Crews prepare to shore up Lake Oroville emergency spillway

Crews at Lake Oroville on Monday prepared bags of boulders they will use to shore up the reservoir’s emergency spillway the morning after erosion on the overflow area threatened the nearly 200,000 residents downstream from the lake.

The water level in the lake fell below the lip of the emergency spillway overnight, slowing rapid erosion on the hillside that appeared to be nearly collapsing.

Officials hope to stabilize the emergency spillway in time for another round of winter storms that are forecast to sweep over the Sacramento Valley starting Wednesday.

The rain and runoff from streams and rivers that feed Lake Oroville could push water back over the emergency overflow area, officials said.

Water and debris rush down the side of the hill as thousands of gallons of water rush over the auxiliary spillway at Oroville Dam in Oroville, Calif., on Sunday. Crews hope to shore up the spillway that was in danger of eroding, threatening nearly 200,000 downstream from Lake Oroville. less Water and debris rush down the side of the hill as thousands of gallons of water rush over the auxiliary spillway at Oroville Dam in Oroville, Calif., on Sunday. Crews hope to shore up the spillway that was in ... more Photo: Carlos Avila Gonzalez / The Chronicle Photo: Carlos Avila Gonzalez / The Chronicle Image 1 of / 84 Caption Close Crews prepare to shore up Lake Oroville emergency spillway 1 / 84 Back to Gallery

“Obviously, we don’t want to have to use it,” said Bud England, a firefighter working at an emergency command center in Oroville. “They want to prepare it to be used if necessary. This is part of the mitigation to prepare it for future use if necessary.”

An estimated 188,000 people in Butte, Sutter and Yuba counties were ordered to evacuate around 4:30 p.m. Sunday when water resource managers detected the trouble with the emergency spillway.

A hole in the reservoir’s primary concrete spillway that was first detected on Tuesday had forced officials to allow water on Saturday to cascade over the emergency spillway. The overflow area was used for the first time since the Oroville dam was put into operation in 1968.

Water resource managers increased the volume of water flowing out of the primary spillway to 100,000 cubic feet per second to relieve the emergency spillway, despite erosion problems caused by the hole in the 3,000-foot concrete slide.

The California National Guard was ready to deploy more than 20,000 soldiers and airmen should an emergency arise.

Evan Sernoffsky is a San Francisco Chronicle staff writer. Email: esernoffsky@sfchronicle.com Twitter: @EvanSernoffsky