Thousands of people are being evacuated from their homes in California amid warnings part of the tallest dam in the United States is in danger of collapse.
Residents below the dam near Oroville in Butte County, California, have been ordered to evacuate after authorities said an auxiliary spillway is 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."
Yuba City, with a population of 65,500, has received evacuation orders, as have all downstream areas in Yuba County.
Evacuation centres have been set up at the Silver Dollar Fairgrounds in nearby Chico.
State authorities and engineers have begun carefully releasing water from the Lake Oroville Dam some 105km 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 230 metres high, the structure, built between 1962 and 1968, is the tallest dam in the US, besting the famed Hoover Dam by more than 12 metres.
Butte County Sheriff Kory Honea said the flow of Oroville Dam's "auxiliary spillway" has decreased, and erosion hasn't progressed as quickly as first feared. He said that a plan is currently being developed to fill the hole and repair the spillway.
The hole had grown to about 15 metres deep, 91 metres wide and 150 metres long.
Officials are racing to relieve water pressure on the dam, which is so far continuing to hold.
California's governor has activated an Emergency Operations Center in response to the emerging crisis.
The local sheriff's office has told media that helicopters are being prepared to drop loads of rocks at critical points along the spillway in an effort to strengthen the structure.
Concerns were first raised yesterday when water started flowing over an emergency spillway at Lake Oroville for the first time after erosion damaged the Northern California dam's main spillway.
Officials hoped to avoid using Oroville Dam's emergency spillway, fearing it could cause trees to fall and leave debris cascading into water that rushes through the Feather River, into the Sacramento River and on to the San Francisco Bay.
It was the first time the emergency spillway has been used in the reservoir's nearly 50-year history.
Agency spokesman Eric See said at a news conference yesterday: "This is a very unusual event for us here in Oroville."
Unexpected erosion chewed through the main spillway last week, sending chunks of concrete flying and creating a 60m long, 10m deep hole that continues growing.
Engineers don't know what caused the cave-in that is expected to keep getting bigger until it reaches bedrock.
Bill Croyle, the Department of Water Resources' acting director, said yesterday officials were continuously monitoring the erosion both on site and through cameras.
"This is mother nature kind of kicking us a few times here," he said. Croyle said the main spillway will need a "complete replacement" from the damage. Officials noted earlier this week that the cost of repairing the dam could approach $US100 million, but they noted the estimate was an early, ballpark figure.
About 240km northeast of San Francisco, Lake Oroville is one of California's largest man-made lakes, and the 235-meters-tall Oroville Dam is the nation's tallest. The lake is a central piece of California's government-run water delivery network, supplying water for agriculture in the Central Valley and residents and businesses in Southern California.