Almost 200,000 people were under evacuation orders in northern California Monday after a threat of catastrophic failure at the United States’ tallest dam.
Officials said the threat had subsided for the moment as water levels at the Oroville Dam, 120 kilometers north of San Francisco, have eased. But people were still being told to stay out of the area.
Several weeks of heavy rains had filled the 235-meter high dam to capacity.
The threat comes not from the dam itself, which the California Department of Water Resources said was not in danger of collapse, but from an emergency spillway that channels off excess water.
A giant hole opened up in the dam’s main spillway last week, forcing authorities to activate the emergency spillway on Saturday for the first time ever.
But it began eroding, threatening a rupture that would have sent water surging towards cities in the valley below, US media reported.
Authorities then released 2,830 cubic meters of water per second from the main spillway, bringing down the level of reservoir Sunday, the Sacramento Bee newspaper said, citing Department of Water Resources spokesman Doug Carlson.
Although the most pressing threat had passed, the evacuation order remained in place as authorities evaluated the state of the two spillways.
Helicopters readied overnight to drop rocks into eroded areas in the emergency spillway, according to local media, ahead of rain that is forecast for Wednesday and Thursday that could fill the reservoir again.
The California National Guard said Monday morning on Facebook that it had alerted its 23,000 members to be ready to deploy.
“Now that there is no more water going over the emergency spillway, though it brings stability to the situation, there are still a lot of unknowns,” Butte County Sheriff Kory Honea told a news conference Sunday night.
“We’re not at the point yet where we can make decisions about whether or not it is safe to repopulate areas. “
About 188,000 people in downstream communities had been ordered to flee on Sunday afternoon as water was still gushing over the top of the wide auxiliary spillway.
In a Facebook post ordering the “immediate evacuation” of low-lying areas of the city of Oroville as well as downstream communities, the Butte County Sheriff’s department warned that a “hazardous situation is developing with the Oroville Dam auxiliary spillway. “
“Failure of the auxiliary spillway structure will result in an uncontrolled release of flood waters from Lake Oroville,” it said, adding three times that this was “NOT A Drill. “
To the south, Yuba County also ordered residents to move to safety.
“Yes, an evacuation has been ordered,” the Yuba County Office of Emergency Services said in a Facebook post.
“All Yuba County on the valley floor. The auxiliary spillway is close to failing… Take only routes to the east, south, or west. DO NOT TRAVEL NORTH TOWARD OROVILLE!!!!! “
Water from the dam flows down the Feather River that runs through Oroville, a city of about 20,000 people.
Governor Jerry Brown issued an emergency order to deploy resources to the affected area.
“I’ve been in close contact with emergency personnel managing the situation in Oroville throughout the weekend. It’s clear the circumstances are complex and rapidly changing,” he said in a statement.
“The state is directing all necessary personnel and resources to deal with this very serious situation. “
The Oroville Dam has been in use since 1968. Less famous than America’s iconic Hoover Dam near Las Vegas, Oroville is still the tallest.
