At least 130,000 people have been told to leave their homes amid fears the tallest dam in the US is about to break.
The Oroville Dam, in northern California, has been weakened by heavy rainfall and snow following years of drought.
Officials said the 770 ft-tall dam could collapse at any moment, ordering everyone living in flood risk areas to leave immediately.
Workers first noticed damage to the dam’s spillway at around 3pm (11pm GMT) on Sunday.
The spillway, which controls the flow of water from the dammed area downstream, had a hole in it and concrete chunks were falling away.
Residents of Oroville, a town of 16,000 people, were told to head north, as well as other towns and cities in the vicinity.
It is the first time an emergency state has been declared at Lake Oroville since the dam was built almost 50 years ago.
Butte County sheriff Kony Honea posted on social media: “Immediate evacuation from the low levels of Oroville and areas downstream is ordered.
“This is NOT A Drill. This is NOT A Drill. This is NOT A Drill.”
However, he later said the damage may not be as bad as initially thought, with the erosion “not advancing as rapidly” as feared.
The evacuation orders still remained in effect into Sunday night though, according to CNN.
"I would rather be safe than sorry," said Sheriff Kony Honea.
"I would rather have people move out of the area hopefully to safety."
The California Department of Water Resources said it was releasing 100,000 cubic feet (2,830 cubic metres) of water per second from the main spillway in an attempt to drain the lake.
