Workers are expected on Monday to begin repairing erosion at the emergency spillway at Lake Oroville that threatened to flood downstream towns and forced more than 100,000 people from their homes Sunday.
Officials said they would use bags of rocks to try to plug the hole at the emergency spillway. Preparations were underway Monday morning, with workers gathering supplies for the repair job and trucks bringing in rocks and other materials.
They emphasized the situation remains dangerous at the reservoir and urged residents in communities along the Feather River to evacuate to higher ground.
The flood danger emerged suddenly when a hole developed Sunday in the auxiliary spillway that was being used to lower the levels of the full-to-the brim reservoir, the second-largest in California.
The erosion could undermine the concrete top of the spillway, allowing torrents of water to wash downhill into the Feather River and flood Oroville and other towns in Yuba, Sutter and Butte counties.
Sunday night, officials said the threat had diminished because the lake level had dropped and water was no longer washing over the emergency spillway.
But the situation at Oroville remained precarious. The two main avenues for getting water out of the lake – the unpaved emergency spillway and the main concrete spillway – were both damaged.
Both spillways are separate from Oroville Dam itself, which state officials continued to say was not in danger. The main spillway, a long concrete chute off to the side of the dam, has a gaping gash in it that forced officials to reduce releases last week.
But a new storm system forecast for later this week put water officials on a race against time. Bill Croyle, the acting director of the state Department of Water Resources, said they planned to continue discharging flows at a rate of 100,000 cubic feet per second, with the hope of lowering the reservoir level by 50 feet.
Gov. Jerry Brown late Sunday issued an emergency order aimed at speeding up state aid for the Oroville efforts.
"I've been in close contact with emergency personnel managing the situation in Oroville throughout the weekend and it's clear the circumstances are complex and rapidly changing," Brown said in a statement. "I want to thank local and state law enforcement for leading evacuation efforts and doing their part to keep residents safe. The state is directing all necessary personnel and resources to deal with this very serious situation."
Earlier in the day, Butte County Sheriff Kory L. Honea said that the hole was developing near the lower edge of the emergency spillway and eroding "at a rather significant rate."
"There was significant concern that [the hole] would compromise the integrity of the spillway, resulting in a substantial release of water," Honea said. "We had to make a very critical and difficult decision to initiate the evacuation of the Oroville area."
Those in Oroville, a city of about 16,000 people, were asked to flee northward toward Chico, along with Gridley and Biggs. In Yuba County, those in Marysville and other communities in the county's valley floor were urged to take routes to the east, south or west. In Sutter County, evacuations were ordered for Yuba City, Live Oak, Nicolaus and all communities around the Feather River basin.
"This is not a drill. This is not a drill. Repeat, this is not a drill," proclaimed a Sheriff's Department statement posted on social media. Authorities urged residents to contact neighbors and family members and reach out to the elderly and assist them in evacuating.
To slow the erosion on the emergency spillway, state Department of Water Resources officials doubled the flow of water down the main spillway to 100,000 cubic feet per second. The rapid increase drastically reduced the water coming over the emergency spillway.
The dam itself is structurally sound, officials said.
The evacuations marked a dramatic turn of events at the Oroville Dam, located about 75 miles north of Sacramento.
For several days, officials have been trying to figure out how to get water out of Lake Oroville after the main spillway was damaged. A massive hole was discovered last week on the main spillway and eventually spread across the full width of the concrete-lined channel.
Saturday marked the first time the emergency spillway was used since the dam was finished in 1968, and until Sunday afternoon it seemed to be working well. But water from rain and snow continued to flow into Lake Oroville at a rapid pace, causing water levels to rise to emergency levels.
Lake Oroville is the linchpin of California's state water movement system, sending water from the Sierra Nevada south to the farms across the San Joaquin Valley and cities in the Southland.
Video from television helicopters Sunday evening showed water flowing into a parking lot next to the dam, with large flows going down the damaged main spillway and the emergency spillway.
Officials feared a failure of the emergency spillway could cause huge amounts of water to flow into the Feather River, which runs through downtown Oroville, and other waterways. The result could be flooding and levee failures for miles south of the dam, depending on how much water is released.
On Sunday, officials said that although they expected the uncontrolled spill to end, they plan to continue using the concrete spillway to create more storage in the reservoir in anticipation of rainfall later in the week.
"We're going to continue to flow water down the spillway and lower the lake," said Eric See, a spokesman for the Department of Water Resources. "You're going to see the lake dropping over the next several days."
Officials emphasized that despite the damage to the spillways, the dam itself was not at risk of failing.
"Believe me, in the last several days there have been a lot of eyes on it," said Bill Croyle, acting director of the water department. "Oroville Dam is not in any way a part of the damage that occurred."
Officials have estimated it could cost $100 million to $200 million to repair the damage to the spillways and other features.
Times staff writers Megerian and St. John reported from Oroville; Hamilton and Phillips reported from Los Angeles.
8:44 a.m.: This story was updated with more information on preparations for repair work.
7:10 a.m.: This story was updated with news that the repair is expected to begin.
1:25 a.m. Feb. 13: This story was updated with news of Gov. Brown's action.
11:35 p.m.: This story was updated with new information from the Department of Water Resources.
8:15 p.m.: This story was updated with comments from evacuating residents.
7:50 p.m.: This story was updated with additional background on the flow of water down the spillway and minor editing.
6:55 p.m.: This story was updated with comments from Butte County Sheriff Kory Honea.
6:30 p.m.: This story was updated with additional details and minor editing.
5:30 p.m.: This article was updated with additional details on the evacuation orders.
4:55 p.m.: This article was updated with news of emergency evacuations.