and Boxall  from Los Angeles.

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