(Video: Monica Akhtar / The Washington Post; Photo: Stephen Lam for The Washington Post)

— Authorities urgently lowered the level of Lake Oroville on Monday ahead of impending rain, stopping the flow of water over the Oroville Dam’s emergency spillway and apparently averting the threat of an immediate flooding disaster.

But law enforcement and water officials said they were not ready to lift a mandatory evacuation order that sent nearly 200,000 people from Oroville and points south of the dam fleeing to Chico and other nearby cities, signaling potentially significant concerns about the infrastructure meant to protect residents in the Northern California valley below.

Though the Oroville Dam’s integrity remained intact, spillways designed to handle overflow began to crumble and erode over the weekend as water drained from the overfull lake, leading authorities to fear that a larger failure could send a torrent of water rushing through the valley below and into area towns.

Lake Oroville is one of California’s largest man-made lakes, with more than 1 trillion gallons of water and 167 miles of shoreline, and the Oroville Dam is the nation’s tallest, at 770 feet. Lake Oroville is a central element of the state’s government-run water system, bringing water from the Sierra Nevada to the Central Valley, where it is crucial for agriculture, and to residents and businesses in Southern California.

The emergency evacuation order Sunday has residents worried about a major piece of California’s infrastructure as the region transitions from a record-setting drought to unusually large amounts of rain and snow. It could portend problems ahead as more rain is in the immediate forecast and as the melting season looms, with more snow piled on the peaks of the Sierra than there has been in years.

Heavy rains forecast for this week could again cause lake levels to rise, put pressure on the damaged spillways and jeopardize area communities. Three storms are lined up to drench Northern California, with up to nine inches of precipitation possible.

It is unclear when residents who were ordered to flee their homes will be able to return. Butte County Sheriff Kory Honea said Monday that he would not lift the mandatory evacuation order until water resources officials have a better grasp of the expected risks.

“This is still a dynamic situation, it’s still a situation we’re trying to assess,” Honea said, noting that he does not want anyone in the community to go back home if there’s a chance that they could be in harm’s way. “Getting those people home is important to me. I want that to happen absolutely as soon as possible.”

An empty Montgomery Street in downtown Oroville, Calif. More than 200,000 residents in Oroville and nearby towns evacuated Sunday evening after officials warned that the weakened emergency spillway at the Oroville Dam might collapse. (Stephen Lam/For The Washington Post)

Oroville and surrounding communities sat as ghost towns Monday, with residents having left at nearly a moment’s notice Sunday, jamming highways as night fell. Many of them gathered here in Chico, at temporary shelters and at the fairgrounds, waiting.

Father and son Pedro and Juan Mota evacuated from Gridley, a small town in the flood zone, along with 12 members of their extended family. Juan, 28, said a friend of his working on the dam’s spillway called him at 4 p.m. Sunday, ahead of the official evacuation order, and said ominously, “You’ve got to get out.”

“It’s crazy. It’s unreal,” Juan Mota, dressed in a black Oakland Raiders sweatshirt, said as he sat on a folding chair in front of a station handing out bottles of water and bags of chips. “You see this stuff in movies, people trying to get gas before it runs out and passing each other on the highway, all of that.”

Evacuee Juan Mota, left, sits with his father, Pedro, at the evacuation center at the Silver Dollar Fairgrounds in Chico, Calif. (Stephen Lam/For The Washington Post)

Cindy Vanneman, who lives in the Golden Oaks Mobile Homes in Oroville and does not have a car, heard about the evacuation order at 9:30 p.m. Sunday. She was whisked out of town by shuttle bus but wasn’t allowed to bring Leo, her 2-year-old tuxedo cat, so she called animal rescue, hoping they could help.

“I felt every emotion you can feel all at one time,” Vanneman said. “You don’t know if you can go back home or if you’ll go back to see your home floating down the river. I’ve been there for 17 years. But now I’m definitely looking into moving. I want to live somewhere high and dry.”

Much of California has felt high and dry for years, the state’s lengthy drought causing Lake Oroville’s water levels to plunge well below capacity and making it an oft-cited example of the extent of the dryness. Now, in a dramatic shift, Northern California has experienced an extraordinarily rainy winter that has caused waters to rise to their highest levels in decades.

