Immediate Evacuations Ordered Below Damaged Dam

Nearly 190,000 people in Northern California were ordered to evacuate after a spillway serving the country's tallest dam developed a hole that threatened to release uncontrolled floodwaters.

California Department of Water Resources

The emergency slipway off the Oroville Dam was the second to fail in a matter of days, after the dam's primary slipway developed a 200-foot-long, 30-foot-deep hole last week.

The dam itself was not damaged. But because its water levels are so high following heavy rain, if the emergency slipway collapses it could unleash a wall of water onto communities and rivers below.

Although no damage to the dam itself has been reported, government officials are dropping water levels in the dam at a rate of 100,000 cubic feet (2,831 cubic metres) per second as a precaution, and if the spillway does collapse, repairs could take as long as 90 days, and cost upwards of US$200 million.

The latest updates from the California State Department of Water Resources say that while the erosion of the Oroville Dam emergency spillway is not advancing as rapidly as feared a couple of hours ago, evacuations of several counties in the area are still being carried out due to imminent flood risks.

Government officials are now racing against the clock to plug a massive crack in the emergency runoff structure using sandbags and rocks dropped from helicopters.

California Department of Water Resources

Several state water and government officials told NBC News that one million acre-feet of water could be released, overwhelming the Feather River and flooding communities in Butte County, Yuba City and Marysville. Cal Fire Deputy Chief Kevin Lawson said at a press conference Sunday night that 188,000 people had been ordered to evacuate from those areas.

It could also breach the network of levees along the way and cause problems as far away as Sacramento.

"We've never seen anything like this in modern times," a state water official told NBC News. "This is a worst case scenario for any water management agency, a worst case nightmare."

Sources: NBC News and Science Alert