State water officials continued to lower the level of Lake Oroville on Tuesday in anticipation of a series of storms that were forecast to begin arriving with new rain late Wednesday.
By 7 a.m. Tuesday, Department of Water Resources crews had lowered the reservoir level by at least 11 feet from the point at which it threatened potential disaster this weekend.
The water level at Oroville peaked on Saturday at about 902 feet, which sent water cascading over a concrete weir and into an unlined emergency spillway that had never been used.
On Sunday however, the earthen emergency spillway began to show signs of heavy erosion. Fearing that the damage could undermine the concrete weir that formed the lake's shore and cause it to fail, and send a wall of water coursing into the Feather River below, officials ordered sweeping evacuations for more than 100,000 people.
Now, according to DWR data, Lake Oroville is at 889 feet and dropping about a foot every three hours, as engineers release water from the lake at about 100,000 cubic feet per second. That water is being released down the reservoir's main spillway, which has also suffered major damage due to erosion. The main spillway was designed for a peak flow water rate of 150,000 cubic feet per second, but a massive break in the lower portion of the concrete chute has complicated matters.
The goal, authorities said after the immediate crisis was averted, is to lower the lake enough to handle an inflow of water from a series of storms expected to move in Wednesday night.
In a break for residents and water officials, the incoming storm system should be far weaker than the one that inundated Northern California last week.
In the five days from Feb. 6 to Friday, Oroville received more than 6 inches of rain, Rowe said. The surrounding mountains and foothills received up to 24 inches of rain and snow in the same time period, he said.
The storm's runoff sent water into the Oroville reservoir at an average rate of 115,260 cubic feet per second, data show. The lake's water level climbed 50 feet in five days.
The reservoir has handled heavier inflows before, according to the Water Resources Department. It normally compensates by increasing flow from the dam's main spillway. But when the gaping hole opened in the spillway last week, the agency slowed the release of Oroville's water to a trickle, boosting storage in the lake and eventually sending water over the weir and into the emergency spillway. To reduce the risk of further erosion of the emergency spillway, the flow in the main spillway was increased, despite its damaged state, to bring the lake level back down.
The storms that are due to begin arriving on Wednesday are far weaker, said meteorologist David Rowe of the National Weather Service's Sacramento office. Only about 2 inches of rain should fall on Lake Oroville between late Wednesday and Feb. 21 and only about 8 inches of rain and snow are expected to drop in the surrounding watershed, he said.
"It's still a lot but quite a bit less than last week," Rowe said.
Oroville is California's second largest reservoir and is impounded by the nation's tallest dam.
For breaking California news, follow @JosephSerna on Twitter.