Thousands Evacuate As Officials Monitor Eroding Oroville Dam Spillway

OROVILLE, Calif. (AP) — Thousands evacuated their Northern California homes Sunday evening after authorities warned an emergency spillway in the country's tallest dam was in danger of failing and unleashing uncontrolled flood waters on towns below.

About 150 miles northeast of San Francisco, Lake Oroville is one of California's largest man-made lakes, and the 770-foot-tall Oroville Dam is the nation's tallest.

At least 130,000 people were asked to evacuate over concerns the dam's emergency spillway could fail. Hundreds of cars were in bumper-to-bumper traffic on highways and at least two shelters said they were at capacity.

The erosion at the head of the emergency spillway threatens to undermine the concrete weir and allow large, uncontrolled releases of water from Lake Oroville, the California Department of Water Resources said. Those potential flows could overwhelm the Feather River and other downstream waterways, channels and levees.

Officials say Oroville Lake levels had decreased by Sunday night as they let water flow from its heavily damaged main spillway but noted that water was still spilling over the dam.

The cities of Oroville, Gridley, Live Oak, Marysville, Wheatland, Yuba City, Plumas Lake, and Olivehurst were all under evacuation orders.

The evacuation order went out around 4 p.m. after engineers spotted a hole that was eroding back toward the top of the spillway.

Butte County Sheriff Koney Honea said engineers with the Department of Water Resources informed him shortly after 6 p.m. that the erosion on the emergency spillway at the Oroville Dam was not advancing as fast as they thought.

"Unfortunately they couldn't advise me or tell me specifically how much time that would take so we had to make the very difficult and critical decision to initiate the evacuation of the Orville area and all locations south of that," he said. "We needed to get people moving quickly to save lives if the worst case scenario came into fruition."

Honea said there is a plan to plug the hole by using helicopters to drop rocks into the crevasse.

Water began flowing over the emergency spillway at the Oroville Dam in Northern California on Saturday for the first time in its nearly 50-year history after heavy rainfall. Officials earlier Sunday stressed the dam itself was structurally sound and said there was no threat to the public.

Residents of Oroville, a town of 16,000 people, should head north toward Chico, and other cities should follow orders from their local law enforcement agencies, the Butte County Sheriff's office said.

The Yuba County Office of Emergency Services asked residents in the valley floor, including Marysville, a city of 12,000 people, to evacuate and take routes to the east, south, or west and avoid traveling north toward Oroville.

The California Department of Water Resources said it is releasing as much as 100,000 cubic feet per second from the main, heavily damaged spillway to try to drain the lake.

Department Kevin Dossey told the Sacramento Bee the emergency spillway was rated to handle 250,000 cubic feet per second, but it began to show weakness Sunday at a small fraction of that. Flows through the spillway peaked at 12,600 cubic feet per second at 1 a.m. Sunday and were down to 8,000 cubic feet per second by midday.

Unexpected erosion chewed through the main spillway during heavy rain earlier this week, sending chunks of concrete flying and creating a 200-foot-long, 30-foot-deep hole that continues growing. Engineers don't know what caused the cave-in, but Chris Orrock, a spokesman for the state Department of Water Resources, said it appears the dam's main spillway has stopped crumbling even though it's being used for water releases.

The lake is a central piece of California's government-run water delivery network, supplying water for agriculture in the Central Valley and residents and businesses in Southern California.

Copyright 2016 The Associated Press.

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• Butte County