There was never any danger of the main Oroville Dam collapsing. The problem instead is with the dam system’s spillways — safety valves along the lake’s shoreline designed to release water in a controlled manner, preventing water from topping over the wall of the colossal dam.

As the lake’s water levels rose to the brim after heavy rain and snow this month, unexpected erosion crumbled the main spillway, pulling off chunks of concrete and creating a large hole. Then sheets of water began flowing over the dam’s emergency spillway for the first time in its nearly 50-year history, coursing down a wooded hillside and carrying murky debris into the Feather River.

“Once we have damage to a structure like that, it’s catastrophic,” said Bill Croyle, acting director of the state’s Department of Water Resources. “We determined we could not fix the hole.”

The damage to the main spillway at the Oroville Dam. (California Department of Water Resources/Getty Images)

More than a decade ago, three environmental groups warned state and federal officials about what they believed was a problem with the dam’s emergency spillway, because it isn’t really a spillway; it’s actually a 1,700-foot-long weir that empties onto a dirt hillside. The concern centered on erosion of that hillside in the event of an emergency.

When the Oroville Dam was going through a re-licensing process, the environmental groups filed a motion in October 2005, urging a federal regulatory agency to require state officials to armor the emergency spillway with concrete so that, in case of extreme rain and flooding, water wouldn’t freely cascade down the hillside and tear it away. The upgrade would have cost millions of dollars, and no one wanted to foot the bill, said Ronald Stork, senior policy advocate for Friends of the River, one of the groups that filed the motion.

“When the dam is overfull, water goes over that weir and down the hillside, taking much of the hillside with it,” Stork said Monday. “That causes huge amounts of havoc. There’s roads, there’s transmission lines, power lines that are potentially in the way of that water going down that auxiliary spillway.”

Federal officials determined that nothing was wrong with the emergency spillway, which they said could handle 350,000 cubic feet of water per second and “would perform as designed” in the event of its use, according to a July 2006 memo from a senior engineer with the regulatory agency.

“The emergency spillway meets FERC’s engineering guidelines for an emergency spillway,” the engineer wrote. “The guidelines specify that during a rare flood event, it is acceptable for the emergency spillway to sustain significant damage.”

Croyle said Monday at a news conference that he hadn’t seen the 2005 reports and declined to comment on them.

Water surges down the main spillway at Lake Oroville. The water level dropped Monday behind the nation's tallest dam, reducing the risk of a catastrophic spillway collapse. (Randy Pench/Associated Press)

When officials decided to use the emergency spillway in recent days, flows that were a tiny fraction of the apparent limit caused serious enough erosion on the hillside to warrant the evacuation.

Anticipating a possible catastrophe for the Oroville area, about 75 miles north of Sacramento and about 25 miles southeast of Chico, the Butte County Sheriff’s Office ordered evacuations, emphasizing in a news release that it was “NOT a drill.”

Honea called the evacuation order a “critical and difficult decision” and said he recognized that it would cause significant dislocations and traffic jams — which it did. Residents of Oroville, a town of 16,000, were ordered to head north toward Chico, while other nearby residents drove south toward Sacramento.

“I recognize how tough this situation is on people,” Honea said. “I recognize that we’ve had to displace a lot of people.”

Stork believes none of that would have happened had officials listened to his and others’ concerns and built a proper spillway 12 years ago.

“They told us not to worry. All was good. Everything was fine. It’s all safe,” he said. “First of all, they’re not supposed to fail. That’s not what we do in a first-world country. We don’t do that. We certainly don’t do that with the nation’s tallest dam. An auxiliary spillway isn’t supposed to cause lots of havoc when it’s being used.”

Adriana Weidman of Marysville, Calif., said she heard about the evacuation around 5 p.m. Sunday. Fearing that nearby rivers would overflow, she rushed to pack as much as she could, then got into the car with her husband and two children, she said. They headed to Colfax, Calif., about 45 miles east.

“It’s scary,” Weidman said. “I’m terrified I’m not going to have a home to come home to.”

A man sits outside the evacuation center at the Silver Dollar Fairgrounds in Chico, Calif. (Stephen Lam/For The Washington Post)

Romeo reported from Chico. Schmidt and Guerra reported from Washington. Angela Fritz and Derek Hawkins in Washington contributed to this report.