California officials are frantically trying to rapidly reduce water levels behind Oroville Dam — the nation's tallest — after issuing evacuation orders for more than 100,000 people who live downstream.
Here's an explainer for the current crisis, and how authorities are racing to prevent catastrophic flooding from affecting the Central Valley.
What’s the biggest concern?
The biggest concern was that a concrete wall, built atop a hillside, that keeps water in Lake Oroville — California's second largest reservoir — would suddenly crumble Sunday afternoon, threatening the lives of thousands of people by flooding communities downstream.
With Lake Oroville filled to the brim, such a collapse could have caused a "30 foot wall of water coming out of the lake," Cal-Fire incident commander Kevin Lawson said at a Sunday night press conference.
What’s so special about this concrete wall ?
This particular concrete wall was chosen to act as an emergency spillway for Lake Oroville — a pathway for excess water to drain when the reservoir is filled to the brim.
If Lake Oroville became too full, and a main concrete spillway couldn't drain the reservoir fast enough, water would then empty out of this emergency spillway, flowing over this low concrete wall — known as a weir — washing past a roadway and down a tree-lined hillside.
The emergency spillway was designed so that water would always drain at this location when the dam was full. That would keep Oroville Dam, built at a different location, safe from being overtopped by water.
So on Saturday, rising reservoir water put the emergency spillway to use for the first time since Oroville Dam was completed in 1968.
When did officials realize something was terribly wrong?
Officials had thought things were going fine until suddenly, on Sunday afternoon, authorities were alerted to what they described as erosion developing near the emergency spillway.
The erosion was occurring so fast that officials feared the concrete wall would be undermined, and that it would cause a catastrophic release of water downstream. Officials said they couldn't wait to act if the worst-case scenario struck, and ordered sweeping evacuations.
What has happened since the evacuations were ordered?
The situation has improved. There has been no catastrophic collapse of the concrete wall on top of the emergency spillway, which would have resulted in an overwhelming release of water downstream.
Officials decided to focus their efforts on draining the lake to funneling more water down the main spillway, a concrete slide that is supposed to be the primary way of draining a full Lake Oroville. The main spillway itself was hobbled last week, as parts of it began disintegrating as a giant pothole formed underneath a section.
Authorities knew they had to take pressure off the emergency spillway, so they began sending more water down the main spillway late Sunday. They increased the flow from 55,000 cubic feet per second to 100,000 cubic feet per second, and hoped for the best.
They were in luck on Sunday night. Sunday's increased flows did not appear to damage the main spillway further. And before midnight, the main spillway had drained the lake enough so that water was no longer flowing down the emergency spillway.
The dry weather early this week helped. As of Sunday night, only 40,000 cubic feet of water per second was flowing into Lake Oroville, and because 100,000 of cubic feet of water per second is flowing out, lake levels are being reduced.
So what’s the plan Monday morning?
Officials need to assess the emergency spillway this morning. Authorities haven't been able to begin to make a fix.
And they're hoping to drain as much water as they can before a new round of storms is forecast to arrive in Northern California later this week.
If officials believe they can send an even faster torrent of water down the main spillway, they will, said Bill Croyle, the acting director of the California Department of Water Resources, at a late night news conference.
What level does the water in Lake Oroville need to get down to for officials to start feeling better?
At the height of the crisis on Sunday, the lake was topped out at 902 feet. Officials would like the lake to fall by an elevation of 50 feet.
Why is it a bad sign for officials to even need to use the emergency spillway?
It's a bad sign because officials don't have any ability to control how much water goes into the emergency spillway, as can be done with the main spillway.
Authorities want to keep a speed limit on how fast water flows from Lake Oroville down the Feather River, which runs past Oroville, Marysville and Yuba City before merging with the Sacramento River and heading to California's capital.
California's flood control system along Oroville can handle only a speed of 150,000 cubic feet of water per second. If water flows faster than that, catastrophic flooding can happen.
"We don't like to press it to the edge if we don't have to," Croyle said.
Have officials had to push the Feather River to its limit before?
Yes, during the historic 1997 rains. There was flooding in and around the valley, "but we did pass water through this region without too much trouble," Croyle said.
What happened in 1997?
Several levee breaks were reported in the Sacramento and San Joaquin valleys. But it was nothing compared with the worst-case scenarios envisioned Sunday.
Is there any way this situation could have been avoided?
That's a question many people will be wondering about for some time. 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.
What were they reacting to?
The first situation officials reacted to was news of the damage in the main concrete spillway Tuesday. Officials stopped water draining out of the lake to inspect the damage, and studied it to see if they could fix it.
But "we determined we could not fix the hole," Croyle said. It was 250 feet long, 170 feet wide, and almost 40 to 50 feet deep. There wasn't enough time to keep the chute dry and fix it. Officials thought they had no choice other than to use the main spillway even in its crippled state, even though it would be further damaged by resuming its use.
The result is a balancing act — drain as much water as quickly as possible while trying not to further damage the spillway as much as can be helped. After all, the crippled main spillway needs to last for the remainder of the rainy season.
By Friday, officials had held out hope that they wouldn't need to use the 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.
It was only after the emergency spillway was revealed to be in dire condition that officials roughly doubled the amount of water flowing out of the main spillway. By that point, luckily, the deterioration of the main spillway had largely stabilized, although there is still cause for concern.
How much time do officials have before the next rainstorm arrives?
The National Weather Service forecasts Monday and Tuesday will be dry and mild, allowing river levels to recede.
Light to moderate rain is expected Wednesday into Thursday, which is forecast to cause limited rising in rivers.
The situation gets worse by the weekend, however, as new storms arrive. The river could begin rising again as early as Friday, and area-wide flooding could happen between Sunday and Tuesday.