Mass evacuations of up to 200,00 citizens have been ordered in Oroville, California over fears that the Oroville Dam would overflow.

The dam is the tallest in the United States, standing at 770 feet. Californian authorities are trying to fix erosion of the dam’s emergency spillway amid fears the problem could unleash uncontrolled flood waters if it fails.

About 150 miles north-east of San Francisco, Lake Oroville — one of California’s largest man-made lakes — had water levels so high that an emergency spillway was used Saturday for the first time in almost 50 years.

People were evacuated to maintain public safety, according to local sheriff Kony Honea.

Other solutions to fix the erosion being considered include the option of plugging the hole by dropping rocks into the crevice from helicopters.

California Fire and Rescue chief Kim Zagaris said at least 20 police officers from throughout the West Coast state are already within the area or en route to the dam.

With the water spillage no longer overflowing at the top, Bill Coyle, the acting directer at the department of water resources, said at a news conference that officials will now be able to assess that damage to the emergency overflow.

He added that the integrity of the dam has not been impacted.