"This is NOT a drill." The Butte County Sheriff's Office repeated the warning three times, telling residents near California's Oroville Dam to evacuate before potential disaster strikes.
The dam's emergency spillway could be on the verge of failing due to erosion, the sheriff's office said in a statement. The structure was expected to give way by 6 p.m. local time (9 p.m. ET).
A spillway controls the flow of water from the lake or river being dammed to the area downstream. It also ensures, during times of high water levels, that the water does not rush over the top of the dam or damage the dam.
"Failure of the auxiliary spillway structure will result in an uncontrolled release of flood waters from Lake Oroville," a statement on Facebook said. "This is an evacuation order. Immediate evacuation from the low levels of Oroville and areas downstream is ordered."
At 770 feet, Oroville Dam is the country's tallest. In addition to flood control, it also serves to provide drinking water and generate hydroelectric power.
In an effort to mitigate the situation, the California Department of Water Resources is increasing water releases from the main spillway to 100,000 cubic feet per second in an effort to lessen the amount of water traveling down the emergency spillway, the sheriff's office said.
The DWR instructed Oroville residents to head north, toward Chico.
The evacuation order also includes "all Yuba County on the valley floor" and the city of Marysville, authorities said.
"Take only routes to the east, south, or west. DO NOT TRAVEL NORTH TOWARD OROVILLE!!!!!" the Yuba County Office of Emergency Services warned residents.
Evacuation shelters have been set up at fairgrounds in Chico and Colusa, CNN affiliate KCRA reported. A California Office of Emergency Services spokesperson told the station that it could be "potentially catastrophic" if the emergency spillway fails.
Marysville is about 40 miles north of Sacramento, Oroville about 70 miles.
The Sacramento Fire Department warned residents that the spillway failure could have effects, including flash flooding, downstream in Sacramento.
By Eliott C. McLaughlin