Evacuation orders for nearly 200,000 people living below the tallest dam in the United States remained in place after residents were abruptly told to flee when a spillway appeared in danger of collapse.
Authorities issued the evacuation order on Sunday, saying that a crumbling emergency spillway on Lake Oroville Dam in north California could give way and unleash floodwaters onto rural communities along the Feather River.
"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.
The California Department of Water Resources said on Twitter at about 1030 AEDT Monday that the spillway next to the dam was "predicted to fail within the next hour."
Several hours later the situation appeared less dire, as the damaged spillway remained standing.
By 1700 AEDT, state and local officials said the immediate danger had passed with water no longer flowing over the eroded spillway. But they cautioned that the situation remained unpredictable.
"Once you have damage to a structure like that it's catastrophic," acting Water Resources director Bill Croyle told reporters. But he stressed "the integrity of the dam is not impacted" by the damaged spillway.
Butte County Sheriff Korey Honea told an earlier news briefing he was told by experts that the hole forming in the spillway could compromise the structure. Rather than risk thousands of lives, the decision was made to order evacuations.
Officials said they feared the damaged spillway could unleash a nine-metre wall of water on Oroville, north of the state capital Sacramento.
They said evacuation orders remained in place for some 188,000 people in Oroville, Yuba County, Butte County, Marysville and nearby communities and would be re-evaluated at dawn.
Evacuation centres were set up at a fairgrounds in Chico, California, northwest of Oroville, but major highways leading south out of the area were jammed as residents fled the flood zone and hotels quickly filled up.
The Oroville dam is nearly full following winter storms that brought relief to the state after four years of drought.
At 230m high, the structure, built between 1962 and 1968, is the tallest US dam, exceeding the Hoover Dam by more than 12m.