At a Monday news conference, Butte County Sheriff Kory Honea did not specify how long the repairs would take and offered no timetable for when the evacuation order would be lifted.
Meanwhile, water levels dropped Monday at Lake Oroville, stopping water from spilling over the 770-foot-tall dam's emergency spillway, slightly easing the fears of a catastrophic spillway collapse.
But with more rain expected later in the week, time was running short to fix the damage ahead of the storms.
California Department of Water Resources officials said they were planning on using helicopters to drop loads of rock on the eroded spillway at Lake Oroville, which is about 150 miles northeast of San Francisco.
The evacuation was ordered Sunday afternoon after engineers told authorities that a hole on the concrete lip of the secondary spillway for the dam could fail within the hour.
If the emergency spillway failed, it could send a 30-foot wall of water roaring downstream into communities. That's why 188,000 residents from three counties were told to evacuate.
As of late Sunday night, no more water was flowing over the emergency spillway. Also late Sunday, Gov. Jerry Brown issued an emergency order to fortify authorities' response to the emergency at the dam and help with evacuations.
The hole started out small last week, but the problem grew worse.
When asked if the spillway was supposed to handle far more water, Bill Croyle, the acting head of the California Department of Water Resources, said he was "not sure anything went wrong" on the damaged spillway.
Croyle said sometimes low-flow water can be high energy and cause more damage than expected. His comments came after officials assured residents for days that the damage was nothing to be concerned, then ordered everyone to get out in an hour.
The sudden evacuation panicked residents, who scrambled to get their belongings into cars and then grew angry as they sat in bumper-to-bumper traffic hours after the evacuation order was given.
When asked why both spillways didn't do their job, as one reporter called it a "double failure," Honea said he saw things differently.
"Public safety professionals did their job to protect the public, and we were able to get people evacuated out of the community in a fairly timely fashion," Honea said. "At the same time, our partners at the department of water resources were working to save that structure and prevent further damage so that we could go on. That's a double success."
Officials said crews will continue releasing as much as 100,000 cubic feet per second from the main spillway to try and reduce the dam's level by 50 feet ahead of storms forecast to reach the area Wednesday and Thursday.
"The idea behind that strategy was to reduce the erosion and stop the erosion, and now the site is starting to de-water and drain out," Croyle said.
Heavy equipment moving thousands of rocks; will be flown by helicopter to try to shore up emergency spillway pic.twitter.com/bIsG16yBDn— Rob McMillan (@abc7robmcmillan) February 13, 2017
Lake Oroville had water levels so high due to recent storms that an emergency spillway was used Saturday for the first time in the dam's nearly 50-year history.
A Red Cross spokeswoman said more than 500 people showed up at an evacuation center in Chico, California.
The shelter had run out of blankets and cots, and a tractor trailer with 1,000 more cots was stuck in the gridlock of traffic fleeing the potential flooding Sunday night, said Red Cross shelter manager Pam Deditch.
A California Highway Patrol spokesman said two planes would fly Monday to help with traffic control and possible search and rescue missions.
At least 250 California law enforcement officers were posted near the dam and along evacuation routes to manage the exodus of residents and ensure evacuated towns don't face looting or other criminal activity.
Lake Oroville is the main reservoir of California's State Water Project, which supplies water for more than half the state's 39 million residents and for millions of acres of farmland in the leading agricultural state. It's not clear how damage to the two spillways will affect long-term water releases from the dam.
The Associated Press contributed to this report.