The rain has started to return in Northern California and will continue over the next few days, but officials aren't as concerned about the upcoming weather so much as the damage already done to the Oroville Dam's already compromised main spillway.

The risk of flooding has dropped substantially, but Butte County Sheriff Kory Honea warned residents Wednesday that they remain in "an emergency situation."

As storm moves in, engineers reduce outflow from Lake Oroville to clean up debris

Spillway repairs continue at Oroville Dam on Wednesday. (Brian van der Brug / Los Angeles Times)
Spillway repairs continue at Oroville Dam on Wednesday. (Brian van der Brug / Los Angeles Times)

Confident that a series of incoming storms won’t overwhelm the Oroville reservoir a second time, state officials said Thursday that they would slow drainage of the lake so they can do work on an adjacent power plant.

The reservoir exceeded its capacity over the weekend, which sent water overflowing into an unlined, emergency spillway. That overflow sent soil, rock and forest debris into the Feather River below.

With the reservoir’s water level down more than 30 feet since Sunday and getting lower, state Department of Water Resources acting director Bill Croyle said at a news conference Thursday that engineers will slow the flow down the Oroville Dam's damaged main spillway from 100,000 cubic feet of water per second to 80,000 cfs over a period of several hours.

The reduction will allow crews to move into the concrete channel to clear out trees, branches and other debris that has clogged the spillway and forced the downstream hydroelectric plant to go offline, Croyle said.

There was no estimate on when the power plant would be back up and running, but it will probably not be before Monday, Croyle said.

Meanwhile, the Herculean effort to reinforce the emergency spillway before more rain arrived used a caravan of helicopters and trucks to fill three deep fissures in the dirt hillside with rocks and cement.

As of Thursday, repairs on one erosion site was completed, the second was 25% filled and the third was 69% filled, Croyle said.

As long as the lake doesn’t reach capacity the emergency spillway won’t be used, Croyle said.

The incoming storm system is weaker than the one that overwhelmed the lake last week after the dam’s main spillway eroded to the point of fracture, Croyle said.

More than 100,000 residents south of the dam remain under an evacuation advisory and should be prepared to flee to higher ground should the dam overflow and the spillways collapse, authorities said.

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