With both spillways badly damaged and a new storm approaching, America's tallest dam on Tuesday became the site of a desperate operation to fortify the massive structures before they face another major test.
A swarm of trucks and helicopters dumped 1,200 tons of material per hour onto the eroded hillside that formed
"This is an aggressive, proactive attack to address the erosion," said Bill Croyle, acting director of the state Department of Water Resources. "There's a lot of people, a lot of equipment, a lot of materials moving around, from the ground and from the air."
At the main spillway, a different and riskier operation was underway: Despite a large hole in the concrete chute, officials have been sending a massive amount of the swollen reservoir's water down the chute to the Feather River in a desperate attempt to reduce the lake's level.
The structure continued to hold Tuesday without sustaining more significant damage, officials said.
The idea is to get the reservoir's water level low enough that it can take in rain from an upcoming series of storms without reaching capacity. If the reservoir filled up again, water would automatically flow down the emergency spillway, which on Sunday appeared to be nearing collapse, forcing the evacuation of more than 100,000 people downstream.
Crews releasing 100,000 cubic feet of water per second through the main spillway have lowered the lake's level by about one foot every three hours. This rate of flow has not caused more damage to the main spillway, engineers said. Meanwhile, hundreds of construction workers used thousands of tons of concrete and rock to shore up the erosion that had carved fissures into the unpaved slope next to the dam.
The reservoir's water line is expected to fall 50 feet by late Saturday or early Sunday, providing a buffer capacity of half a million acre-feet, officials said. That would avert the risk of using the eroded hillside as an emergency spillway again, officials believe.
In a sign of the progress made Tuesday, officials downgraded the evacuation order to a warning, allowing all evacuated residents to return home.
"They have to be vigilant," Butte County Sheriff Kory L. Honea said at a news conference. "They have to pay attention to what's going on. There's the prospect that we can issue another evacuation order if circumstances change."
Workers concentrated their efforts on shoring up the hillside just below the emergency spillway's weir, a low concrete wall that is designed to be the last defense for Lake Oroville.