Crews continue to repair erosion areas with a concrete mixture over large rocks below the Lake Oroville emergency spillway.  Ryan McKinney / California Department of Water Resources,
Crews continue to repair erosion areas with a concrete mixture over large rocks below the Lake Oroville emergency spillway.  (Ryan McKinney / California Department of Water Resources,)

Oroville Update: Evacuation Advisory Finally Lifted

Oroville Update: Evacuation Advisory Finally Lifted

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Update, 1:45 p.m., Wednesday, March 23: Nearly six weeks after downstream residents were ordered to flee their homes because of trouble with Oroville Dam's spillways, Butte County Sheriff Kory Honea has lifted all evacuation warnings and advisories.

Residents of Oroville were given just an hour to leave their homes on the afternoon of Feb.12. That was the day after Lake Oroville, rising rapidly after flood-control releases were reduced down the dam's main spillway, flowed over an ungated emergency weir. Severe erosion on the slope below raised concerns that the emergency structure would collapse and unleash a catastrophic flood down the Feather River.

Residents as far as 35 miles downstream were told to leave immediately, and an estimated 180,000 people in Butte, Sutter and Yuba counties were under evacuation orders. They were cleared to return home Feb. 14, but Butte County had remained under an evacuation advisory while the California Department of Water Resources worked to lower Lake Oroville, shore up the emergency spillway and clear a mountain of debris from the adjacent river channel. The blocked river channel had shut down the dam's hydroelectric power plant and further limited managers' ability to release water from the reservoir.

Wednesday, Sheriff Honea said he was satisfied with the progress of the DWR's work, which has employed an army of contractors and cost something on the order of $200 million to date.


In a statement, Honea acknowledged that the evacuation had been chaotic. Residents complained about not being notified they needed to leave, and there was at least one case in which a disabled Oroville resident was left behind for hours after the evacuation warning because no emergency transport was available.

"These past six weeks have been a very difficult and unsettling time for many individuals and families affected by the danger posed by fast-moving erosion to the emergency spillway," Honea said. "I couldn't be more proud of this community and the countless unsung heroes who helped their neighbors and cared for those who needed it most."

Honea's mantra at virtually every media briefing and public appearance over the last month and a half has been a request for residents to sign up for the county's emergency notification system. And despite lifting the evacuation advisory, county officials are working on developing new evacuation plans in case of a future emergency.

The county has designated 11 new flood evacuation zones, complete with assembly points and emergency departure routes, along the Feather River from Oroville to the town of Gridley. Officials are holding informational meetings in each zone.

The potential for future trouble with the Oroville Dam was highlighted in a report from a board of experts appointed to review the situation at the facility and oversee the process of repairing or rebuilding the spillway.

The report, obtained earlier this week by The Associated Press, says dam managers are facing a "very significant risk" if the main spillway is not operational in time for this fall's rainy season. The panel also said it's "absolutely critical" to avoid further flows over the emergency weir and down the hillside below.

The Department of Water Resources has placed thousands of tons of rock in eroded sections of the eroded hillside, "armored" sections of the slope with concrete, and built a series of walls and check dams to slow any flow of water down to the river channel below.

The current operational status of the spillway and reservoir: Releases down the damaged concrete structure continue at about 40,000 cubic feet per second. Water is also being released through two of the five operational units in the dam's hydroelectric plants, for a total flow of about 45,000 cfs.

DWR officials said when flows were resumed down the main spillway last Friday that they intend to lower the lake's surface elevation to between 835 and 838 feet above sea level. That would represent a drop of 26 to 29 feet from last week and would put the lake level 63 to 66 feet below the now-dreaded emergency weir. The agency said it plans to shut down spillway releases at that point to allow resumption of preliminary work to repairing or replacing the structure.

One aspect of that work started this week, with crews drilling for rock and soil samples near the spillway to assess underlying conditions.

Flood-control releases down the main spillway are just one part of the equation determining how fast the lake level drops, of course. The other principal factor is the amount of water flowing into the lake from the Feather River watershed, or "inflow."

Inflow peaked during the February spillway crisis at about 190,000 cfs. After a long run of mostly dry, clear and cool weather in the first half of March, it fell and leveled off between 15,000 cfs and 20,000 cfs. Now, with a series of storms marching through Northern California, inflow has periodically risen into the 45,000 to 50,000 cfs range -- meaning the lake's level has fallen very slowly, and some hours not at all, during the last several days.

One more big storm is expected in the week ahead -- a cold system that DWR forecasters say could drop 2 to 3 inches of rain or its snow equivalent on the Feather River basin over the weekend. Back to top.

Update, 1:45 p.m. Friday, March 17: The Department of Water Resources has reopened the Oroville Dam's badly damaged main spillway to make room for an expected surge of runoff amid a return of stormy weather and the onset of the spring runoff season.

The flow of water resumed down the spillway just after 11 a.m. Friday. Bill Croyle, the agency's acting director, said during a media briefing that flows would be increased to 50,000 cubic feet per second during the day. He said managers aimed to lower Lake Oroville, the state's second-largest reservoir, from its current surface level of 864 feet above sea level to between 835 and 838 feet. That would be 63 to 66 feet below the level of the dam's emergency spillway, which overflowed Feb. 11 and triggered a mass evacuation of Oroville and other communities along the Feather River.

Croyle said the relatively high rate of flow from Lake Oroville into the Feather River will continue for five or six days, depending on the amount of runoff coming into the lake. He said DWR would "continuously evaluate the condition of the flood-control spillway to see how it's performing, and then we'll make decisions during the week on how we'll step down from 50,000 to 40,000 (cfs) and ultimately back down to zero."

Croyle said dam managers anticipate they will have to conduct as many as three releases during the spring as snow in the higher elevations of the Feather River melts and flows into the lake.

After nearly three weeks of mostly dry, sunny weather, a series of storms is expected to roll across Northern California over the next week.

The weather systems are expected to start out relatively warm, with freezing levels beginning about 7,500 feet over the Feather River watershed that feeds Lake Oroville, then falling to 4,000 to 5,500 feet as the heavier storms move in next week. The colder storms mean most precipitation will fall as snow over the watershed and slow the rush of runoff into the reservoir.

Releases down the shattered spillway chute were halted Feb. 27 so crews could bring in heavy equipment to clear a mountain of rubble, rock and sediment from the adjacent river channel. At the same time, workers have been scrambling to reinforce what remains of the main spillway -- grouting and cementing cracks and seams, bolting sections of the spillway to underlying rock, and enclosing an eroded area at the lip of the surviving structure in concrete.

DWR says the work -- which at various points has meant marshaling a contractor army of helicopters, cranes, bulldozers, loaders, trucks and barges -- has cost about $4.7 million a day. If that figure is accurate, the effort to deal with the broken spillway and severe erosion below the dam's emergency weir has cost about $180 million so far.

DWR has advised residents of downstream communities that the increased releases will trigger a rise of 13 to 15 feel along the Feather River. That has renewed fears among farmers along the stream whose land suffered severe erosion when river levels fell rapidly in late February.

