The emergency spillway of the Oroville Dam in northern California is severely eroded and people living downstream should leave immediately, officials have warned.
Around 16,000 people who live in the town of Oroville and should head north towards Chico.
Emergency evacuations have also been ordered for Yuba City and other towns and villages along the Feather River in Sutter County.
Oroville dam is expected to fail within the hour
Water began flowing into the emergency spillway at the dam on Saturday after heavy rainfall blasted the main spillway.
Bits of concrete were sent flying out of the main spillway which created a 200-foot long, 30-foot deep hole which is only growing.
Thousands of lives could be at stake
The Butte sheriff confirmed that helicopters will drop rock-filled containers in a desperate attempt to plug potential gaps in the emergency spillway.
He added that the dam is so far holding up better than first expected and that emergency work was underway to ease pressure on the structure.
The dam is holding up better than expected
A flash flood warning has been given through until 12.15am GMT (4.15pm local time).
Heavy rainfall on Saturday, along with unexpected erosion which caused a massive sinkhole in the dam’s main spillway have led to the structure overflowing for the first time in its 49-year history.
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Thousands of residents leaving the town has seen traffic reduced to a slow crawl.
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Officials are trying to drain water out of the main spillway at a speed of 100,000 cubic feet per second.
The LA Times estimates it could cost up to £160 million ($200million) to repair.
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