The emergency spillway at California's swollen Oroville Dam was activated Saturday, as water levels from heavy rains this week caused the reservoir to rise above its capacity during an unusually plentiful rain season.
Saturday marked the first time the emergency spillway has been used in the dam's 48-year history. Normally the dam would use its concrete-lined spillway to discharge water, but that primary channel is severely damaged. However, utilizing the emergency spillway—essentially an unlined hillside—is likely to send mud and other debris into the water of rivers and channels downstream.
The California Department of Water Resources said in a release that the "the volume of water is expected to pose no flood threat downstream, and should remain well within the capacity of Feather River and other channels to handle."
Even so, the state agency cautioned that "the rate of flow into the ungated emergency spillway may change quickly."
The use of the emergency spillway follows a series of so-called atmospheric river storms that have dropped huge amounts of rain into Northern California—a region tracking to have its wettest year ever recorded.
The mountains surrounding the Oroville Dam received between 10 and 20 inches of rain from Wednesday to Friday, according to the National Weather Service in Sacramento. The dam is located in the foothills of the western Sierra Nevada mountain range.
"This is tracking to be perhaps the wettest season in Northern California ever," DWR spokesman Doug Carlson said Friday.
According to Carlson, the Northern California region as of Friday was at 228 percent of normal for this time year. This year's wet season, which began on October 1, is on track to be wetter than the 1982-83 season, which was the wettest.
Oroville Dam, the state's second-largest reservoir, suffered damage to its primary concrete-lined spillway due to erosion. The primary spillway was designed to divert rising water out of the dam, but damage to the channel was discovered Tuesday just as major storms were approaching.
"Oroville Dam itself remains safe, and there is no imminent threat to the public," DWR said Saturday. "DWR is coordinating closely with state and federal wildlife and dam safety officials at Oroville Dam."