Officials have ordered over 162,000 residents near the Oroville dam in Northern California to evacuate the area after an emergency spillway severely eroded, the media reported.
According to the Butte county Sheriff's Office, the main spillway of the dam, the nation's tallest, was damaged in storms this week, NBC News reported.
"A hazardous situation is developing with the Oroville Dam auxiliary spillway. The operation of the auxiliary spillway has led to severe erosion that could lead to a failure of the structure," the sheriff's office posted on its Facebook page on Sunday night, emphasising that it was not a drill.
"Failure of the auxiliary spillway structure will result in an uncontrolled release of flood waters from Lake Oroville."
"In response to this developing situation, DWR (Department of Water Resources) is increasing water releases to 100,000 cubic feet per second. Immediate evacuation from the low levels of Oroville and areas downstream is ordered," the post added.
Thousands of people evacuated below the Oroville Dam, jamming roads, the Los Angeles Times reported.
TV news footage showed long delays out of Oroville, with officials urging people to move to higher ground. Gas stations were also packed.
Butte county officials said they evacuated jail inmates due to the situation.
After discovering the damage, officials said that they will attempt to plug it using sandbags and rocks. But they stressed the situation remains dangerous.
If the dam collapses, water would get into Oroville within an hour.
Shortly thereafter, the nearby towns of Briggs, Gridley and Live Oak would be affected. It would take eight to 12 hours for water to reach the cities of Marysville and Yuba city.