Part of the tallest dam in the US is on the verge of collapsing -- and California was completely unprepared

Here’s what Oroville Dam, the tallest dam in the United States, looked like in August 2015, during the height of California’s raging five-year drought:

Here’s what the same dam looks like on Monday, February 13:

Oroville damCalifornia Department of Water ResourcesAn aerial photo taken Saturday by the California Department of Water Resources shows the damaged spillway with eroded hillside in Oroville, California.

Not surprisingly, the state was somewhat unprepared for the dam’s water levels to reach capacity, which they did — rapidly — on February 9.

After the dam’s water levels steadily rose from Thursday through the weekend, authorities on Sunday evening ordered the evacuation of nearly 200,000 residents in several northern California towns.

“Immediate evacuation from the low levels of Oroville and areas downstream is ordered,” the Butte County sheriff’s office posted on Facebook.

Officials have not released information on what exactly caused the dam’s structural problems, including a gaping hole in the middle of the spillway. But issues in the area may have been brewing since as early as least March, when the Sacramento Bee reported that both of California’s two largest reservoirs — which had been decimated for years by the drought — bumped up to historically average levels after a weekend of heavy storms.

It was, at first, good news.

Experts said California’s five-year drought was the worst the state had seen in 1,200 years. Dwindling reservoirs, shrinking lakes, and dried-up farm fields dominated the landscape. The state’s snowpack reserves, which typically supply California’s farmers and residents with roughly a third of their water, lingered at their lowest level in history in 2015.

The state responded slowly, with some urban conservation mandates requiring residents to use less water while supplies were so low.

In March of last year, Felicia Marcus, chair of the State Water Resources Control Board, told the Sacramento Bee that those mandates would finally be loosened in the coming months as water levels looked to be returning to normal — the first time they’d done so since 2013.

“In May, we’ll be either lifting it or changing it significantly,” she said in March. “The more precipitation we get, the more snowpack we have, the better it is.”

That month, the combined supply at all of California’s biggest reservoirs stood around 78% of their average levels, but officials remained cautiously optimistic, saying there was no guarantee that the state would see enough continuous storms to raise water levels completely.  

On Tuesday, February 7, engineers discovered a 250-foot-long pothole in the Oroville Dam’s main spillway, which forced them to bring all water releases there to a halt. The spillway is traditionally used to drain some of the excess water into surrounding areas, so with it closed, the reservoir began to fill. In less than a day, it took on some 150,000 acre-feet of water, the Sacramento Bee reported, filling to roughly 85% capacity.

At the time, the California Department of Water Resources reported that the dam itself was safe and did not pose a threat to downstream populations. “These are things we can repair,” Eric See, the agency’s environmental program manager, told the Sacramento Bee on Tuesday. 

But the water kept coming. 

Two days later, after the gaping hole in the spillway had continued to grow, the spillway gave in completely, and a complete break could be seen all the way across the structure:

As of Thursday afternoon, the dam continued to release thousands of turbo-charged cubic feet of water each second, including from the broken spillway. It wasn’t enough to compensate for all the excess water brought in by storms, however, and heavy rainfall on Saturday caused the dam to overflow for the first time in its 49-year history, according to The Associated Press.

Authorities ordered the evacuation of nearly 200,000 residents in several northern California towns Sunday night.

“Immediate evacuation from the low levels of Oroville and areas downstream is ordered,” the Butte County Sheriff’s Office posted on Facebook. 

“Operation of the auxiliary spillway has lead to severe erosion that could lead to a failure of the structure. Failure of the auxiliary spillway structure will result in an uncontrolled release of flood waters from Lake Oroville.”

The evacuation warning was “NOT a drill,” the sheriff’s office added.

Anticipating the failure of the dam’s auxiliary spillway, officials in the northern California town frantically attempted to drain water from the main spillway, at a clip of 100,000 cubic feet per second, according to the Sacramento Bee. Helicopters dropped sand and rocks into a sinkhole in the spillway to stem the erosion.

“It’s uncontrolled. It’s uncontrolled,” Department of Water Resources spokesman Chris Orrock said, when asked how much water could be released should the spillway fail.

Later Sunday evening, officials said the threat of collapse due to erosion had diminished, according to the Sacramento Bee. Water levels fell to a point at which little or no water flowed out of the emergency spillway, which Orrock said was the main factor in its erosion.

An evacuation center was set up in Chico, a nearby town north of Oroville, the sheriff’s office said on Twitter. Traffic heading out of Oroville appeared to slow to a crawl as thousands of residents attempted to flee. Residents of seven towns in neighbouring Yuba and Sutter counties were also instructed to evacuate.

The number of residents ordered to evacuate totaled 188,000, according to AFP.

At 770 feet, Oroville Dam is the tallest dam in the United States. It could take up to $200 million to repair the damaged spillway, according to the Los Angeles Times.

This story is developing.

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