“Immediate evacuation from the low levels of Oroville and areas downstream is ordered. This is NOT A Drill. This is NOT A Drill. This in NOT A Drill,” says the statement posted on the Butte County Sheriff’s Facebook page.

The statement refers to the Lake Oroville Dam, located 105 km (65 miles) north of Sacramento.

Butte County Sheriff Kory Honea said the evacuation orders affect 188,000 people, AP reports.

The dam’s spillway was “predicted to fail within the next hour,” the California Department of Water Resources (DWR) said at around 4:30pm PST Sunday (00:30 GMT Monday).

“DO NOT TRAVEL NORTH TOWARD OROVILLE,” the Yuba County Office of Emergency Services said on Facebook, urging evacuees to travel safely in all other directions and help the elderly.

Thousands of people from Yuba County and Sutter County have been filmed being evacuated from local towns and communities, with vehicles creating long traffic jams.

DWR issued a statement, saying that there is no danger that the dam itself will collapse, as the incident had affected only an emergency spillway.

“Oroville Dam itself is sound and is a separate structure from the auxiliary spillway,” the statement read.

The emergency is the first time in the 48-year-old dam’s history that an uncontrolled spillway is being used, local NBC affiliate KCRA-TV reported.

Heavy rains and snow that struck California this winter have resulted in the dam’s reservoir being filled up to the point of overspill. On Thursday, the works to release water from the dam began after chunks of concrete from the spillway were discovered in the channel below.

On Friday, only 7 feet (2 meters) were left until the dam was to reach its full capacity.

The DWR has been using the emergency spillway instead of the damaged one to release the water.

Meanwhile, a series of statements from the DWR included a seemingly bizarre plan to fill in the widening gorge in the spillway by dropping rocks from helicopters.

‘Imminent danger’

“Around 3pm we were advised of a hole that was developing near the lower edge of the alternative slash emergency spillway, which has been used to reduce the level of the lake given that the main spillway was damaged a couple days ago,” Sheriff Kory Honea told a press conference, adding that he was advised of the “imminent danger” that it would collapse.

Explaining his decision to promptly evacuate the area, the sheriff said the authorities had “significant concern that [the erosion] would compromise the integrity of the spillway.”

“We had to make a very critical and difficult decision to initiate evacuation,” he said, adding that he “could not risk the lives of thousands of people.”

He added, however, that while the threat to the area remains significant, there is a chance to "avert the crisis" and efforts to do so are currently under way.

“About 10 minutes ago, the DWR told me that the erosion that caused all this concern was not advancing as rapidly as they thought,” the sheriff said.

He announced that helicopters will be engaged in the operation to drop large rocks to “plug the hole.”

Acting Water Resources director Bill Croyle also confirmed to reporters at around 10pm local time that the immediate danger had passed, with water no longer flowing over the eroded spillway. 

He cautioned, however, that the situation remained unpredictable.

Officials are set to re-evaluate the evacuation orders at dawn, Reuters reported. 

Measuring 770 feet (230 meters) high, the Oroville dam is the tallest dam in the United States. It exceeds the Hoover Dam by more than 40 feet (12 meters).