San Francisco — California officials are frantically trying to rapidly reduce water levels behind Oroville Dam — the nation’s tallest — after issuing evacuation orders for more than 100,000 people who live downstream.
Here’s an explainer for the current crisis, and how authorities are racing to prevent catastrophic flooding from affecting the Central Valley.
What’s the biggest concern?
The biggest concern was that a concrete wall, built atop a hillside, that keeps water in Lake Oroville — California’s second largest reservoir — would suddenly crumble Sunday afternoon, threatening the lives of thousands of people by flooding communities downstream.
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With Lake Oroville filled to the brim, such a collapse could have caused a “30 foot wall of water coming out of the lake,†Cal-Fire incident commander Kevin Lawson said at a Sunday night press conference.
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)
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What’s so special about this concrete wall ?
This particular concrete wall was chosen to act as an emergency spillway for Lake Oroville — a pathway for excess water to drain when the reservoir is filled to the brim.
If Lake Oroville became too full, and a main concrete spillway couldn’t drain the reservoir fast enough, water would then empty out of this emergency spillway, flowing over this low concrete wall — known as a weir — washing past a roadway and down a tree-lined hillside.
The emergency spillway was designed so that water would always drain at this location when the dam was full. That would keep Oroville Dam, built at a different location, safe from being overtopped by water.
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So on Saturday, rising reservoir water put the emergency spillway to use for the first time since Oroville Dam was completed in 1968.
When did officials realize something was terribly wrong?
Officials had thought things were going fine until suddenly, on Sunday afternoon, authorities were alerted to what they described as erosion developing near the emergency spillway.
(Raoul Ranoa/Los Angeles Times)
The erosion was occurring so fast that officials feared the concrete wall would be undermined, and that it would cause a catastrophic release of water downstream. Officials said they couldn’t wait to act if the worst-case scenario struck, and ordered sweeping evacuations.
What has happened since the evacuations were ordered?
The situation has improved. There has been no catastrophic collapse of the concrete wall on top of the emergency spillway, which would have resulted in an overwhelming release of water downstream.
Officials decided to focus their efforts on draining the lake to funneling more water down the main spillway, a concrete slide that is supposed to be the primary way of draining a full Lake Oroville. The main spillway itself was hobbled last week, as parts of it began disintegrating as a giant pothole formed underneath a section.
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Authorities knew they had to take pressure off the emergency spillway, so they began sending more water down the main spillway late Sunday. They increased the flow from 55,000 cubic feet per second to 100,000 cubic feet per second, and hoped for the best.
They were in luck on Sunday night. Sunday’s increased flows did not appear to damage the main spillway further. And before midnight, the main spillway had drained the lake enough so that water was no longer flowing down the emergency spillway.
The dry weather early this week helped. As of Sunday night, only 40,000 cubic feet of water per second was flowing into Lake Oroville, and because 100,000 of cubic feet of water per second is flowing out, lake levels are being reduced.
Officials need to assess the emergency spillway this morning. Authorities haven’t been able to begin to make a fix.
And they’re hoping to drain as much water as they can before a new round of storms is forecast to arrive in Northern California later this week.
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If officials believe they can send an even faster torrent of water down the main spillway, they will, said Bill Croyle, the acting director of the California Department of Water Resources, at a late night news conference.
What level does the water in Lake Oroville need to get down to for officials to start feeling better?
At the height of the crisis on Sunday, the lake was topped out at 902 feet. Officials would like the lake to fall by an elevation of 50 feet.
Why is it a bad sign for officials to even need to use the emergency spillway?
It’s a bad sign because officials don’t have any ability to control how much water goes into the emergency spillway, as can be done with the main spillway.
Authorities want to keep a speed limit on how fast water flows from Lake Oroville down the Feather River, which runs past Oroville, Marysville and Yuba City before merging with the Sacramento River and heading to California’s capital.
California’s flood control system along Oroville can handle only a speed of 150,000 cubic feet of water per second. If water flows faster than that, catastrophic flooding can happen.
“We don’t like to press it to the edge if we don’t have to,†Croyle said.
Have officials had to push the Feather River to its limit before?
Yes, during the historic 1997 rains. There was flooding in and around the valley, “but we did pass water through this region without too much trouble,†Croyle said.
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What happened in 1997?
Several levee breaks were reported in the Sacramento and San Joaquin valleys. But it was nothing compared with the worst-case scenarios envisioned Sunday.
Is there any way this situation could have been avoided?
That’s a question many people will be wondering about for some time. Croyle was asked this question at Sunday night’s news conference: why didn’t officials increase the flow down the damaged main spillway earlier?
Croyle’s answer, essentially, was that officials were reacting to the best information they had at the time.
What were they reacting to?
The first situation officials reacted to was news of the damage in the main concrete spillway Tuesday. Officials stopped water draining out of the lake to inspect the damage, and studied it to see if they could fix it.
But “we determined we could not fix the hole,†Croyle said. It was 250 feet long, 170 feet wide, and almost 40 to 50 feet deep. There wasn’t enough time to keep the chute dry and fix it. Officials thought they had no choice other than to use the main spillway even in its crippled state, even though it would be further damaged by resuming its use.
The result is a balancing act — drain as much water as quickly as possible while trying not to further damage the spillway as much as can be helped. After all, the crippled main spillway needs to last for the remainder of the rainy season.
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By Friday, officials had held out hope that they wouldn’t need to use the emergency spillway. But then it rained Friday night.
“It came in a little wetter. The storm system parked over this region of California was parked a little longer,†Croyle said.
It was only after the emergency spillway was revealed to be in dire condition that officials roughly doubled the amount of water flowing out of the main spillway. By that point, luckily, the deterioration of the main spillway had largely stabilized, although there is still cause for concern.
How much time do officials have before the next rainstorm arrives?
The National Weather Service forecasts Monday and Tuesday will be dry and mild, allowing river levels to recede.
Light to moderate rain is expected Wednesday into Thursday, which is forecast to cause limited rising in rivers.
The situation gets worse by the weekend, however, as new storms arrive. The river could begin rising again as early as Friday, and area-wide flooding could happen between Sunday and Tuesday.
Rong-Gong Lin II is a Metro reporter based in San Francisco who specializes in covering statewide earthquake safety issues and the COVID-19 pandemic. The Bay Area native is a graduate of UC Berkeley and started at the Los Angeles Times in 2004.
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