Oroville Dam: Residents Advised to Remain Vigilant as Storm Advances

Rain is expected Thursday in the Northern California area around Lake Oroville, as crews work to reduce the water level there.

The heavy flows from the 3,000-foot main Oroville dam spillway continues to push debris into the turbid Feather River as the concrete span further eroded and the jagged fracture in its midsection sends water flowing over an adjacent hillside. (Credit: Florence Low / California Department of Water Resources)

The heavy flows from the 3,000-foot main Oroville dam spillway continues to push debris into the turbid Feather River as the concrete span further eroded and the jagged fracture in its midsection sends water flowing over an adjacent hillside. (Credit: Florence Low / California Department of Water Resources)

Earlier this week, authorities ordered mandatory evacuations over concerns that an emergency spillway at the Oroville Dam could fail and threaten nearby communities. On Tuesday, officials downgraded the evacuation order to a warning, allowing 188,000 evacuees from Butte, Sutter and Yuba counties to return home.

In a news conference Wednesday, Butte County Sheriff Kory Honea emphasized that even though the mandatory evacuation has been lifted, this remains an emergency situation.

"The risk level was reduced that I felt people could return to the area, however they need to maintain vigilance," Honea said.

A series of storms are heading into the region, but they are smaller than previous ones. These storms are expected to persist through the weekend. A flash flood watch is in effect until Sunday.

The National Guard has 150 members in the area to provide support.

Evacuation order lifted

With the water level at the lake dropping, authorities became more confident that Lake Oroville could handle the upcoming storms.

Heavy rains last week caused the lake level to rise until the water began to pour down the emergency spillway on Saturday. Authorities ordered people to leave the area on Sunday when a hole developed in an eroded area of the spillway, raising concerns that it could fail and flood communities downstream.

While fears of such a disaster have largely been alleviated, crews are continuing repairs by dropping rocks into the gouged hole in the emergency spillway. They're also trying to dump as much water out of the lake.

Lake Oroville was down to a level of 878 feet on Wednesday. Authorities seek to get it down to 850 feet. Excess water starts to flow into the emergency spillway when the lake level rises above 901 feet.

So far, the lake is dropping at about eight inches per hour.

The rainfall over the next week could total 5 to 12 inches and will likely push hundreds of billions of gallons of water back into Lake Oroville, said CNN senior meteorologist Dave Hennen. Bill Croyle, the state's acting director of water resources, said he didn't expect the emergency spillway will be needed this time due to weaker storms.

"We are still removing more water from the reservoir than what we would be receiving from the storm event in the next day or so," Croyle said. "That allows us to continue to prepare for future wet weather and runoff."

He added that the spillway is stable and they are making "great progress."

Two damaged spillways

The Oroville Dam, the tallest dam in the United States, provides flood control in the area. It was constructed in 1960. While the dam itself has no structural issues, the two spillways that prevent water from overflowing have been damaged.

The main spillway, has a giant crater almost the size of a football field and at least 40 feet deep. It's being used to drain the lake at a rate of 100,000 cubic feet per second in an effort to reduce the water level. Normal flows down the main spillway are about 55,000 cubic feet per second.

The emergency spillway, which is an embankment, is a last resort and was used for the first time in its 48-year history on Saturday.