Residents below the tallest dam in the United States near Oroville in Northern California were ordered to evacuate immediately on Sunday after authorities said an auxiliary spillway was in danger of imminent collapse.

“Immediate evacuation from the low levels of Oroville and areas downstream is ordered,” the Butte County Sheriff said in a statement posted on social media. “This in NOT A Drill. This in NOT A Drill. This in NOT A Drill.”

The California Department of Water Resources said on Twitter said at about 4:30 p.m. PST that the spillway was “predicted to fail within the next hour.”

State authorities and engineers on Thursday began carefully releasing water from the Lake Oroville Dam some 65 miles (105 km) north of Sacramento after noticing that large chunks of concrete were missing from a spillway.

The earthfill dam is just upstream and to the east of Oroville, a city of more than 16,260 people.

At 770 feet (230 meters) high, the structure, built between 1962 and 1968, is the tallest dam in the United States, besting the famed Hoover Dam by more than 40 feet (12 meters).

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