Brad Foster, who farms near the Yuba County town of Marysville, told the Los Angeles Times this week:

“My concern right now is erosion,” Foster said. “We have 100-year-old oak trees lying in the river. Everything that was there, old growth that protected the banks, it was just sucked in. … This is all going to go under water and it’s all freshly slipped material. This is all going to start eroding. We don’t know if it’s going to take the banks. … The river could actually start a new channel.”

... Foster said a 300-foot buffer zone of bluffs, trees and vegetation protecting his walnut orchard was wiped out and now the orchard sits in the path of future rising waters. Debris turned the river brown.

“I’ve never seen it so dirty in my life,” he said.

Back to top.

Update, 7 p.m. Friday, March 10: To bring us up to date before the weekend:

  • Power plant: During the course of the week, the Department of Water Resources put all five of the available turbines at the Oroville Dam's Hyatt powerhouse into operation. The result: Releases from Lake Oroville, which had been halted Feb. 27 to allow crews to clear rock, rubble and mud from the river channel below the dam's devastated main spillway, have increased from 0 to about 13,000 cubic feet per second.
  • Lake level: The surface elevation of Lake Oroville, California's second-largest reservoir, is hovering right around 860 feet. That's 41 feet below the emergency spillway weir and right at the level that Bill Croyle, the DWR's acting chief, said last month the agency would consider restarting flows down the main spillway in order to maintain space in the reservoir for any incoming floodwaters. But runoff into the lake has remained modest as Northern California gets a prolonged break from rain and snow, and no new releases down the main spillway have been mentioned. On the other hand, much warmer weather over the next week in the Feather River basin could begin to melt the region's abundant snowpack and renew a rise in lake levels.
  • Main spillway: Crews have been engaged in patching and caulking cracks and holes along the surviving section of the concrete spillway and have also applied spray-on concrete -- shotcrete -- to a section under the concrete chute that showed signs of further erosion. That work is aimed at ensuring the structure can endure further releases without further major erosion.
  • Emergency spillway: Contractors continue to armor eroded areas below the dam's emergency weir, the slope where serious erosion the weekend of Feb. 11-12 threatened to undermine the weir and unleash a wall of water down the Feather River. The work now involves building a series of channels and check dams to slow the flow of water down the hill, should Lake Oroville go over the top of the weir again.
  • Debris removal: DWR has estimated that 1.7 million cubic yards of rubble, enough to cover a football field to a depth of 80 stories, would up in the river channel below the spillway. To get the Hyatt powerhouse running again, it was necessary to at least partially clear the channel. Friday, the agency said the army of contractors working on the job have removed about half the debris out of the channel.
  • Costs: A frequently asked question -- how much is this whole Oroville spillway emergency project costing the taxpayers? Here's an answer, by way of the Chico Enterprise-Record: $4.7 million a day. The details:

    Regarding estimated daily cost of labor, we’re focused on emergency response and recovery efforts. It would be premature to estimate costs at this time,” DWR public information officer Lauren Bisnett wrote in an email Monday.

    Representatives from the California Office of Emergency Services and the state’s Finance Department previously told this newspaper DWR was accountable for keeping track of the costs for the project.

    On Wednesday afternoon, Assemblyman James Gallagher, R-Yuba City, said he was expecting to hear about costs accrued, as the DWR met with the Federal Emergency Management Agency earlier Wednesday to discuss repair and maintenance costs related to damage of the spillways.

    Curtis Grima, Gallagher’s chief of staff, later said in an email that according to conversations with DWR officials, the estimated daily average cost is $4.7 million.

    It is estimated that between 75 percent-90 percent of the cost will be reimbursed by FEMA, Grima’s email said.

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Update, 4:35 p.m. Monday, March 6: The Department of Water Resources reopened the Oroville Dam hydroelectric plant at about 6 p.m. Sunday -- after suspending operations for 32 hours to allow crews to deepen the river channel downstream of the plant.

As of Monday afternoon, just one of the plant's five available turbines was running, resulting in a release of about 1,750 cubic feet per second. The water agency hopes to get all five units running soon, which would increase outflow from Lake Oroville to somewhere in the range of 13,000 to 14,000 cfs (DWR has cited both figures).

The reason the esoteric water release data is important: Higher flows through the powerhouse will allow the agency to limit the reservoir's rise as work continues on assessing the devastated main spillway and clearing debris from the river channel, formally known as the Thermalito Diversion Pool, below the shattered concrete structure.

Lake Oroville's surface level at 4 p.m. Monday was 856 feet above sea level. That's 45 feet below the top of the problematic emergency spillway, where an overflow and severe erosion prompted a mass evacuation of Oroville and other communities downstream on Feb. 12. And it's 18 feet above the lake level a week ago, when flows were halted down the main spillway.

DWR estimates the pile of debris in the channel to be a shocking 1.7 million cubic yards. That's mostly rock blasted out of the terrain beneath and adjacent to the main spillway by emergency reservoir releases that reached a maximum of 100,000 cfs after the emergency spillway crisis. So far, the water agency says, a force of contractors driving cranes, bulldozers, heavy trucks and barges has removed about a quarter of the material to spoils sites on land along the river channels. Back to top.

Update, 1:30 p.m. Saturday, March 4: Friday, the Department of Water Resources declared that resuming operations through the Hyatt Power Plant at the base of Oroville Dam marked a "pivot point" in the effort to get a handle on water levels in Lake Oroville and to proceed with the immense job of recovering from the failure of the dam's main spillway.

After hearing a declaration like that, you might involuntarily say "uh oh," when what you've been told is a big step forward is interrupted without explanation.

That was the case at midday Saturday. The power plant, which gives dam operators a way to let some water out of the reservoir and allows the closure of the crippled main spillway to continue, had been releasing a relatively modest but steady 2,500 cubic feet per second late Friday and early Saturday.

The water was coming through one of the power plant's five available turbines. DWR Acting Director Bill Croyle said the agency planned to have all five running by early next week, which would allow a release of about 14,000 cfs -- enough to minimize rises in the lake during a period of relatively low inflow from the Feather River watershed.

But Saturday, flows through the power plant stopped without a prior announcement. And that led to social media "uh oh" moments like this:

Then, just after midday, DWR announced in a press release that it had shut down the powerhouse again. The reason: Crews need to remove more of the rock, rubble and sediment from the debris-choked channel downstream of the power facility to allow it to operate full bore. From the release:

“We will dig deeper so we can fully ramp the plant up,” said DWR Acting Director Bill Croyle.

Initial flow from the plant on Friday was 1,750 cubic feet per second (cfs) and increased to 2550 cfs. Once fully operational, the plant can release up to 14,000 cfs, which is important for managing reservoir inflows and outflows through the spring runoff season.

DWR engineers have determined that further deepening of the channel will help the power plant reach full capacity and that it will take approximately 1-2 days, at which time the plant will be restarted.

