Inside the frantic fight to protect Oroville dam, nation's tallest, as spillway rapidly erodes

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An aerial view of the water flowing out of the Oroville Dam main spillway, in Oroville, Calif., on Tuesday, February 21, 2017.

(Marcus Yam / Los Angeles Times)

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An aerial view of the water flowing out of the Oroville Dam’s main spillway on Feb. 21. (Marcus Yam / Los Angeles Times)

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Reduced water releases at the Oroville Dam have made damage to its main spillway more visible. (Marcus Yam / Los Angeles Times)

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With a reduced flow on Sunday, most of the water being released from the Oroville Dam is not going down the spillway; it has broken through and is flowing down the hillside. (Marcus Yam / Los Angeles Times)

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With a reduced flow on Sunday, most of the water being released from the Oroville Dam is not going down the spillway, it’s broken through and is going down the hillside.

(Marcus Yam / Los Angeles Times)

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With a reduced flow on Sunday, most of the water being released from the Oroville Dam is not going down the spillway, it’s broken through and is going down the hillside.

(Marcus Yam / Los Angeles Times)

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Juan Alvarez reassures his girlfriend, Sarah Hendrix, after helping her move out of her home in rural Maxwell. Water was a foot high and crews had to evacuate 100 people because of flooding-- some by boat.

(Marcus Yam / Los Angeles Times)

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Ron Chambers lets Duke out of his crate for the first time in hours since the flooding began n Maxwell, Calif. on Saturday.

(Marcus Yam / Los Angeles Times)

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Kevin Anfinson and other volunteers help shovel the muddy sediment that has built up in the salmon raceway at the Feather River Fish Hatchery in Oroville, Calif. (Marcus Yam / Los Angeles Times)

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Fernando Martinez and his mother, not pictured, wade through a road in Gridley, Calif., flooded by the Feather River as it continues to swell from the water being let out of Lake Oroville.

(Marcus Yam / Los Angeles Times)

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A man in a 4x4 truck turns around on Gridley Road after having second thoughts about making it across the flooded road, which had been closed.

(Brian van der Brug / Los Angeles Times)

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Randy Boheim packs up his tools and emergency supplies in anticipation of having to evacuate his whole family as floodwaters creep closer to his home in an Oroville, Calif., mobile home park. The nearby Feather River continued to swell from the water being let out of Lake Oroville ahead of this weekend’s storm.

(Marcus Yam / Los Angeles Times)

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A plantation in Oroville, Calif., sits in floodwaters as the Feather River continues to swell from the water being let out of Lake Oroville ahead of this weekend’s storm.

(Marcus Yam / Los Angeles Times)

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Helicopters ferry sand and rocks to the Oroville Dam’s emergency spillway reconstruction project in Oroville, Calif., ahead of coming rains.

(Brian van der Brug / Los Angeles Times)

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California Water Service district manager Toni Ruggle surveys the Feather River at Bedrock Park downstream from the Oroville Dam.

(Brian van der Brug / Los Angeles Times)

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McKenna Harvey, 9, left, Kylie Atteberry, 11, and Brooklyn Atteberry, 7, hold signs thanking workers in the repair effort at the Oroville Dam.

(Brian van der Brug / Los Angeles Times)

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A helicopter flies over as water flows from the main spillway at Lake Oroville. (Marcus Yam / Los Angeles Times)

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Reconstruction continues in a race to shore up the emergency spillway at Oroville Dam. (Brian van der Brug / Los Angeles Times)

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As rain clouds gather, friends from left, Johnny Eroh, Cody Balmer, Kristien Bravo and Jerel Bruhn hang out by the flooded Feather river in the Bedrock neighborhood of Oroville, Calif., last week. (Marcus Yam / Los Angeles Times)

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Evacuees at the Bangor Community Hall in Bangor, Calif., listen to Butte County sheriff’s deputies in February as the mandatory evacuation order was lifted. An evacuation advisory was lifted Wednesday. (Brian van der Brug / Los Angeles Times)

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Evacuees at the Bangor Community Hall get the news from Butte County sheriff’s Deputy Jeff Heath that the evacuation order has been lifted.

(Brian van der Brug / Los Angeles Times)

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Evacuee Sharon Dalton, right, hugs Raiden Ellis, 10 months, and Chris Ellis as they say their goodbyes as she leaves the Bangor Community Hall in Bangor, Calif., on Tuesday.

(Brian van der Brug / Los Angeles Times)

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David McGlamery returns to his Oroville home with his belongings after the evacuation order was lifted. The family had to retreat to Chico, where they initially stayed at a Walmart parking lot with other evacuees. (Marcus Yam / Los Angeles Times)

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Bill Tirey helps family members move back into their home in the Bedrock neighborhood of Oroville near the Feather River after evacuation orders were lifted.

