Crews at Oroville Dam begin aggressive debris cleanup

Water gushing down the main spillway at the Oroville Dam was completely shut off Monday for the first time since the damaged outlet triggered a mass evacuation of people downstream this month.

The spillway was closed to allow workers to clear out debris at the base of it and fully assess the damage to the spigot and what it will take to get a hydroelectric plant back up and running, officials said.

Bill Croyle, acting director of the California Department of Water Resources, said crews plan to work aggressively on the task at hand before more rain necessitates turning the spillway back on.

But as of Monday, the spillway was expected to be closed for seven days, said Nancy Vogel, a spokeswoman from the Department of Water Resources.

Oroville Dam's crippled spillway is inspected via helicopter after it was shut off Monday, Feb. 27, 2017, in Oroville, Calif. (Paul Kitagaki Jr./The Sacramento Bee via AP) Oroville Dam's crippled spillway is inspected via helicopter after it was shut off Monday, Feb. 27, 2017, in Oroville, Calif. (Paul Kitagaki Jr./The Sacramento Bee via AP) Photo: Paul Kitagaki Jr., Associated Press Photo: Paul Kitagaki Jr., Associated Press Image 1 of / 99 Caption Close Crews at Oroville Dam begin aggressive debris cleanup 1 / 99 Back to Gallery

“We are hoping for seven days,” Vogel said. Crews “are here for the next week and a half. It looks like dry weather.”

A crisis unfolded during a Feb. 7 rainstorm that filled Lake Oroville to capacity. While dumping water from the reservoir, crews discovered a gaping hole in the main concrete spillway and opened up the earthen emergency spillway for the first time in the dam’s 48-year history, a move that led to rapid erosion and a near collapse of the emergency spillway.

The trouble prompted an evacuation on Feb. 12 of 188,000 people living downstream in Oroville, Marysville and other towns along the Feather River.

On Monday, dam operators began reducing the flow from the main flood-control spillway and reached zero by the afternoon, officials said, adding that the reduction would minimize risks to downstream levees.

The inflow of water has decreased to about 22,000 cubic feet per second, down from about 40,000 cubic feet per second during the high flow of Feb. 13, and it will continue decreasing throughout the week, which is forecast to be mostly dry, officials said.

Lake Oroville’s level has dropped to 838 feet — 63 feet below the maximum level of the reservoir. Lake levels aren’t expected to rise more than 15 feet during the period of the shutdown, officials said.

Construction crews are installing rock and concrete foundations that will slow water flows in case the emergency spillway is needed in the future.

Efforts to release water from the main spillway have been ongoing since the evacuations were ordered.

Sarah Ravani is a San Francisco Chronicle staff writer. Email: sravani@sfchronicle.com Twitter: @SarRavani