We've asked for but haven't yet gotten details on how much more excavation needs to be done to prepare the channel for full operation of the power plant.

Meantime, the San Francisco Chronicle's Kurtis Alexander reports a serious problem down the Feather River from the dam.

Steep banks along the river started collapsing after DWR abruptly cut flows down the damaged spillway on Monday from 50,000 cfs to zero. Releases into the river have continued from smaller reservoirs near Oroville, but the Feather River is now flowing at something like a summertime rate of 2,500 cfs.

Here's the result, Alexander reports:

With high water no longer propping up the shores, the still-wet soil crashed under its own weight, sometimes dragging in trees, rural roads and farmland, they said.

“The damage is catastrophic,” said Brad Foster, who has waterfront property in Marysville (Yuba County), about 25 miles south of Lake Oroville.

The farmer not only saw 25-foot bluffs collapse, but also lost irrigation lines to his almonds. “When the bank pulled in,” he said, “it pulled the pumps in with it. It busted the steel pipes.”

Officials at the state Department of Water Resources, which runs the dam, said Friday that they’re monitoring the river for erosion. But they declined to discuss the situation.

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Update, 2:25 p.m. Friday, March 3: The Department of Water Resources halted flows down the shattered main spillway at Oroville Dam earlier this week with one purpose in mind: to begin clearing the monstrous pile of concrete, rock and sediment washed into the river channel below the spillway. That work, in turn, would allow the channel's water level to drop and allow the hydroelectric plant at the base of the dam to resume operations. (How monstrous is that debris pile? We'll get to that.)

Friday, the agency said it's making progress. The water level in the channel, which serves as a tailrace for the hydro plant and is formally called the Thermalito Diversion Pool, has fallen 22 feet over the last several days. That allowed dam managers to start up one of the plant's five available turbines, and they aim to have all of those units online by early next week.

"Pretty exciting day for us," Bill Croyle, DWR's acting director, said during a midday media briefing in Oroville. "This is a pivot point in how we are managing the inflows to the river (and) the reservoir elevation."

The crucial point there: Running water through the power plant gives DWR a route other than the partially obliterated main spillway of releasing water from Lake Oroville.

Keeping water moving down the river also allows the agency to maintain the flow of water for several fish species, including juvenile chinook salmon that have started making their way down the Feather River on their way to the Delta and the Pacific Ocean. The abrupt halt to flows from the spillway earlier this week led to the stranding of both adult and juvenile fish downstream from Oroville.

With one turbine running, about 1,700 cubic feet of water is being discharged through the powerhouse. DWR says that rate will rise to 14,000 cfs when all five available units are online.

Having that water exiting the lake will help balance inflows -- which have stayed in the 14,000-20,000 cfs range most of the week since -- and slow the lake's rise while work continues to clear rubble from the river channel and assess the terrain around the badly damaged spillway.

And now, about that big pile of debris: DWR estimates it's about 1.7 million cubic yards. A cubic yard, as everyone knows, is a cube measuring 3 feet by 3 feet by 3 feet, or 27 cubic feet. How much material is 1.7 million of those cubes?

Our calculations, using our handy cultural reference of a football field -- 120 yards long and 53.33 yards wide: 1.7 million cubic yards would be enough to bury a football field to a depth of 797 feet. That's a little higher than San Francisco's Bank of America building (779 feet).

Update, 2:20 p.m. Monday, Feb. 27: The state Department of Water Resources has, as promised, halted flows down the damaged main spillway at Oroville Dam. Even if you've been following the progress of this incident since it began Feb. 7, and even if you understood the damage to the spillway was catastrophic, the first images of the structure are sobering.

DWR stopped the release of water down the spillway early Monday afternoon with two main goals in mind.

First, it wants contractors to begin the task of removing a staggering amount of rubble, rock and sediment that have wound up at the bottom of the river channel below the spillway. Clearing the debris, in turn, will allow dam managers to resume operations at the hydroelectric power plant at the base of the dam, a facility that was shut down as water rose behind the blockage in the channel.

Second, shutting down the flows will allow geologists and other experts to inspect the shattered spillway structure and the surrounding terrain. That will give DWR officials a better understanding of the work ahead in designing a replacement spillway and the potential for further erosion when flows down the current spillway resume.

The amount of material to be removed from the channel, parts of which are 70 to 80 feet deep, is immense.

"There are a lot of numbers being thrown around, anywhere from 150,000 cubic yards all the way up to a million," DWR Acting Director Bill Croyle said in an interview Monday.

He said with flows down to zero, laser mapping technology will be used to assess just how much debris now obstructs the channel.

"I suspect it's going to be between a half-million and a million cubic yards," Croyle said. "But again we won't know until that mapping tomorrow."

(A million cubic yards, if you're keeping score at home, would be enough material to cover five football fields, complete with end zones, to a depth of 100 feet.)

Croyle said contractors have been tasked with clearing a channel 30 feet deep, 150 wide and 1,500 feet long to help facilitate flows below the dam.

The water level in the Thermalito Diversion Pool early Monday was about 20 feet high to allow operation of the turbines in the dam's hydroelectric powerhouse. Getting the turbines back online will give water managers another way of releasing water from Lake Oroville, the state's second-largest reservoir, as the spring runoff season begins. Back to top.

Update, 4:25 p.m. Sunday, Feb. 26: Having drawn down the level of Lake Oroville 60 feet in the two weeks since a spillway emergency that triggered mass evacuations, and with the prospect of mostly dry weather for at least the next week, state water officials announced Sunday they will halt flows down Oroville Dam's badly damaged main spillway to speed up recovery work there.

The California Department of Water Resources said it would reduce reservoir releases from 50,000 cubic feet per second to zero during the day Monday.

DWR says stopping the flow of water down the main spillway will allow workers to "aggressively attack" a mountain of rubble that now lies submerged in the Feather River channel immediately below the broken concrete chute.

The blockage in the channel, formally called the Thermalito Diversion Pool, has caused water to back up to the hydroelectric power plant at the base of the 770-foot-high dam. That high water, in turn, has forced officials to suspect operations at the plant.

Flows down the main spillway were as high as 100,000 cfs -- 750,000 gallons a second, enough to supply four average California households for a year -- after an emergency at the dam earlier this month.

Damage to the spillway was detected on Feb. 7, just as a series of storms triggered a huge surge of runoff into Lake Oroville, the state's second-biggest reservoir.

With flow rates into the lake peaking at about 190,000 cfs, releases down the damaged spillway were limited to a maximum of 55,000 cfs. The result: The lake rose nearly 50 feet in just four days and, for the first time since Oroville Dam went into service in 1968, flowed over an emergency weir on Feb. 11.

The water cascading over the ungated 1,730-foot-long weir rapidly eroded the adjacent slope. Less than 36 hours after the flow began over the weir, officials became concerned that the erosion was undermining the massive weir structure -- a collapse of which could unleash a devastating surge of water. That concern led to the mass evacuation of Oroville, the town of 16,000 just downstream of the dam, and about 180,000 people along the Feather River in Butte, Sutter and Yuba counties.