(Marcus Yam / Los Angeles Times)

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Maria Alancar returns home to greets her pet pig Bacon, who was left behind when the family moved to higher ground in Honcut, Calif.

(Brian van der Brug / Los Angeles Times)

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Sulet Lopez, 21, left, Melissa Mendoza, 3, and Yeanet Lopez, 18, pack up their car at the Bangor Community Hall in Bangor, Calif., to head home after the evacuation order was lifted.

(Brian van der Brug / Los Angeles Times)

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Erica Stenholm, left, Ronnie Vaughan, and Brooklyn Jackson, 7, unpack their car upon returning home after the evacuation order had been lifted in Oroville.

(Marcus Yam / Los Angeles Times)

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Jonah Avina, left, and his wife, Eileen, pray before lunch at the Maranatha Mennonite Fellowship in Bangor, Calif., on Tuesday. The Bangor-area Mennonites are hosting several immigrant families evacuated from the nearby town of Honcut.

(Brian van der Brug / Los Angeles Times)

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Evacuee Estafani Reynoso, left, colors with Mennonite children at the Maranatha Mennonite Fellowship in Bangor, Calif.

(Brian van der Brug / Los Angeles Times)

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Crews work on a damaged section of the emergency spillway at Lake Oroville on Monday. (Josh Edelson / AFP/ Getty Images)

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An Oroville property is flooded on Monday as thousands were under evactuation orders.

(Josh Edelson / AFP/Getty Images)

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The Marysville cemetery underwater along the Feather River.

(Marcus Yam / Los Angeles Times)

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Nirmal Singh, a Sikh priest, conducts a morning prayer ritual as evacuees sleep in the background at the Shri Guru Ravidass, a Sikh temple that has opened its doors for evacuees of the Oroville Dam crisis in Rio Linda, Calif.

(Marcus Yam / Los Angeles Times)

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Kamlesh Nahar, far left, talks to fellow evacuees at the Shri Guru Ravidass Temple.

(Marcus Yam / Los Angeles Times)

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Maria Lopez reads on her smartphone as she spends a second night in her father’s car in the parking lot of the Bangor Community Hall.

(Brian van der Brug / Los Angeles Times)

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Sharon Dalton finds a quiet spot under a table as she spends a second night with her dog Cruiser inside the Bangor Community Hall.

(Brian van der Brug / Los Angeles Times)

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Verna Chadwick and 10-month-old son Raiden Ellis during a second night in the Bangor Community Hall.

(Brian van der Brug / Los Angeles Times)

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The emergency spillway, left, and the damaged main spillway at Lake Oroville are seen in an aerial photo Monday.

(Elijah Nouvelage / Getty Images)

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A dump truck crosses the primary spillway to deliver boulders to the damaged emergency spillway at Lake Oroville on Monday evening.

(Brian van der Brug / Los Angeles Times)

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Helicopters place large rocks on the damaged emergency spillway at Lake Oroville on Monday evening.

(Brian van der Brug / Los Angeles Times)

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Around-the-clock monitoring continues on the damaged primary spillway at Lake Oroville on Monday evening.

(Brian van der Brug / Los Angeles Times)

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Water rushes down a spillway at the Oroville Dam. (Brian van der Brug / Los Angeles Times)

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California Department of Fish and Game wardens view the damaged spillway on Monday. (Brian van der Brug / Los Angeles Times)

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Bill O’Kelley, 86, and wife Doris O’Kelley, 84, of Oroville sit near a flagpole Monday at the Silver Dollar Fairgrounds evacuation center in Chico, Calif.

(Brian van der Brug / Los Angeles Times)

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Coua Tha, of Oroville prepares a meal for her family in the parking lot at the Silver Dollar Fairgrounds evacuation center in Chico, Calif.

(Brian van der Brug / Los Angeles Times)

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Desiree Garcia and daughter Kay’lee Pearl Garcia, 3, of Oroville look over donated clothing Monday at the Silver Dollar Fairgrounds evacuation center in Chico, Calif.

(Brian van der Brug / Los Angeles Times)

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Members of Oroville’s Jordan Crossing Mission pray with a volunteer service member Monday at the Silver Dollar Fairgrounds evacuation shelter in Chico, Calif.

(Brian van der Brug / Los Angeles Times)

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The swollen Feather River flows through Oroville, Calif., on Monday.

(Rich Pedroncelli / Associated Press)

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A building is submerged in Riverbend Park as more water is released from Lake Oroville.

(JOSH EDELSON / AFP)

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Siblings Zach Soto, 11, left, and Gabby Soto, 13, keep an eye on the Feather River along a railroad bridge in Oroville, Calif., on Monday. The family decided to stay in Oroville as they live on high ground near downtown.

