Some 188,000 people were told to evacuate over concerns that America's tallest dam could burst.
Residents living in cities near California's Lake Oroville Dam were told to leave their homes on Sunday afternoon as concerned officials warned that the emergency spillway could collapse imminently.
"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 is NOT A Drill."
Water levels were less than 7ft (2 metres) from the top of the dam on Friday.
Engineers began carefully releasing water from the dam some 65 miles (105km) north of Sacramento after noticing that large chunks of concrete were missing from the spillway.
The California Department of Water Resources said on Twitter that the spillway next to the dam was "predicted to fail within the next hour," but hours later the situation appeared less dire as the spillway remained standing.
It said it is using helicopters to drop rocks to help fill in the gouge and help the flow stabilise.
The cities of Oroville, Gridley, Live Oak, Marysville, Wheat land, Yuba City, Plumas Lake, and Olivehurst are all under evacuation orders.
Evacuation centres were set up 20 miles away at fairgrounds in Chico, California, but major highways leading south out of the area were congested as residents fled the evacuation zone.
Oroville itself was largely deserted after nightfall.
Butte County Sheriff Korey Honea told a press conference that experts said that the hole that was being created in the spillway could compromise the structure.
Rather than risk thousands of lives, the sheriff said, a decision was made to order the evacuations, he said.
The sheriff said the evacuation order will stand until there is more information on the dam's emergency spillway.
State Fire and Rescue Chief Kim Zagaris says at least 250 law enforcement officers from throughout the state are in the area or en route to help keep things safe for the people who have been evacuated.
A wave of winter storms that swept in to the area after four years of severe drought has filled the dam with water.
At 770 feet (230 metres) high, the structure, built between 1962 and 1968, is the tallest dam in the US, beating the famed Hoover Dam by more than 40 feet (12 metres).
It is the first time the area has experienced an emergency evacuation in the dam's 50-year history.