Water stopped flowing Monday over a spillway that has eroded at the nation’s tallest dam, allowing crews to assess the damage — and figure out how to shore it up — before another round of rain anticipated later this week.
At least 180,000 residents living below the dam were ordered to evacuate late Sunday after officials said they feared the emergency spillway could collapse at any time. The concern appeared to ease Monday as the water level dropped; water was flowing out of the lake at nearly twice the rate as it was flowing into it, officials said.
“We have engineers and geologists and dam safety experts that are on the scene,” Eric See, a public information officer for the California Department of Water Resources, said of the emergency spillway of the Oroville Dam.
Helicopters had flown large rocks and other building materials to the dam for engineers to use to shore up the spillway, Mr. See said. The Federal Emergency Management Agency said it had activated coordination centers in response to the situation.
There is no damage to the dam itself — which, at 770 feet, is the tallest in the country — but recent heavy rains and snowfall raised the water levels at Lake Oroville, the second-largest reservoir in California and a linchpin of the state’s water system.
Last week, a hole opened in the main spillway, a concrete structure that releases water from the dam, so the authorities reduced the flow of water over it and turned to a nearby auxiliary spillway, a mostly earthen structure that had not been used since the dam was built in 1968. The resulting erosion to the emergency spillway led the authorities to warn of a possible failure, prompting the nearby counties of Butte, Yuba and Sutter to urge evacuations.
“The evacuation orders are still in place,” said Jonathan Gudel, the public information officer for the California Office of Emergency Services. Many residents have gone to shelters set up in nearby towns while others have gone to motels or to relatives.
By Monday, Mr. See said, the authorities had increased flows back over the main spillway in an effort to bring down the level of the lake.
“We want to keep the lake level low enough where the water’s not going to have to go over the emergency spillway again,” Mr. See said, adding that crews were trying to get ahead of anticipated rainfall.
“We are tracking this incoming storm,” Mr. See said.
On Monday, officials around the affected counties directed residents to shelters and turned wary eyes upstream.
“The big fear, again, is that potential for catastrophic failure,” said Chuck Smith, the public information officer for Sutter County, a low-lying, agricultural area downstream from the dam with an extensive system of levies.
“We do live with high water around here, we’re used to it,” Mr. Smith said. “We’re not used to having a crippled dam, and that’s raised everybody’s anxiety.”
Kurt Richter, a rice farmer in Yuba City, left for nearby Colusa on Sunday in a two-car caravan with his wife and child, as the sudden evacuation order prompted chaos in the area.
“It was just pandemonium outside,” Mr. Richter said, with people driving on the shoulders, the medians and the wrong side of the road. “It just felt like a scene from a movie where society has collapsed and people start acting crazy.”
And it was a sharp about-face for a region that has spent the last several years dealing with a prolonged drought.
“We go from hard-core conservation mode — curtailments, restrictions, all these regulations that are imposed on us, justifiably so — and now we’ve got more water than we can physically manage,” Mr. Richter said. “And there’s an element of human danger that’s hanging over our head for who knows who long. We’ve got another huge storm system that’s coming this week.”
Some of Mr. Richter’s farmland is already under water.
“The drought was basically limiting the water we would have access to,” Mr. Richter said. “Now there’s going to be so much water out there we might not even be able to put the crops in.”
But he said his major concern Monday was not his farm but his home and the safety of his family.
“It’s a flood plain — we know the history of the area and understand sort of the realities of it. I think if anything, the drought maybe caused a lot of us to kind of ease into forgetting about that,” Mr. Richter said. “We’ve all just gotten slapped back into reality about what really can go down in this part of the valley.”