The rain has started to return in Northern California and will continue over the next few days, but officials aren't as concerned about the upcoming weather so much as the damage already done to the Oroville Dam's already compromised main spillway.
The risk of flooding has dropped substantially, but Butte County Sheriff Kory Honea warned residents Wednesday that they remain in "an emergency situation."
- Engineers are racing to lower the water level at Lake Oroville.
- These graphics explain what is happening at the Oroville Dam.
- Could the crisis have been prevented?
- Here is Butte County's emergency information website.
- PHOTOS: Crisis at the Oroville Dam
- VIDEOS: The Lake Oroville emergency explained | An evacuee waits to return home
As work continue to prevent disastrous flooding at Oroville Dam, one big question keeps occurring: How did we get here?
The operators at North America’s tallest dam found themselves in a precarious position, with both spillways used to release water compromised and the reservoir still filled almost to capacity after a winter of record rain and snow. It's a drama that began a week ago and got worse day by day.
Here's how it happened:
Hole in the main spillway
The first situation officials faced was damage in the main concrete spillway last Tuesday. Officials stopped water draining out of the lake to inspect the damage. They then studied the problem to see if they could fix it.
But “we determined we could not fix the hole,” Bill Croyle, the acting director of the state Department of Water Resources, said at a news conference Sunday. It was 250 feet long, 170 feet wide and 40 to 50 feet deep. There wasn’t enough time to halt the flow down the spillway chute dry and then repair the damage. Officials thought they had no choice other than to use the main spillway, even in its crippled state, though it would be further damaged.
A delicate balancing act
The result is a balancing act — drain as much water as possible as quickly as possible while trying to minimize further damage to the spillway. The main spillway was returned to action, but at reduced capacity compared to what it could normally do under the current conditions. After all, the spillway needs to last for the remainder of the rainy season.
More rain than expected
Officials had held out hope that they wouldn’t need to use the dam's emergency spillway, but then it rained Friday night.
“It came in a little wetter. The storm system parked over this region of California was parked a little longer,” Croyle said.
Crisis at emergency spillway
So on Saturday, water started flowing down the emergency spillway for the first time since the dam was completed in 1968. Initially, it seemed the operation was going fine. But on Sunday afternoon, officials detected a hole — earth was eroding away in a path that could dig a canyon or tunnel underneath a concrete retaining wall holding in a 30-foot wall of water in Lake Oroville. If the hole grew, there could suddenly be a path for that water to escape the reservoir uncontrolled, causing the failure of the concrete wall and resulting in a massive flood. More than 100,000 people were evacuated downstream.
Frantic to lower the reservoir level and stop water flowing down the emergency spillway, officials approximately doubled the amount of water flowing out of the main spillway.
Luck when they needed it most
By that point, luckily, the deterioration of the main spillway had largely stabilized, although there is still cause for concern.
Croyle was asked this question at Sunday night’s news conference: Why didn’t officials increase the flow down the damaged main spillway earlier?
Croyle’s answer, essentially, was that officials were reacting to the best information they had at the time.
On Monday, he described the situation at Oroville as unprecedented.
"I'm not sure anything went wrong," he said. "This was a new, never-happened-before event."
More than a decade ago, several environmental groups argued that substantial erosion would occur on the hillside during a significant emergency spill. They asked a federal commission to order the state “to armor or otherwise reconstruct the ungated spillway.”
Gov. Jerry Brown on Monday welcomed calls for more scrutiny about Oroville’s spillway system.