Q: Where is Oroville Dam?
A: The earthen dam is a few miles northeast of Oroville in Butte County, 65 miles north of Sacramento. Lake Oroville is California’s second-largest reservoir, and its dam is the tallest in the United States. The reservoir is capable of holding 3.5 million acre-feet of water. An acre-foot is equivalent to about 326,000 gallons, or enough to cover an acre of land with a foot of water.
Q: What does Oroville Dam do?
A: Completed in 1968, the dam captures and stores rain and snowmelt that washes in from the mountains in the Feather River watershed, a vast area of the Sierra Nevada that stretches across the state’s eastern border from nearly Truckee to Lassen Volcanic National Park. The dam regulates flows on the Feather River, the Sacramento River’s largest tributary. The structure provides flood control for the eastern edge of the Sacramento Valley. The dam’s main outlet outside of the rainy season is a hydroelectric power plant at the base of the dam.
The reservoir is the principal storage facility for the State Water Project, which delivers water to Central Valley farms and urban water agencies from the Silicon Valley to as far south as San Diego. One third of Southern California’s water supply comes from the State Water Project, whose members – water agencies that include the mammoth Metropolitan Water District of Southern California – helped finance the dam’s construction and pay for upkeep. The dam is managed by the California Department of Water Resources.
Q: Why is there suddenly a crisis?
A: To make room in the reservoir for flood protection, officials had been releasing water from the dam’s main spillway, a 3,000-foot concrete span built on soil and rock that leads to a channel that flows into the Feather River. A huge gash was discovered on the bottom half of the chute Feb. 7.
Fearing the spillway would further erode and become inoperable, dam operators dialed back flows to avert further damage. With stormwater and runoff from the snow-packed Sierra rushing in last week, the lake climbed to the point that water began overtopping an emergency spillway adjacent to the damaged main spillway. It marked the first use of the emergency spillway in the 48-year history of the dam.
Unlike the main spillway, which is lined in concrete and controlled via release gates, the emergency spillway dumps water down a wooded hillside. Water that reaches its lip – a 1,700-foot span of concrete – pours uncontrolled down the hill. Engineers had frantically tried to avoid allowing water to reach the lip because they were worried that the flows would scrape mud, trees and debris into the Feather River below the dam, creating clogs and other problems downstream.
On Sunday afternoon – just hours after officials declared the worst of the crisis over – engineers saw a deep gash forming in the hillside beneath the lip of the emergency spillway. The concern is that the erosion could continue to eat into the hillside, undercutting the upper 30 feet of the spillway, forcing a collapse. Such a collapse would release the top 30 feet of water held by the reservoir, and inundate much of Butte and Sutter counties. Nearly 200,000 people who live in the floodplain below the dam were ordered to evacuate.
Q: What’s the risk?
A: Officials say the earthen face of the dam, which is separated by a hillside from the main and emergency spillways, has not been compromised. But experts say that if the emergency spillway collapses, it wouldn’t be much different than a total dam failure. The hillside could quickly erode and empty the lake.
The massive release of water could blow out levees along the Feather River and inundate Sacramento Valley communities that are home to tens of thousands of people, including Oroville, Marysville and Yuba City. The city of Sacramento also could be affected, though experts said a levee breach along the Feather River likely would relieve pressure in the system enough that Sacramento’s flood-control infrastructure could absorb the influx.