Because of concerns that water levels were nearing capacity at Lake Oroville, officials turned to the emergency spillway for the first time in the dam's 48-year history. He said the emergency agency is supporting other state agencies and local officials to coordinate efforts at the dam's spillway and "monitoring it very closely".
The dam's 3,000-foot main spillway was being used for the controlled release of water, but on Tuesday this past week, the concrete spillway split open and is continuing to erode.
The department said it preferred not to use the emergency spillway because it would dump water onto trees and put debris into the Feather River, a source of water for parts of California.
By this morning, the DWR said the emergency spillway would indeed be needed. The emergency spillway is a hillside and could cause potential debris flow downstream into the Feather River where a fish hatchery is located.
Crews expect more erosion but the releases will help the operators absorb rising water levels as storms hit Northern California Thursday and Friday.
Stay with KCRA for updates.
The section of collapsed spillway has grown to more than 200 feet wide and 30 feet deep, according to the San Francisco Chronicle. Oroville Dam itself remains safe and there is no imminent threat to the public.
Faced with a little choice, the department resumed ramping up the outflow from Lake Oroville over the damaged spillway to partially keep up with the torrential rainfall flowing into the reservoir from the Sierra foothills.
It wasn't clear why the dam spillway was not more closely inspected, said Chris Orrock, a Division of Dam Safety spokesman.
Officials said they expected flows along the Feather River, which runs through tiny Oroville, to reach about.
Lake Oroville, the second largest reservoir in California, is in the foothills just east of the city of Oroville, about 70 miles north of Sacramento. They say the dam is sound and there is no public danger.
Use of the emergency spillway would only increase damage to the hatchery, Morse said.
"Our maintenance, we hope, has prevented any possibility of this", Moore said. The dam came close to being overtopped in 1964, as it filled at a rate of 250,000 cfs, and in 1997, when it filled at a rate of 331,000 cfs.