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(1 of ) In this Saturday, Feb. 11, 2017, photo, water flows down Oroville Dam's main spillway, in Oroville, Calif. Water began flowing over the emergency spillway on Saturday for the first time in its nearly 50-year history after heavy rainfall. In addition to the emergency spillway, water also flowed through the main spillway that was significantly damaged from erosion. Officials said they'll assess the damage starting Monday. (AP Photo/Rich Pedroncelli) (2 of ) California Highway Patrol officer Ken Weckman directs traffic as residents evacuate Marysville, Calif., Sunday, Feb. 12, 2017. Thousands of residents of Marysville and other Northern California communities were told to leave their homes Sunday evening as an emergency spillway of the Oroville Dam could fail at any time unleashing flood waters from Lake Oroville, according to officials from the California Department of Water Resources. (Paul Kitagaki Jr./The Sacramento Bee via AP) (3 of ) Kendra Curieo waits in traffic to evacuate Marysville, Calif., Sunday, Feb. 12, 2017. Thousands of residents of Marysville and other Northern California communities were told to leave their homes Sunday evening as an emergency spillway of the Oroville Dam could fail at any time unleashing flood waters from Lake Oroville, according to officials from the California Department of Water Resources. (AP Photo/Rich Pedroncelli) (4 of ) In this Saturday, Feb. 11, 2017, water flows down Oroville Dam's main spillway, near Oroville, Calif. Water began flowing over the emergency spillway at the Oroville Dam on Saturday for the first time in its nearly 50-year history after heavy rainfall. In addition to the emergency spillway, water also flowed through the main spillway that was significantly damaged from erosion. Officials said they'll assess the damage starting Monday. (AP Photo/Rich Pedroncelli) (5 of ) In this Saturday, Feb. 11, 2017, water flows down Oroville Dam's main spillway near Oroville, Calif. Water began flowing over the emergency spillway at the Oroville Dam on Saturday for the first time in its nearly 50-year history after heavy rainfall. In addition to the emergency spillway, water also flowed through the main spillway that was significantly damaged from erosion. Officials said they'll assess the damage starting Monday. (AP Photo/Rich Pedroncelli) (6 of ) Jason Newton, of the Department of Water Resources, takes a picture of water going over the emergency spillway at Oroville Dam Saturday, Feb. 11, 2017, in Oroville, Calif. Water started flowing over the emergency spillway, at the nation's tallest dam, for the first time Saturday morning after erosion damaged the Northern California dam's main spillway.(AP Photo/Rich Pedroncelli) (7 of ) This Friday, Feb. 10, 2017 image from video provided by the office of Assemblyman Brian Dahle shows water flowing over an emergency spillway of the Oroville Dam in Oroville, Calif., during a helicopter tour by the Butte County Sheriff's office. About 150 miles northeast of San Francisco, Lake Oroville is one of Californias largest man-made lakes, and the 770-foot-tall Oroville Dam is the nation's tallest. (Josh F.W. Cook/Office of Assemblyman Brian Dahle via AP) (8 of ) Jason Newton, left, of the Department of Water Resources, takes a picture of water going over the emergency spillway at Oroville Dam Saturday, Feb. 11, 2017, in Oroville, Calif. Water started flowing over the spillway,at the nation's tallest dam, for the first time Saturday morning after erosion damaged the Northern California dam's main spillway.(AP Photo/Rich Pedroncelli) (9 of ) A employee of the Department of Water Resources watches as water flows over the emergency spillway at Oroville Dam Saturday, Feb. 11, 2017, in Oroville, Calif. Water started flowing over the spillway, at the nation's tallest dam, for the first time Saturday morning after erosion damaged the Northern California dam's main spillway.(AP Photo/Rich Pedroncelli) (10 of ) Muddy water rushes down the emergency spillway at Oroville Dam, Saturday, Feb. 11, 2017, in Oroville, Calif. Water started flowing over the emergency spillway at the nation's tallest dam for the first time Saturday after erosion damaged the Northern California dam's main spillway.(AP Photo/Rich Pedroncelli) (11 of ) E. Knight uses his smartphone to record muddy water rushing down the emergency spillway at Oroville Dam, Saturday, Feb. 11, 2017, in Oroville, Calif. Water started flowing over the emergency spillway at the nation's tallest dam for the first time Saturday after erosion damaged the Northern California dam's main spillway. (AP Photo/Rich Pedroncelli) (12 of ) In this Feb. 11, 2017, photo released by the California Department of Water Resources, water flows over an emergency spillway at the Oroville Dam at Lake Oroville in Oroville, Calif. Water will continue to flow over the emergency spillway at the nation's tallest dam for another day or so, officials said Sunday. (Albert Madrid/California Department of Water Resources via AP) (13 of ) This Feb. 11, 2017, photo released by the California Department of Water Resources shows the main spillway, bottom, and an auxiliary spillway, upper, of the Oroville Dam at Lake Oroville in Oroville, Calif. Water will continue to flow over the emergency spillway at the nation's tallest dam for another day or so, officials said Sunday. (Albert Madrid/California Department of Water Resources via AP) (14 of ) A driver waits in traffic to evacuate Marysville, Calif., Sunday, Feb. 12, 2017. Thousands of residents of Marysville and other Northern California communities were told to leave their homes Sunday evening as an emergency spillway of the Oroville Dam could fail at any time unleashing flood waters from Lake Oroville, according to officials from the California Department of Water Resources. (AP Photo/Rich Pedroncelli)

OLGA RODRIGUEZ AND DON THOMPSON

ASSOCIATED PRESS | February 12, 2017, 5:33PM

| Updated 2 minutes ago.

OROVILLE Thousands evacuated their Northern California homes Sunday evening after authorities warned an emergency spillway in the countrys tallest dam was in danger of failing and unleashing uncontrolled flood waters on towns below.

About 30 miles southeast of Chico, Lake Oroville is one of Californias largest man-made lakes, and the 770-foot Oroville Dam is the nations tallest.

At least 130,000 people were asked to evacuate over concerns the dams 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, Wheat land, 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 couldnt 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 Sheriffs 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 dont know what caused the cave-in, but Chris Orrock, a spokesman for the state Department of Water Resources, said it appears the dams main spillway has stopped crumbling even though its being used for water releases.

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

Read more:
Oroville Dam's backup spillway set to fail; evacuations ordered - Santa Rosa Press Democrat