At least 188, 000 people were evacuated from several northern California counties after authorities warned an emergency spillway in the countrys tallest dam was in danger of failing and unleashing uncontrolled flood waters on towns below.

The erosion prompted concerns that it could endanger the communities living downstream and evacuation orders were made on Sunday for cities and counties near Lake Oroville, which remain in effect, CNN reported.

"I would rather be safe than sorry. I would rather have people move out of the area hopefully to safety," Sheriff Kory Honea said.

"A hazardous situation is developing with the Oroville Dam auxiliary spillway. Operation of the auxiliary spillway has led to severe erosion that could lead to a failure of the structure," the sheriff's office posted on its Facebook page on Sunday night, emphasising that it was not a drill.

California Governor Jerry Brown also issued a state emergency order to help local authorities with the emergency situation and the evacuations.

The cities of Oroville, Gridley, Live Oak, Marysville, Wheat Land, Yuba City, Plumas Lake and Olivehurst were all under evacuation orders. The order was sent out after engineers discovered a hole that was eroding back towards the top of the spillway.

It's possible that crisis could be averted, however, because the California Department of Water Resources told the sheriff that the erosion "was not advancing as rapidly as we thought."

A plan is currently in place to employ helicopters to drop rocks into a crevice atop the erosion, which would plug the hole, said the sheriff.

As of about 6:30 p.m, about two inches of water was coming over the spillway, which was down significantly from when authorities first issued the evacuation orders, according to the report.

Department of Water Resources officials said they expect the flow to soon stop entirely, which will reduce the pace of erosion.

"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 was increasing water releases from the main spillway to 100,000 cubic feet per second in an effort to lessen the amount of water going down the emergency spillway, the sheriff's office said.