The mass evacuations underway below the Oroville Dam capped a week of frantic efforts to prevent flooding as America’s tallest dam reached capacity and its main spillway was severely damaged.
Here is a brief explanation of the events so far.
What happened Sunday?
On Saturday, water levels reached so high that an emergency spillway was used for the first time. Officials initially believed the measure worked. But on Sunday afternoon, as more water from record storms flowed into Lake Oroville, officials detected a hole in the emergency spillway. That prompted the evacuation order.
Officials are trying to reduce water levels at the dam and repair the emergency spillway.
Why are the flood fears so high?
Officials worry that a failure of the emergency spillway could cause huge amounts of water to flow into the Feather River, which runs through downtown Oroville and other waterways. The result could be flooding and levee failures for miles south of the dam, depending on how much water is released.
Officials have said they don’t know how much water would run into the Feather River. But a huge release could flood many communities. River levels currently are still below flood stage.
What is the state of the dam?
Officials say the dam is structurally safe.
How did the crisis start?
Bloated with storm runoff, Lake Oroville had gone from 80% full to overflowing in less than a week when managers were forced to reduce releases on Oroville’s heavily damaged concrete spillway.
The emergency spillway was doing what it was supposed to do: let water out of the huge lake so it wouldn’t top the dam.
Could more storms put more pressure on dams?
With northern Sierra precipitation levels tracking ahead of the wettest year on record so far this winter, Oroville managers have a nerve-racking several months ahead of them. “There’s a lot of snow up there,” said Bill Croyle, acting director of the Department of Water Resources.
The snowpack in the northern Sierra, which includes the Feather River watershed, is 150% of normal for the date. Statewide it is 180% of average.
Reservoirs around the state are making flood-control releases.
In Mariposa County, the small reservoir behind Mariposa Dam was overflowing, sending water down its spillway for the first time since the 1950s, said Merced County spokesman Mike North. About 25 homes were flooded Saturday by the swollen Mariposa Creek.
This winter’s turnaround from five parched years has been dramatic. A year ago at this time, Oroville was slightly less than half full. In 2014, it was a little more than a third full and its receding shoreline served as a vivid symbol of the drought’s punishing toll on the state.
How much will it cost to fix the damaged main spillway?
Officials estimated that it will cost $100 million to $200 million to fix the spillway, adding that could not be done until the end of rain season this spring.