The emergency prompted federal dam safety authorities to order the Department of Water Resources to immediately form a panel of experts to investigate the cause of the main spillway failure and the performance of the emergency spillway. The federal order directs DWR to report to the panel throughout the process of designing and building a replacement for the main spillway and enhancements for the emergency spillway. Back to top.

Update, 8:15 a.m. Wednesday, Feb. 22: With runoff from our most recent spate of stormy weather dwindling, it appears that Lake Oroville's level is also falling again. According to Department of Water Resource's hourly data, the reservoir surface peaked at 852.93 feet above sea level at 5 a.m. and had fallen to 852.89 feet by 8 a.m.

OK, that's not much -- the decline amounts to a half-inch, a change imperceptible to all but the DWR's instruments. Overall, though, the lake is about 48 feet below the edge of Oroville Dam's emergency spillway and 4 feet above the low point it reached Monday amid managers' efforts to restore space in the reservoir to receive incoming floods.

A couple of notable Wednesday news pieces on the Oroville situation:

Back to top.

Update, 12:05 p.m. Tuesday, Feb. 21: Lake Oroville is on the rise again in the wake of a series of storms that soaked most of the state.

The rise, however, is very gradual. The lake remains 49 feet below the top of the emergency weir at the center of the Oroville Dam crisis that resulted in the Feb. 12 evacuation order for about 188,000 people in Butte, Sutter and Yuba counties.

The 120 hours of weather systems that culminated in the very wet Presidents Day storm dropped as much as a foot of precipitation -- rain or its snow equivalent -- in the Feather River watershed upstream of Lake Oroville. The gauge at Oroville Dam recorded 4.04 inches.

The precipitation triggered a spike in runoff into the giant reservoir. The volume of water flowing in had remained in the range of 15,000 to 45,000 cubic feet per second for most of the week. On Monday, though, it increased to as much as 90,000 cfs. That's 673,000 gallons, or 2 acre-feet per second -- enough water to supply about four average California household for a year.

Dam managers reduced the volume of water going down the facility's damaged main spillway from 100,000 cfs last week to about 60,000 cfs. The lower level allows crews to begin the work of clearing rubble, rock and sediment from the channel below the main spillway. That work, in turn, is designed to allow the hydroelectric power plant at the base of Oroville Dam to resume operations.

DWR has been quick to point out in each and every press release on the situation that work continues to "armor" and reinforce the severely eroded hillside below the emergency weir. That erosion occurred when floodwaters flowed across the structure for the first time since the dam was finished in 1968. Back to top.

Update, 1:45 p.m. Sunday, Feb. 19: The significant weekend news at Oroville Dam: The Department of Water Resources decreased flows down the damaged main spillway to 55,000 cubic feet per second on Saturday, then announced it would ramp them up again, to 60,000 cfs, on Sunday afternoon.

Those levels are far lower than the 100,000 cfs released down the spillway starting a week ago, amid fears that the dam's emergency spillway system was about to fail. Those very high flows, maintained for four straight days, helped lower the lake from a foot above the 1,700-foot emergency weir last Sunday afternoon to 50 feet below it.

The flow reductions over the last couple of days were intended to help crews assess how much rubble, rock and sediment has been swept into the 80-foot-deep channel beneath the main spillway and begin the process of removing it. The debris has dammed the channel and made it impossible to use the hydropower plant at the base of Oroville Dam.

Incoming weather will no doubt play a part in releases over the next several days, with a storm expected to drop 8 inches or more of water by early Wednesday on the Feather River watershed above Lake Oroville. Snow levels are forecast to remain low, however, which will help slow down runoff into the reservoir.

Below: DWR drone video showing the state of work to reinforce the badly eroded slope beneath the emergency weir, as well as the condition of the main spillway as of Saturday afternoon.

Back to top.

Update, 3:25 p.m. Friday, Feb. 17: To start with the numbers: Department of Water Resources data show that despite cutting back releases down Oroville Dam's shattered spillway and the return of storms to the Feather River basin, Lake Oroville continues to empty.

At 3 p.m. Friday, DWR's running statistics on the reservoir show that its surface is now a little more than 42 feet below the lip of the dam's emergency spillway. The lake is falling at a rate of roughly 3 to 4 inches an hour.

The second in a series of winter storms arrived in the region on Friday, dropping moderate amounts of rain and snow on the 3,600-square-mile Feather River watershed. Forty-eight-hour rain totals in the area ranged from 1.36 inches at Oroville Dam to 2.44 inches at the Humbug gauge in the mountains north of the reservoir.

Thursday, DWR cut releases from Lake Oroville from 100,000 cubic feet per second to 80,000 cfs. The reduction was designed to give crews a chance to begin removing the mass of concrete rubble, rock and sediment that tumbled into a channel that issues from the bottom of the dam. The agency said Friday it would cut spillway flows further -- down to 70,000 cfs -- as part of the effort to clear the channel.

As we've reported every day this week, work continues to repair erosion damage to the hillside below the dam's emergency spillway structure. That erosion, which occurred when the water rose above the weir at the top of the emergency spillway and gouged out huge sections of the slope below as it rushed downhill, prompted last Sunday evening's mass evacuation from Oroville and communities as far as 35 miles downriver from the dam.

Butte County Sheriff Kory Honea reiterated during a press briefing Friday that those who live downstream from the dam need to be prepared to leave if trouble recurs at the dam.

"The likelihood (of trouble) is low," Honea said. "But -- and I don't want to sound like a broken record, but that's my job. My job is to keep people prepared. So they've got to pay attention, they've got to be vigilant, they've got to be prepared, they've got to sign up for their emergency warning notification system. And if you're tired of hearing my say that, I'm sorry, but I'm going to keep saying it until this situation is well past us."

Honea also addressed again a question that has arisen in the aftermath of last weekend's evacuation: Whether Oroville or other communities in the evacuation zone had experienced looting after people left town.

The sheriff has said while there had been burglaries and thefts during the roughly 48-hour evacuation, there had been no looting. Friday, he clarified that a little.

"Now that my staff has had a better opportunity to talk with me, we find that a couple of those burglary- or theft-related crimes, we can charge ... the individuals responsible with an enhancement of looting," Honea said. He did not immediately offer specific details of those episodes. Back to top.

Update, 3:20 p.m. Thursday, Feb. 16: The Feather River watershed has gotten its first dose of rain and snow from an expected series of storms, with moderate amounts of precipitation that haven't yet caused a major increase in flows into the reservoir behind Oroville Dam.

For the 12 hours ending at 9 a.m., precipitation totals ranged from about a half-inch at the dam itself to 1.50 inches near Bucks Lake, in the higher country of the Feather River watershed..

At the same time, the Department of Water Resources announced today it was reducing flows down the dam's main spillway as crews get ready to remove the large volume of debris that has fallen into the channel below.

Rubble from the main release structure, and rock and sediment eroded from the adjacent slope, have filled the 80-foot channel immediately below the spillway chute.

