A 90-ton gate at Folsom Dam buckled Monday, unleashing a thundering white-water torrent that forced the evacuation of boaters, hikers and anglers along the American River.

None of the towns below the dam, about 20 miles northeast of Sacramento, was in danger of flooding but law enforcement officials ordered people away from the river’s edge.

Dam operators said they may not be able to stop the water roaring out of the broken gate for up to a week, until the water level in the reservoir drops 40 feet to below the spillway.

The buckled gate will drain nearly half the water from the reservoir, which holds about 1 million acre-feet used for drinking water, irrigation and to sustain wildlife in marshlands downstream.