At least 188,000 people have been evacuated from several Northern California counties after damage to a spillway at the Oroville Dam.
The dam, which is the nation's highest, remains intact. But the emergency spillway, which helps to ensure that the water does not rush over the top of the dam when levels are high, was eroding Sunday.
The erosion prompted concerns that it could endanger the communities living downstream and evacuation orders were made for cities and counties near Lake Oroville, which remain in effect.
"I would rather be safe than sorry. I would rather have people move out of the area hopefully to safety," Sheriff Kory Honea said.
California Governor Jerry Brown also issued a state emergency order to help local authorities with the emergency situation and the evacuations.
Around 3 p.m. local time (6 p.m. ET), authorities learned that the dam's emergency spillway -- which has been in use since the primary spillway was recently damaged -- had a hole in it and was eroding, the sheriff said.
It's possible that crisis could be averted, however, because the California Department of Water Resources told Honea that the erosion "was not advancing as rapidly as they thought."
A plan is currently in place to employ helicopters to drop rocks into a crevice atop the erosion, which would plug the hole, he said.
As of about 6:30 p.m. (9:30 p.m. ET), about 2 inches of water was coming over the spillway, which was down significantly from when authorities first issued the evacuation orders.
DWR officials expect the flow to soon stop entirely, which will reduce the pace of erosion.
Evacuation orders issued
About an hour prior, Honea's office had used the most urgent of language -- "This is NOT a drill" -- in imploring residents to evacuate ahead of an impending disaster.
The erosion could result in "large, uncontrolled releases of water from Lake Oroville," which could exceed the capacities of various channels downstream, the sheriff's office said.
"This is an evacuation order. Immediate evacuation from the low levels of Oroville and areas downstream is ordered," a Facebook post said.
The evacuation order also includes "all Yuba County on the valley floor" and the city of Marysville, authorities said.
"The next several hours will be crucial in determining whether the concrete structure at the head of the auxiliary spillway remains intact and prevents larger, uncontrolled flows," the Butte County sheriff's office said in a statement.
In an effort to mitigate the situation, the DWR is increasing water releases from the main spillway to 100,000 cubic feet per second in an effort to lessen the amount of water traveling down the emergency spillway, the sheriff's office said. Normal flows down the main spillway are about 55,000 cubic feet per second.
Thousands ordered to leave
The DWR instructed Oroville residents to head north, toward Chico.
Conversely, the Yuba County Office of Emergency Services warned its residents, "Take only routes to the east, south, or west. DO NOT TRAVEL NORTH TOWARD OROVILLE!!!!!"
Evacuation shelters have been set up at fairgrounds in Chico and Colusa, CNN affiliate KCRA reported. A California Office of Emergency Services spokesperson told the station that it could be "potentially catastrophic" if the emergency spillway fails.
Marysville is about 40 miles north of Sacramento, Oroville about 70 miles. It's unclear how many residents could be affected by the evacuation orders, but the combined population of Yuba and Butte counties is roughly 300,000 people.
The Sacramento Fire Department warned residents that the spillway failure could have effects, including flash flooding, downstream in Sacramento.
In Oroville, Judy Brandt had been at her father-in-law's house, which is located on a ridge over the dam. Although she and her family are not in danger because they are not downstream, she said she saw concerning scenes at the dam.
"Before it got dark, I watched the river increase in size. I saw the spray change and whitecaps," she said. "It's very worrisome for the town [of Oroville]."
Why Oroville Dam is affected this year
At 770 feet, Oroville Dam is the country's tallest. In addition to flood control, it also serves to provide drinking water and generates hydroelectric power.
The lake has returned to near its pre-drought levels.
The 7,000-foot wide dam sits on Lake Oroville, which "is located where the North, Middle, and South Forks, and the West Branches of the Feather River join," according to the California Department of Water Resources.
This winter season, Northern California has received a glut of rain, filling up Lake Oroville to the verge of overflowing. This has put stress on its spillways which let water out so it doesn't gush over the dam.
Lake Oroville receives water from the northern Sierra Nevada mountain range which is having one of the wettest seasons this year.
More rain is in the forecast starting Wednesday.