By Grant Smith, National News Writer
On February 12, the emergency spillway at Northern California’s Oroville Dam began to suffer effects of erosion that forced 188,000 people to evacuate their homes due to the anticipated failure of the dam. However, on February 14, following state efforts to lower the water levels at Oroville Lake, residents were allowed to return the their homes.
Problems began to surface at the nation’s tallest dam on February 7, when officials noticed a crater in the main Oroville Dam spillway that eventually grew to 300 feet wide, 500 feet long, and 45 feet deep, according the Los Angeles Times. The Sacramento Bee reported that damage to the downstream Feather River Fish Hatchery forced dam operators to reduce the main spillway discharge from 65,000 cubic feet per second to just 55,000 cubic feet per second despite continued heavy rainfall.
Because of these factors, the water level at Oroville Lake rose to 901 feet, one foot over the 900-foot capacity, forcing the emergency spillway to carry water for the first time since the dam’s conception in 1968, as reported by CNN. Within one day, there were warning signs that the ground underneath the emergency spillway’s 30-foot wall was eroding away, compromising the structural integrity of the wall itself and bringing about the evacuation of all low-lying areas nearby.
In response, dam operators increased discharge at the damaged main spillway to 100,000 cubic feet per second in an attempt to drop water levels by a minimum of fifty feet, according to CBS News Sacramento. State officials announced on the morning of February 17 that water levels had indeed been reduced by forty feet, just as three new storm systems were headed toward Northern California. Mercury News quoted Bill Croyle, acting director of the California Department of Water Resources, as saying “The threat level is lower. It’s much, much, much lower than what it was on Sunday.”
While it is still too early to investigate the cavity on the main spillway, there has been speculation that the cause of the gaping hole involved tiny bubbles known as cavitation. Discovered in 1983 when the Glen Canyon Dam on the Colorado River almost failed; cavitation is the process initiated by turbulence, caused by the billions of gallons of water tumbling over concrete at high speeds, as it generates bubbles of water vapor that collapse with powerful force equivalent to that of a jackhammer.
In response to this discovery, aeration slots were innovated and implemented at vulnerable spots in the spillway for air pockets, to disperse and weaken water vapor concentrations along Grand Canyon Dam. Both the Hoover Dam and the Blue Mesa Dam were retrofitted with aeration slots, but Oroville Dam’s spillways were not retrofitted with aerators according to the Sacramento Bee. Essentially, this decision in the late 1980s left the 178-foot wide and only 15-foot thick concrete chute’s structural integrity up to chance. While threat levels have fallen tremendously, state officials have warned nearby citizens to remain alert and be prepared to evacuate should the situation change for the worse in coming weeks.
A version of this article appeared in the Tuesday, February 21st print edition.
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