What happened at Lake Oroville?

What happened at Lake Oroville?

A wet winter brought to light problems at the nation’s tallest dam, which controls water delivered across California. As storms raged in early February, dam operators from the state Department of Water Resources released heavy flows down the Feather River, but a crisis began when a crater opened on the main spillway. Days later, water poured over an emergency spillway – essentially a hillside – that had never been used. The erosion of the hillside prompted the evacuation of nearly 200,000 people downstream.

Where is Lake Oroville?

Lake Oroville is located in the Sierra Nevada foothills about 75 miles north of Sacramento in Butte County. The reservoir is fed by mountain runoff from the forks of the Feather River and is the centerpiece of the California State Water Project, which supplies 29 urban and agricultural water agencies. The lake also provides flood control, generates hydroelectic power and helps control salinity in the Sacramento-San Joaquin River Delta.

In drier times

During the drought, Lake Oroville saw its level dip to historic lows. In this photo from Aug. 19, 2014, the reservoir was at 32 percent of capacity. When it is full, the reservoir can hold 3.5 million acre-feet of water. An acre-foot is the amount needed to cover an acre with a foot of water — enough to supply one or two households for a year.

Then came the rains

The unusually wet winter pushed lake levels up through January, and in early February a series of warm storms barreled through. By the end of February, the Oroville area had seen nearly 39 inches of rain, and the mountains that feed runoff to Lake Oroville were hit with similarly heavy snow.

Water surges into Lake Oroville

This season’s big storms caused an almost unprecedented inflow of water in the reservoir, peaking in early February at more than 150,000 cubic feet per second.

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Lake level climbs

The inflows of water caused Lake Oroville to fill toward its brim, which was great news for a drought-parched state, but put pressure on dam operators to release water.

2017 outflow Reservoir level

Water releases grow

As Lake Oroville filled, dam operators had to bump up releases down the main spillway, with some limitations. Sending more water down the spillway can stress it. Meanwhile, a maximum of 150,000 cubic feet per second can flow down the Feather River without flooding downstream communities.

Years: 2010 2011 2012 2013 2014 2015 2016 2017

Crisis hits

On Feb. 7, dam operators noticed an unusual flow down the main spillway, halted releases and saw that erosion had opened a crater in the main spillway. Officials reduced releases but had to continue them as storm runoff pushed the reservoir toward capacity, causing the crater to grow. On Feb. 11, with lake levels still rising, water started to flow over the dam’s emergency spillway.

Emergency spillway erodes

On Feb. 12, dam operators were shocked to learn that water pouring over the emergency spillway was chewing up the hillside beneath it. Officials feared the erosion could cause the emergency spillway to fail, sending a catastrophic amount of water downstream. They began to fortify the slope with boulders and poured concrete, while increasing releases from the damaged main spillway.

Evacuations ordered

At about 4:45 p.m. on Feb. 12, as damage continued to grow below the emergency spillway, the Butte County Sheriff's Office initiated evacuation orders for nearly 200,000 people. Authorities initially said the emergency spillway could fail within an hour, but it held. The evacuation order first affected residents in low-lying areas of Butte County, but was expanded to places farther south along the Feather River, including Yuba City (Sutter County) and Marysville (Yuba County).

Residents stranded

Evacuated residents of the downstream communities were forced to stay with family members, friends or at emergency shelters. At the Butte County Fairgrounds in Chico, Oroville residents Georgia Robert King, Jerry Lee Huggins and Anna Gibson slept outside, because pets weren't allowed inside.

Lake levels fall

By releasing water down the damaged main spillway, and by taking advantage of a break in the storms, dam operators were able to bring the lake level down, calming the crisis.

2017 outflow Reservoir level

Evacuees return

By Feb. 13, with the main spillway releasing 100,000 cubic feet of water per second, the water level dropped. It was clear the emergency spillway would not have to be used again, at least anytime soon. The evacuation order was lifted Feb. 14, and people streamed back to their homes.

Carlos Avila Gonzalez / The Chronicle

Damage assessed

On Feb. 27, with the lake level down significantly, dam operators were able to stop water releases on the devastated main spillway. That allowed geologists to begin inspecting the vast canyon that had been dug out of the earth by the heavy outflows. The state Department of Water Resources must now figure out how the crippled chute, which is as wide as a 15-lane freeway, can be fixed. The job may cost hundreds of millions of dollars.

Snowpack still worrisome

This year’s trouble isn’t over. Snowpack in the mountains is near record highs across California, and as it melts this spring, the inflow to Lake Oroville and other reservoirs is likely to be large.

Current Sierra snowpack level:

Current reservoir levels:

Total reservoir capacity Current reservoir storage

Big test ahead

While dam managers at Lake Oroville have been able to sufficiently release water down the Feather River using the crumbled main spillway, they wonder whether the chute will hold up to heavy spring runoff. Just in case, the state has been working to increase outflow capacity elsewhere at the reservoir. The dam’s hydroelectric plant, which was shut down after the main spillway fractured, is being reactivated and will not only generate power but offer another way to release water. Construction crews are also fortifying the emergency spillway.