DWR reduced the spillway flows from 100,000 cubic feet per second to 80,000 cubic feet per second.

The 100,000 cfs rate, which commenced Sunday afternoon as fears mounted that the dam's emergency spillway system might fail and unleash an uncontrolled surge of water down the Feather River, helped lower the lake's level 34 feet over the past four days.

The DWR has said the reduced releases will be sufficient to continue lowering the lake and make room for runoff from future storms and snowmelt.

The wettest storm in the series of storms that began Thursday is expected to arrive Monday. One precipitation forecast, from NOAA's California-Nevada River Forecast Center, says that system could drop as much 6 inches of water -- either rain or snow -- on the higher elevations of the Feather River watershed. Back to top.

Update, 4:30 p.m. Wednesday: Officials raced to drain more water from Lake Oroville as new storms began rolling into Northern California on Wednesday.

The three storms were expected to stretch into next week. Forecasters said the first two storms could drop a total of 5 inches of rain in higher elevations.

However, the third storm, starting as early as Monday, could be more powerful.

"There's a potential for several inches," National Weather Service forecaster Tom Dang said. "It will be very wet."

Nonetheless, California Department of Water Resources chief Bill Croyle said water was draining at about four times the rate that it was flowing in and the repairs should hold at the nation's tallest dam.

About 100,000 cubic feet of water was flowing from the reservoir each second, enough to fill an Olympic-size swimming pool.



Croyle said work crews had made "great progress" cementing thousands of tons of rocks into holes in the spillways.

"We shouldn't see a bump in the reservoir" from the upcoming storms, he said.

The reservoir has dropped 20 feet since it reached capacity Sunday. Croyle said officials hope it falls 50 feet by this Sunday.

Still, officials warned residents who have returned to their homes that the area downstream of the dam remained under an evacuation warning and they should be prepared to leave if the risk increases.

KQED's Dan Brekke hosted a live Facebook video below the Oroville Dam spillway earlier Wednesday:

Update, 2:45 p.m. Tuesday, Feb. 14: The evacuation order affecting about 180,000 residents along the course of the Feather River below Oroville Dam has been reduced to a warning, allowing residents to return to their homes, Butte County Sheriff Kory Honea said during a press conference.

“Taking into account the current level of risk, the predicted strength of the next round of inclement weather and the capacity of the lake to accommodate increased inflow associated with those storms, we have concluded that it is safe to reduce the immediate evacuation order currently in place to an evacuation warning,” Honea said.

The Department of Water Resources indicated during the conference that the inflow of water to the reservoir continues to drop and that about 100,000 cubic feet of water per second is being released.

“We’re continuing to make significant gains in removing water from the reservoir,” acting DWR chief Bill Croyle said.

DWR officials said the goal is to get the level of the reservoir down to flood control storage, which is about 850 feet. Back to top.


Update, 8:15 a.m. Tuesday, Feb. 14: Large-scale releases of water continue at Oroville Dam, and the level of the giant reservoir there has dropped to about 12 feet below the emergency spillway structure that engineers believed was on the verge of failure on Sunday.

The Department of Water Resources and other agencies are continuing to assess the condition of the slope below the dam, parts of which were scoured down to rock by the force of water rushing over the emergency release structure over the weekend.

The crisis was triggered a week ago, when serious damage to the dam's main spillway was detected just as runoff began cascading into the nearly full lake after a series of wet, warm storms.

Gov. Jerry Brown has issued an emergency declaration to help speed up state agencies' response to the Oroville crisis. On Monday, he told reporters at a Sacramento-area media briefing with emergency officials that he's confident the Trump administration will respond promptly to the state's requests for aid.

Responding to questions about whether the Department of Water Resources should have done more to reinforce the emergency spillway system -- as suggested by environmental groups during a 2005 relicensing process -- Brown said:

"Every time you have one of these disasters, people perk up and start looking at analogous situations and things that you might not have paid as much attention to. But we live in a world of risk – the earthquake shook the Bay Bridge, and then we the state and all the different governors had to put up a new bridge."

Tuesday morning, 180,000 people remain evacuated along the course of the Feather River in the east-central Sacramento Valley. At a media briefing Monday, Butte County Sheriff Kory Honea said the evacuation order, issued hurriedly on Sunday, would be in place until agencies handling the situation at the dam say the danger of a catastrophic emergency spillway failure has passed.

A series of storms expected to begin rolling across Northern California on Wednesday night are expected to trigger a new rise in Lake Oroville -- the reason dam managers are continuing to try to lower the lake as fast as the damaged main spillway will allow. Back to top.

Update, 1:20 p.m. Monday, Feb. 13: Here are four big takeaways from the Department of Water Resources (with other local officials' noontime briefing on the situation at Oroville Dam:

  • First: The evacuation order that forced 180,000 people from their homes on Sunday will remain in place for now. Butte County Sheriff Kory Honea, whose jurisdiction includes the dam and the communities immediately downstream, said he is depending on the advice of "subject-matter experts" from the DWR and other agencies before people are allowed to return home.
  • Second: The desperate effort to lower Lake Oroville's level after an imminent failure of the dam's emergency spillway continues. With water pounding down the severely damaged main spillway at nearly 100,000 cubic feet per second -- that's about 750,000 gallons, for those of us who don't measure water in cubic feet -- the giant reservoir is falling at about 4 inches per hour and is now about 5 feet from the top of the emergency spillway.
  • Third: Dam and water managers are preparing for the resumption of winter storms over the Feather River watershed above Lake Oroville. The DWR's 10-day precipitation forecast, based on analysis of weather models, suggests that the next round of storms will be much colder and drop less than half the precipitation than the very warm weather systems that helped trigger the Oroville crisis.
  • Fourth: The DWR and other state and federal agencies are going to face very tough questioning about whether something should have been done years ago to shore up the emergency spillway structure and adjacent hillside. Those questions will be prompted by a story by KQED Science Managing Editor and San Jose Mercury News reporter Paul Rogers, who details concerns raised about the soundness of the emergency spillway system back in 2005.

Back to top.

Update, 5:40 p.m. Sunday, Feb. 12: Officials say the emergency spillway at Oroville Dam could fail at any time and are ordering evacuations from Oroville to Gridley.

The California Department of Water Resources urged residents of Oroville to head north, toward Chico. Residents elsewhere downstream should follow the orders of their local law enforcement, the department said. Officials have set up an evacuation shelter at the Silver Dollar Fairgrounds in Chico.

The emergency spillway is separate from the main dam structure. It's a massive, ungated concrete weir that stretches for one-third of a mile to the north of the dam and began overflowing Saturday morning. Below an initial concrete lip, water courses over bare earth all the way to the river channel below, scouring the slope of earth, rocks and trees.

Erosion on the hillside has increased beyond expectations. Oroville Dam contains California's second-largest reservoir, and is currently holding back more than 3.5 million acre-feet of water.

Update, 9:40 a.m. Sunday, Feb. 12: After rising to record high levels, the water level in Lake Oroville appears to be dropping.

