Nearly 200,000 people living below the tallest dam in the United States, near Oroville in Northern California, were urgently ordered to flee their homes after a spillway appeared to be in danger of imminent collapse.
Authorities issued the abrupt evacuation orders in the mid-afternoon, saying that a crumbling emergency spillway on the Lake Oroville Dam could give way and unleash raging floodwaters onto a string of 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. "This is NOT A Drill."
The Oroville dam is nearly full after weeks of heavy rains and snow brought relief to the state after some four years of devastating drought.
The California Department of Water Resources said on Twitter at about 1.30pm (NZT) that the spillway next to the dam was "predicted to fail within the next hour."
However it was still standing hours later as the Water Resources department said crews would use helicopters to drop rocks to fill a gouge in the spillway. Authorities were also releasing water to lower the lake's level.
"DO NOT TRAVEL NORTH TOWARD OROVILLE," the department said on Twitter.
State authorities and engineers on Thursday began carefully releasing water from the Lake Oroville Dam some 105 km north of Sacramento after noticing that large chunks of concrete were missing from a spillway.
Water levels were less than two metres from the top of the dam on Friday as levels in the reservoir rose because of a California winter of heavy rain and snow that followed several years of severe drought in the state.
California Governor Jerry Brown asked the Federal Emergency Management Agency on Friday to declare a major disaster due to flooding and mudslides brought on by the storms.
The earthfill dam was just upstream and to the east of Oroville, a city of more than 16,260 people.
At 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 12 metres.