Officials in Northern California ordered more than 100,000 people to evacuate to safer ground on Sunday as the country’s tallest dam threatened to release uncontrolled floodwaters downstream.

Motorists jammed highways as thousands of residents rushed to flee the area around the Oroville Dam, about 150 miles northeast of San Francisco, after authorities warned that the emergency spillway could fail, the National Weather Service.

Hundreds of cars lined up on Highway 99, creating a bumper-to-bumper traffic jam as people streamed north, the Associated Press said.

Lake Oroville, one of California’s largest man-made lakes, has been swollen by more than a month of heavy rains. Water began topping the emergency spillway on the 770-foot-tall dam, the nation’s tallest, on Saturday, causing erosion damage that could lead to a surge of water being released, officials said.

Officials from California’s Department of Water Resources said they planned to use helicopters to drop rocks to fill in the gouge in the spillway.

Residents of Oroville, Gridley, Live Oak, Marysville, Wheat land, Yuba City, Plumas Lake, and Olivehurst were all ordered to evacuate immediately, authorities said.

Butte County Sheriff Kory Honea said officials made the decision to nearly double the volume of water being released from the dam to 100,000 cubic feet per second to drain the lake quickly and stop erosion at the top of the auxiliary spillway.

“Hopefully, that will release pressure on the emergency spillway and they’ll find a repair to prevent a complete failure,” Honea said on Sunday. “[The] situation is dynamic and could change anytime.”

Officials announced shortly before 11 p.m. that the water was no longer topping the damaged spillway.

This is a breaking news story. Please check back for updates.

The Associated Press contributed to this report.

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