Data from the California Department of Water Resources -- see real-time Lake Oroville levels here -- show the reservoir's surface crested at 902.59 feet above sea level at 3 a.m. Sunday.

With the volume of runoff into the lake decreasing and about 500,000 gallons of water flowing out of the lake every second down the badly damaged main spillway and the emergency outlet, reservoir levels had dropped to 902.39 feet by 9 a.m. That drop is equivalent to about 2.5 inches.

The lake is considered full at 901 feet, and it's at that level that it began pouring over an emergency spillway early Saturday. The emergency outlet is being used for the first time since the dam went into operation in 1968.

DWR managers say water should stop flowing over the emergency spillway sometime Monday. Back to top.

Update, 4:45 p.m. Saturday, Feb. 11: The real news of this afternoon came from a media briefing with acting Department of Water Resources chief Bill Croyle, who gave new details about the work ahead to replace Oroville Dam's shattered spillway.

Before we get to that, though, let's take a glance once more at Lake Oroville, which has continued to rise and spill over on this sparkling midwinter Saturday. The giant reservoir, California's second-largest, is now a foot over the dam's never-before-used emergency spillway.

DWR officials say that with several days of dry weather in store and the volume of runoff dropping, they expect water to continue to flow over the emergency weir until sometime Monday. Video posted Saturday afternoon (see below) showed a muddy, debris-laden torrent pouring into the waterway below the spillway.

At his noon-hour media briefing, Croyle said the damaged main spillway will need to be completely rebuilt. He said he told Gov. Jerry Brown in a discussion on Friday the cost would come to $100 million to $200 million.

"My objective is to get a spillway back in operation before the wet season next year, which is typically Oct. 15 or so," Croyle said.

Croyle said he can only give "a very rough range" of the eventual cost because of the many unknowns involved in the project, including exactly where the replacement spillway will be built.

"We haven't gone in and looked at it, we don't know how much more damage we're going to do, decisions have to be made on a new one ... so the range is huge," Croyle said. "What we told the governor yesterday afternoon is a hundred to two hundred million. Again, with the caveats we don't know a lot about the site itself."

He added that while the agency has the resources it needs to carry out the new spillway project and associated cleanup and repairs, he's hoping for support from the federal government.

Croyle said dam managers face a long, complex juggling act to deal with the impact of the spillway failure amid a continuing very wet winter.

One of the biggest challenges engineers and work crews face is how to clear the Thermalito Diversion Pool immediately below the wrecked spillway of a large volume of concrete debris and sediment that have dammed the waterway and forced closure of the hydroelectric plant at the base of Oroville Dam.

Muddy water rose and backed up toward the powerhouse as the lower section of the main spillway disintegrated under high flows. To avoid contaminating the power facility, it was shut down early Friday. That had an unfortunate side effect: Outflows through the plant, which can handle a maximum of 12,000 cubic feet per second, were halted. That, in turn, limited the amount of water managers could release from the fast-filling reservoir.

To remove the debris blocking the waterway, Croyle said, flows down the damaged main spillway will probably need to be halted temporarily. With another series of storms forecast to arrive in the region starting Thursday, that's not something that can be done immediately.

One piece of good news about the forecast, though: The next round of storms is expected to be colder, meaning they are far less likely to unleash the torrents of runoff produced by the last group of extremely warm weather systems.

Update, 11:30 a.m. Saturday, Feb. 11: Floodwaters began flowing over Oroville Dam's emergency spillway early Saturday morning.

It's the first time since the dam went into operation in 1968 that the emergency outlet from Lake Oroville has been used. The lake filled rapidly this week after severe damage to the main spillway forced dam managers to decrease the volume of water being released at the same time a series of warm storms triggered heavy runoff into the reservoir.

California Department of Water Resources officials said water began moving over the 1,700-foot-long emergency weir just before 8 a.m.

TV helicopter video soon after showed sheets of water cascading over the concrete structure, although heavy flows did not appear to have begun downhill.

And the lake continues to rise. By 11 a.m., the reservoir's surface was 901.55 feet, 6 inches over the top of the emergency spillway.

DWR spokesman Doug Carlson said the rate of flow over the auxiliary release structure was expected to increase from an estimated 660 cubic feet per second at 9 a.m. to 6,000 to 12,000 cubic feet per second.

He said dam and water managers estimate the flow will continue for 40 to 56 hours -- a time frame that runs roughly between midnight Sunday and 4 p.m. Monday.

Update, 12:35 a.m. Saturday, Feb. 11: Anyone who's been watching the numbers associated with Oroville Dam and Lake Oroville this evening -- how much water is flowing into the lake, how much is flowing out through the partially destroyed spillway -- probably has come to a conclusion similar to this one: At some point during the next few hours, water from the state's second-largest reservoir is likely to start pouring across the dam's emergency spillway and start racing down an adjacent slope toward the waterway below.

At midnight Friday, Lake Oroville had risen to within about 18 inches of the lip of the emergency spillway. With water still coming into the lake from the Feather River watershed faster than it can be released down the damaged spillway, the level is rising at about 3 inches per hour. At that rate, simple spectator arithmetic tells you that the lake will overtop the emergency spillway as early as 6 a.m. Saturday.

Dam managers with the Department of Water Resources had calculated releasing 65,000 cubic feet of water per second down the damaged spillway would slow the lake's rise enough to keep water from reaching the emergency structure. Those hopes dimmed Friday evening when releases were cut to 55,000 CFS to lower the risk of erosion that would threaten the stability of nearby power line towers.

DWR is unable to use another release point in the dam, a hydroelectric generating station that can handle another 12,000 CFS. Debris from the shattered spillway wound up in the channel just downstream from the power plant, causing water to back up and forcing officials to shut it down.

This would mark the first time water has flowed over the emergency facility since the dam began operating in 1968. (The closest call since then: June 2011, when late-season runoff from a lush snowpack brought the lake to within 15 inches of the emergency spillway.)

Beyond the history, the event brings uncertainty about what happens next. Crews from DWR, Cal Fire and private contractors scurried over the landscape immediately below the emergency weir over the last two days, trying to prepare the way for the cataract that soon might be pouring down the slope. Preparations included clearing trees and brush and cementing boulders into place at the edge of the emergency spillway. (See KCRA-Channel 3's helicopter footage of the scene Friday afternoon.)

To corral any debris that comes tumbling down the slope as the water comes down, log booms have been placed in the channel below the spillway (a waterway known as the Thermalito Diversion Pool) with crews ready to tow large objects to a nearby cove.

Of the highest immediate concern to people residing downstream is whether the water coming over the emergency spillway will represent a flood threat. The Department of Water Resources says it will not.

Longer term, the deeper interest will be finding out whether DWR did everything it could and should have to ensure the integrity of the spillway, and what it will do to design and build a repaired structure.

And now, just after midnight early Saturday morning, we'll sign off by saying: We'll see what happens after day breaks. Back to top.

