Nearly 200,000 people remained under evacuation orders Monday in the western U.S. state of California while authorities tried to repair erosion of an emergency spillway at the country's tallest dam to keep it from failing and unleashing uncontrolled flood waters.
The immediate emergency that led to the evacuation appeared to have waned; but, officials were assessing how to fix the overflow channel at the Oroville Dam before deciding whether it was safe to let people return to their homes in rural communities 240 kilometers northeast of San Francisco.
"Now that there is no more water going over the emergency spillway, though it brings stability to the situation, there are still a lot of unknowns," Butte County Sheriff Kory Honea told reporters late Sunday. "We're not at the point yet where we can make decisions about whether or not it is safe to repopulate areas."
The 235-meter dam, which holds back Lake Oroville, was not in danger of collapse. Authorities were releasing 2,830 cubic meters of water per second from the main spillway on Sunday, bringing down the reservoir so that there was no more flow into the emergency channel. Officials are trying to drop the dam's water level by 15 meters ahead of more storms forecast to hit the region on Wednesday.
The emergency is the result of winter storms that brought relief to the most populous U.S. state after four years of drought, but, water levels behind the dam forced officials to use the emergency spillway for the first time in almost 50 years. The evacuation was ordered after engineers spotted a hole in the concrete lip of the emergency spillway.
In addition, there was unexpected erosion in the main spillway, with heavy rains creating a 60-meter-long crevice 9 meters deep, although authorities said that cave-in appears to have ended.
California Governor Jerry Brown said, "It's clear the circumstances are complex and rapidly changing. The state is directing all necessary personnel and resources to deal with this very serious situation."
Evacuees jammed local roads to escape the region Sunday after officials warned them of the emergency spillway's possible imminent collapse, telling them the evacuation order for the valley communities below the dam was "not a drill."
"I'm just shocked," one resident, Greg Levias, said as he left with his wife, two sons and a dog just three weeks after moving to the community.
Lake Oroville plays a role in California's government-run water delivery network, supplying water for vast agricultural fields in the middle part of the Pacific coastal state and water for residents and businesses in the populous southern part.