(Brian van der Brug / Los Angeles Times)

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Water flows down the damaged main spillway of the Oroville Dam at 55,000 cubic feet per second into the Feather River. (Brian van der Brug / Los Angeles Times)

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Water from Lake Oroville flows down the damaged main spillway.

(Brian van der Brug / Los Angeles Times)

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Water from Lake Oroville flows down the emergency spillway of the Oroville Dam toward the Feather River.

(Brian van der Brug / Los Angeles Times)

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Pacific Gas & Electric crews aided by a helicopter removed transmission lines and insulators from towers standing in the bed of the emergency spillway of Lake Oroville.

(Brian van der Brug / Los Angeles Times)

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A rainbow appears over Feather River as water cascades down the damaged spillway at Lake Oroville Dam.

(Brian van der Brug / Los Angeles Times)

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Water cascades down the spillway below Oroville Dam. The water is being released by authorities to avoid flooding at Lake Oroville because of recent heavy rain. (David Butow / For The Times)

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Pacific Gas and Electric Co. crews move two electric transmission line towers on the bank of Feather River as a precaution if the Lake Oroville Dam emergency spillway needs to be used.

(Brian van der Brug / Los Angeles Times)

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A California Highway Patrol cruiser patrols Lake Oroville Dam, which is closed to the public due to the damaged spillway.

(Brian van der Brug / Los Angeles Times)

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Water cascades down the spillway below the Oroville dam. The water is being released by authorities to ease flooding in Lake Oroville because of recent heavy rain.

(David Butow / For the Times)

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Butte Country Sheriff Kory L. Honea speaks with department of water and power workers at an overlook as the observe runoff from the Oroville Dam.

(David Butow / For the Times)

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Water trickles down as workers inspect part of the Lake Oroville spillway failure in Oroville, Calif.

(Randy Pench / Associated Press)

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A boat launch at Bidwell Canyon is still hundreds of yards above the current lake level on Jan. 21, 2016.

(Brian van der Brug / Los Angeles Times)

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California State Park Ranger Bryan Taylor searches for signs of disturbance or theft as California’s severe drought conditions are revealing historic artifacts at Lake Oroville, June 21, 2014.

(Allen J. Schaben / Los Angeles Times)

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Severe drought conditions are evident as a family treks across a long path back to their car at Lake Oroville, June 21, 2014.

(Allen J. Schaben / Los Angeles Times)

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Houseboats are dwarfed by steep banks that show the water level down 160 feet from the high water mark at the Bidwell Bar Suspension Bridge over Lake Oroville on June 21, 2014.

(Allen J. Schaben / Los Angeles Times)

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Houseboats at Bidwell Canyon Marina at Lake Oroville, January 21, 2016.

(Brian van der Brug / Los Angeles Times)

By Rong-Gong Lin IIStaff Writer

Feb. 10, 2017 11 AM PT

Oroville Dam is the center of California’s massive water project, which sends water collected from the Sierra Nevada north of Sacramento south to cities and farms.

It’s an epic engineering project that now faces a crisis.

For the record:

8:46 a.m. May 7, 2024An earlier version of this story incorrectly listed the height of Oroville Dam as 7,700 feet. It is 770 feet tall.

What is the problem?

Flow from heavy rains has damaged the reservoir’s spillway, creating dramatic scenes of muddy torrents gushing over a concrete chute marred by a massive sinkhole.

Officials have stressed that the dam itself suffered no damage and that the spillway problems don’t pose a imminent threat to the public. Still, they have been frantically working to reduce the amount of water in the Lake Oroville reservoir, which is near capacity. It’s now at about 96% of capacity, and more water has been flowing in than is draining out.

If the reservoir exceeds capacity, it could damage the dam. Officials plan to avoid that, if necessary, by using an emergency spillway to release water. It’s unclear whether they will have to do that.

What are the contingencies?

The emergency spillway has never been used since the dam was completed in 1968.

Crews were clearing trees and brush from the emergency pathway Thursday. The state also moved a couple of million fish out of the downstream Feather River Hatchery because water from the eroded spillway is too muddy for hatchery use.

Right now, they continue to use the damaged spillway.

Why is Oroville Dam such an important part of the state’s water works?

The dam is the tallest in the United States, exceeding even Hoover Dam. It stands 770 feet, 44 feet taller than Hoover. Lake Oroville is the second-largest reservoir in the state, behind Shasta Lake, which is the center of the federal water project that serves California.

The lake takes in water from the major branches of the mighty Feather River.

If the state keeps sending water down the spillway, it will worsen the damage. (Feb. 9, 2017)

What caused the situation?

Northern California has seen the wettest rain season in years. And that has sent huge amounts of water into Lake Oroville. Data show water levels for this time of year have not been this high in decades.