Update, 12:45 p.m. Friday, Feb. 10: State Department of Water Resources officials now say they believe the volume of water rushing into Lake Oroville is slowing enough -- and releases down a badly damaged spillway have increased enough -- that the giant reservoir will not flow over an emergency spillway as feared.

Dam managers increased the flow of water down the broken main spillway to 65,000 cubic feet per second -- 486,000 gallons -- in the early morning hours Friday. While department officials say damage to the structure is continuing, the erosion does not appear to pose a threat to the spillway gates or other critical infrastructure.

At the same time, DWR officials noted at a noon media briefing, runoff into the lake is decreasing. The inflow hit a peak of 190,000 cubic feet per second Thursday evening and had fallen to 130,000 by midnight Friday.

The difference between the inflow and outflow means the lake is still rising -- about 4 inches per hour at noon. Lake Oroville's surface is about 5 feet below the lip of the emergency spillway. But DWR officials say with rains having stopped for the time being, the volume of water coming into the lake should continue to drop and the lake's rise will stop short of overflowing.

Update, 9:20 a.m. Friday, Feb. 10: Two things have changed overnight at Oroville Dam and the giant reservoir behind it.

First: Inflow from the Feather River watershed into Lake Oroville, while still very high, has dropped from its peak levels Thursday.

Second: California Department of Water Resources managers followed through with a plan to ramp up releases down the dam's wrecked spillway (for their rationale for doing that, see our earlier updates, below).

The rate of rise in the lake -- see the DWR's real-time data for yourself --has decreased from nearly a foot an hour at times Thursday to about 4 or 5 inches an hour Friday morning. The reservoir surface at 9 a.m. was reported to be 895 feet -- up 45 feet from Tuesday when the spillway damage was discovered and just 6 feet below the dam's emergency spillway.

The net result: That rate of increase would mean water from the reservoir would begin cascading over the emergency spillway sometime early Saturday morning. The lake, which has a stated maximum capacity of 3.5 million acre-feet, is now 98 percent full.

Mostly light rain and snow are expected across the Feather River watershed today before clear weather Saturday. Colder weather and a break from heavy rain could help reduce the volume of water flowing into the lake. Back to top.

Update, 7:15 p.m. Thursday, Feb. 9: The situation surrounding the damaged spillway at Oroville Dam has escalated into a crisis, with state water managers hoping they can dump enough water down the badly compromised structure to prevent the state's second-largest reservoir from pouring over an emergency release point that has never been used before.

Flow rates down the collapsing spillway were increased late Thursday morning to 35,000 cubic feet per second. The result was a spectacle of churning mud and water and further damage to the concrete structure.

But with storms continuing to pound the northern Sierra and torrents of water quickly filling Lake Oroville, the huge reservoir behind the dam, crews from the Department of Water Resources and Cal Fire are getting ready for what officials previously called "a very last-ditch measure."

Crews on Thursday began cutting down trees and bulldozing brush on the steep slope below an emergency spillway to try to minimize downstream debris flows should the lake exceed its 3.5 million acre-feet capacity.

"We have crews out there just as a precaution," said DWR spokesman Eric See during a media briefing at midday Thursday. "We're still taking every measure we can to not have to use the emergency spillway, but if we do, we're actually removing that debris right now so it doesn't get mobilized" into an adjacent waterway.

But the possibility that Lake Oroville would overflow for the first time in its half-century history grew stronger as the day progressed, despite the water being released down the damaged spillway.

Acting DWR chief Bill Croyle said at an evening press conference that it was becoming more and more likely that water would pour uncontrolled over the emergency spillway.

"To be very clear, with the hydraulic conditions we have now, and with the flow that we have coming down out of the spillway chute, unless conditions change, we anticipate there may be a release of water over the emergency spillway," Croyle said. "Maybe sometime on Saturday."

That event has become imminent because the volume of water flowing into the lake increased dramatically during the day as heavy rain fell across the Feather River watershed. Some locations in upstream mountains had received 4 to 5 inches of rain in the last 24 hours, with another inch or two expected before clear weather arrives Saturday.

Lake Oroville will overflow the emergency spillway if it reaches an elevation of 901 feet above sea level. On Tuesday, when the spillway damaged was first noted, the lake's surface was at about 850 feet. With the spillway shut down for most of the last 48 hours, the lake has risen to 887 feet as of 7 p.m. Thursday. (See DWR's real-time Lake Oroville statistics.)

"The downside of having water go over the emergency spillway is that it would go down the hillside and take out trees and soil and create a big mess in the diversion down below," the DWR's See said.

See said the severe erosion seen on and around the spillway structure is being closely monitored by crews on the ground, remote cameras and drones. Engineers believe the heavy flow of water will scour its way down to bedrock before long, See said, but acknowledged there are risks to allowing the erosion to continue.

"Erosion is occurring in multiple ways," See said. "You can have erosion to the side and erosion going down the hill, and then you can have 'head cutting,' which is erosion that can actually work its way back upstream. So that's the one that's of most concern."

If engineers detect that uphill erosion, See said, it would be "a trigger point" that would prompt another shutdown of releases down the spillway.

The erosion has already released massive flows of sediment into the adjacent waterway, a canal called the Thermalito Diversion Pool. The canal carries water from the dam down to and around the city of Oroville. Among the facilities to which it conveys water is the Feather River Hatchery, which raises millions of chinook salmon and steelhead trout.

Heavy sediment in the water can kill juvenile salmonids. With muddy water cascading into the hatchery facility Thursday morning, the Department of Fish and Wildlife began an emergency rescue of salmon and steelhead, trucking the young fish to a satellite hatchery on the Thermalito Afterbay, west of Oroville.

From the Sacramento Bee's account of the fish rescue:

At the hatchery Thursday, workers waded waist-deep through concrete holding ponds filled with water the color of chocolate milk. They used screens to push baby fish toward tanker trucks that would transport them a few miles southwest to Thermalito.

[Department of Fish and Wildlife spokesman Harry] Morse said that wild steelhead and salmon are spawning in the Feather River, fueling concern that sediment could overwhelm their nests and kill eggs and juvenile fish.

Officials at the media briefing repeated further reassurances that the integrity of Oroville Dam, one of the largest in the United States, has not been affected by the spillway collapse.

Butte County Sheriff Kory Honea said that while local emergency agencies are preparing for evacuations downstream of the dam, he didn't believe the spillway situation posed an imminent threat.

Update, 11:55 a.m. Thursday, Feb. 9: The California Department of Water Resources is fast running out of time and options for dealing with the badly damaged spillway at Oroville Dam.

With Lake Oroville rapidly approaching full, water managers increased flows down the spillway Wednesday afternoon and early Thursday to test the effect on the damaged structure. The result was both unsurprising and sobering.

The department said it expected the test, which involved releasing about 20,000 cubic feet per second down the long concrete spillway chute, would cause further damage to the structure.

