More than 100,000 people in communities downstream of Lake Oroville were told to evacuate Sunday evening after authorities grew concerned that dangerous flood waters would start surging out of the huge reservoir.
The flood threat emerged suddenly Sunday afternoon when a hole developed in the auxiliary spillway that was being used for an emergency spill to lower the level of the full-to-the brim reservoir, the second-largest in California.
If the erosion advanced quickly uphill, it could undermine the concrete top of the spillway, allowing torrents of water to wash downhill into the Feather River and flood nearby Oroville and other downstream towns.
Kevin Lawson of CalFire said it had threatened to "unleash a 30-foot wall of water coming out of the lake."
But by 10 p.m. Sunday, officials said the immediate threat had passed because water had stopped washing over the emergency spillway.
Nonetheless, the situation at the reservoir 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.
That caused the lake to quickly rise with heavy storm runoff, triggering emergency releases down the auxiliary spillway, which consists of a concrete weir at the reservoir's edge that sends flows down a hillside into the Feather River.
Although the emergency spill was small, it started to erode the hillside Sunday afternoon.
"There was significant concern that [the erosion] would compromise the integrity of the spillway, resulting in a substantial release of water," Butte County Sheriff Kory Honea said at a Sunday evening news briefing.
"We had to make a very critical and difficult decision to initiate the evacuation of the Oroville area," he added.
"Immediate evacuation from the low levels of Oroville and areas downstream is ordered," proclaimed a Sheriff's Department statement posted on social media. "This is NOT A Drill."
The order affected rural communities located along the Feather River and included the counties of Butte, Yuba and Sutter. Oroville residents were told to make their way north of the lake to Chico, where an impromptu evacuation shelter had been set up at the fairgrounds.
The Yuba County Office of Emergency Services urged evacuees to travel only to the east, south or west. "DO NOT TRAVEL NORTH TOWARD OROVILLE," the department said on Twitter.
As traffic slowed to a crawl, travelers reported encountering road blocks heading north on California highways 70 and 99. For those headed south, the driving was no easier as thousands poured onto the highways in an attempt to evacuate.
Belen Castaneda, 23, a preschool teacher from Biggs, fled her home around 4:30 p.m. for the Silver Dollar fairgrounds.
"We just grabbed everything we could," she said. "Everyone was freaking out."
Castaneda and her family — three sisters and her mom and dad — drove in two separate cars, bringing along their elderly neighbor who doesn't speak English.
Castaneda said her family took special care to bring irreplaceable items. "Old family pictures we had brought from Mexico that we had for a long time," Castaneda said, "and our birth certificates."
She also grabbed a couple items from her sprawling makeup collection.
"Some of that stuff is limited edition," she explained.
By Sunday night, experts were planning to plug the crevice in the emergency spillway with bags of rocks dropped from helicopters.
They had also doubled discharges down the main spillway to help lower the lake level and reduce the overflow.
Lake Oroville is the keystone of the State Water Project that sends Northern California water hundreds of miles south to the southern San Joaquin Valley and the Southland.
In addition to flooding concerns, if operators can't easily get water out of the lake that could interfere with deliveries to contractors, including the Metropolitan Water District of Southern California.
A series of powerful storms in Northern California sent runoff rushing into the lake just as a gaping hole developed in the main spillway Tuesday, forcing managers to reduce releases.
That pushed the reservoir to overflow Saturday, marking the first time the emergency spillway was used since the dam was finished in 1968. Until Sunday afternoon, it seemed to be going smoothly.
James Nash, 86, heard about the evacuation order from his apartment building manager. A retired chef and Korean War vet, he wasn't sure where to go.
He couldn't get to Chico on his bike. He had a small bag with shaving gear, a washcloth and paper towels. "No blankets. No water," he said.
He remembered the 1997 evacuation order, which he ignored. There was no flooding then and he was pretty sure nothing would happen this time.
"I don't believe it's going to happen," he said.
But he wasn't comfortable enough to go back home, so he kept watching the water in the river below.
On Sunday evening, Christopher Cruz, 21, and his girlfriend, Jessica Isaacson, 18, waited outside of a CVS surrounded by the plastic jugs they had filled with water and the snacks and clothes they'd packed.
Stranded in Oroville, where they rent a room, they had hoped Isaacson's mother would pick them up, but she was stuck in Chico. Finally, they called local law enforcement, who told them to wait outside the pharmacy until someone came for them.
Cruz said that when Lake Oroville started to hit its overflow point a few years earlier, he had thought about packing a bag.
"I just thought everything was going to blow over," he said.
This time, with no car, he and Isaacson had set out walking, only to run into a police officer who told them in no uncertain terms that they were going in a dangerous direction.
"Heading that way is heading for death," he told them.
Times staff writers Anna Phillips and Matt Hamilton contributed to this report.