But they may not have anticipated the extent of the damage that daylight revealed early Thursday. Photos from the scene showed that the massive cavity in the face of the spillway had grown several times larger and that the adjacent slow had suffered extensive new erosion. Here are a couple of views tweeted out early Thursday:

With the spillway mostly out of commission since major releases were curtailed, Lake Oroville has been rising at the rate of about half a foot an hour since midday Tuesday. Its level has increased 30 feet since then, with the reservoir's surface now 20 feet below an emergency spillway.

The emergency spillway, which would release water down a steep slope adjacent to the spillway, has never been used in the dam's half-century of operation. DWR officials and others say water flowing down the slope will likely result in a large volume of debris being dumped into the Feather River, which flows through the city of Oroville on its way to the Sacramento Valley.

That's one reason dam managers are willing to risk the destruction of the concrete spillway, calculating that would be preferable to the unknowns involved in an uncontrolled emergency spillover.

"It's going to be rocks, trees, mud -- liquid concrete -- going down that river," retired DWR engineer Jerry Antonetti told Sacramento's KCRA as he watched the spillway Wednesday night. "I'd open 'er up, sacrifice the bottom of that thing -- it's going to go in the river -- clean it out next year and build a new spillway."

Update, 8:45 a.m. Thursday, Feb. 9: State water officials say they may be forced to continue using a badly damaged spillway at Oroville Dam to prevent the lake from reaching capacity in the next few days.

Doing that would likely cause further damage to the spillway structure and continue eroding the surrounding area, Department of Water Resources spokesman Doug Carlson said Wednesday afternoon. But that could be preferable to allowing the lake to begin flowing over an emergency spillway on the dam.

Carlson called the alternate spillway -- which would send water cascading down a long tree- and brush-covered slope containing roads and power lines, a "very last-ditch measure."

"It's an outcome that DWR is committed to not allowing to happen," Carlson said. Like other DWR officials, he was quick to add that the spillway damage does not pose a threat to the dam itself, one of the largest ever built in the United States.

The department conducted an experiment during the day Wednesday in which it began sending a limited amount of water -- about 20,000 cubic feet per second -- down the damaged concrete spillway structure. The purpose of the test, Carlson said, was to see how much additional damage was done.

"We may just let the spillway do its job" despite the damage, Carlson said. Then, after the rainy season, "we could shut off the spillway, keep it dry, put construction people in there, whatever has to be done -- rocks, fill, concrete mix, whatever -- and get it back to 100 percent efficiency."

The DWR's spillway test came as Lake Oroville, the state's second-largest reservoir, is filling rapidly with runoff from recent storms.

In order to maintain enough space in the lake to accommodate in-rushing floodwaters, managers would normally release water down the dam's massive concrete spillway. That was just what was happening Tuesday when bystanders alerted dam personnel that there appeared to be damage to the structure.

Releases that were being ramped up to about 60,000 cubic feet per second were abruptly halted so that Department of Water Resources crews could assess the situation.

In the meantime, a high volume of runoff into the lake has continued, raising it more than 20 feet since early Tuesday. Late Wednesday afternoon, the reservoir was just 30 feet below an emergency spillway that has never been used in the dam's half-century of use.

"It's quite serious," Carlson said of the dam and reservoir's status. "The good news is that we think we have it under control."

Below: DWR photo gallery depicting damage to spillway and erosion to adjacent area.

Back to top.

Update, 12:25 p.m. Wednesday, Feb. 8: State water officials say engineers are still in the process of assessing damage to the spillway at Oroville Dam and figuring out what they can do to fix it.

"They're evaluating the situation intensively this morning," said Ted Thomas, the chief spokesman for the Department of Water Resources. "They're looking at what their options are for repair."

An extensive section of concrete on the spillway, which is used to manage the level of Lake Oroville, has peeled away or collapsed.

At the time the problem was spotted at midday Tuesday, water managers were in the process of ramping up the volume of water being dumped down the spillway into the Feather River. That was necessary to make room for high flows coming into the reservoir, the state's second largest, from a series of storms that have dumped very heavy rain over the Feather River watershed.

Releases were reduced from about 60,000 cubic feet per second to just 5,000 cfs -- the amount being routed through the dam's hydroelectric generating facility.

The immediate result of curtailing the releases while huge amounts of runoff stream into the reservoir has been a very rapid rise in the lake's level. In the 20 hours after releases were reduced at midday Tuesday, Lake Oroville has risen 10 feet and added 150,000 acre-feet.

If current release and flow rates persisted -- and that's not a sure thing by any means -- the reservoir would reach its 3.5 million acre-foot capacity in the next three or four days.

If that happens, Thomas said, the dam's emergency spillway -- which has not been used since the dam was finished in the late 1960s -- would channel floodwaters down a hillside into the river.

Thomas said he expected details on a proposed fix for the spillway damage later Wednesday. Back to top.

Original post, 5:35 p.m. Tuesday, Feb. 7: California Department of Water Resources crews are assessing a potentially serious problem with Oroville Dam, the giant structure that impounds the Feather River to create the state's second-largest reservoir.

Tuesday morning, the spillway that managers use to release water from Lake Oroville into the river appeared to suffer a partial collapse. That led to the shutdown of the spillway while engineers assess its condition.

Department officials say the dam itself, perched above the Sacramento Valley about 130 miles northeast of San Francisco, is not in danger.

The timing of the shutdown is critical: A huge amount of runoff is coming into Lake Oroville from the Feather River watershed after recent storms. To maintain room in the reservoir to contain the incoming flows, a high volume of water --- about 55,000 cubic feet per second -- was being released down the spillway.

With the spillway closed for the time being, there's no way to release water from the dam except through a hydroelectric powerhouse built into the structure. Only about 5,000 cubic feet per second can be released through the powerhouse.

The net effect is that with releases virtually halted and heavy inflows from a series of very wet winter storms continuing to pour into the reservoir, the lake is rising steadily.

As of 3 p.m. Tuesday, Lake Oroville was 82 percent full and was 150,000 acre-feet above the storage level prescribed to maintain room for incoming floodwaters.

The Department of Water Resources said in a statement that "sufficient capacity exists within the reservoir to capture projected inflows for at least days, and DWR expects to resume releases from the gated spillway at a rate deemed later today after a thorough inspection is performed."

Oroville Dam is an earth-fill dam and was dedicated in 1968. At 770 feet high, it's the highest dam in the United States.

What does the spillway look like under normal conditions? Here's a video shot Monday, when managers had ramped up releases from 25,000 cubic feet per second top 50,000 CFS (see below for some perspective on the flow numbers):

The flow perspective: One cubic foot of water is 7.48 gallons. So 55,000 cubic feet per second, roughly the volume being released down the spillway before problems were detected Tuesday, comes out to 411,400 gallons a second. That equals 1.26 acre-feet -- enough water to flood a football field to a depth of 15 inches.

An acre-foot, in turn, is roughly the amount of water used each year by two "average" California households. So the volume of water pounding down the spillway every second is close to what three households would use in a year.

Miranda Leitsinger, Don Clyde, Kat Snow, Craig Miller and David Marks of KQED contributed to this post.