For the first time since Oroville Dam was completed in 1968, water from its storm-swollen reservoir overtopped the emergency spillway Saturday, sending sheets of water down a forested hillside and adding to the murk and debris churning in the Feather River below.
State officials said they did not expect the flows to cause flooding in Oroville or other communities downstream, but the emergency releases underscored the perilous situation confronting the operators of California’s second-largest reservoir for the rest of this extraordinarily rainy winter.
Unable to release enough water from the dam’s 3,000-foot main spillway, which split open Tuesday and continues to erode, the California Department of Water Resources announced that stormwaters reached the top of the dam at around 8 a.m Saturday and began flowing over the concrete lip of the adjacent emergency spillway onto a wooded ravine below.
The flow began as a steady, smooth spill across the 1,700-foot-wide lip of the emergency structure, and was expected to peak at 6,000 to 12,000 cubic feet of water per second at around midnight Saturday. With dry weather in the near-term forecast for the Sierra and inflows to the reservoir slowing, the lake level should fall below the emergency spillway as of Monday night, said Doug Carlson, a DWR spokesman.
The emergency spillway exists to prevent water from topping the wall of the massive earthen dam that holds back Lake Oroville, a situation that could cause problematic erosion. Department of Water Resources Acting Director Bill Croyle stressed Saturday that the dam itself “is not threatened by these conditions.”
But the crisis at Lake Oroville won’t abate any time soon. Northern California is on pace for its wettest winter ever, and Croyle said an estimated 2.8 million acre-feet of snow blankets the Sierra above the dam. Depending on how quickly that melts, it will put additional strain on Oroville Dam in the months to come.
“Our next 60 to 90 days will be critical, how we route this (snow) runoff through this reservoir,” Croyle said. “There’s a lot of snow up there.”
On Saturday, working to relieve pressure on the dam, operators continued to blast water down the battered main spillway, which developed a pothole in the past week that has expanded to a cavernous 300-foot fissure. The main spillway was releasing 55,000 cfs, meaning a total of 60,000 to 65,000 cfs was pouring out of the dam.
For a second day, the explosive flows from the main spillway scraped brush and earth into the turbid, churning Feather River below, as the jagged gash in the spillway’s midsection sent water sloshing over an adjacent hillside.
“There is a fair amount of debris in the diversion pool at the bottom of the dam,” Carlson said, referring to the body of water that leads to the Feather River.
Saturday morning, helicopters flew back and forth over the dam area, as Pacific Gas and Electric crews worked to dismantle cables and electrical components from electrical towers on a hillside adjacent to the main spillway, concerned that they, too, could be dragged into the channel by the crashing flows.
Denny Boyles, a spokesman for PG&E, said the work was being done in “an abundance of caution” to minimize materials that might get sucked into the river if the land around the towers is eroded by the powerful flows.
Unlike the main spillway, which is lined in concrete, the emergency spillway dumps water onto an open hillside. DWR officials had worked feverishly in recent days to try to lower reservoir levels enough to avoid overtopping the emergency structure, concerned that the outflows would scour the hillside, dumping additional trees, mud and debris into the Feather River. Work crews spent Thursday and Friday removing some of the trees from the ravine below the emergency spillway as a contingency.
Cal Fire spokesman Mike Smith said DWR officials had laid out floating structures known as booms in the waterways to catch debris.
Despite the unprecedented conditions, Smith said, “DWR is confident that there should be no flooding” along the Feather River, which cuts through Butte, Yuba and Sutter counties.
DWR spokesman Eric See said the flows on the Feather aren’t expected to top 75,000 cfs, well within the carrying capacity of its levees and channels. About twice as much water was released from the dam during the major storms in 1997, which produced considerable flooding in Yuba and Sutter counties after several levees broke. Since then, hundreds of millions of dollars have been spent upgrading levees and other flood control facilities in the region.
“I don’t believe we’re in an emergency situation in terms of flooding,’’ said Butte County Sheriff Kory Honea. But he urged residents to stay alert “if the situation changes and there is need for additional response.” Residents can call 530-538-7826 for updates.
Briefing reporters a few miles from the dam, Croyle said he has told Gov. Jerry Brown that fixing the main spillway could cost $100 million to $200 million, although those figures are preliminary. He said DWR engineers will spend weeks calculating whether it makes sense to fix the existing concrete structure or build an entirely new spillway nearby. He wants the repairs done by the start of the next rainy season in October.
Lake Oroville can store 3.5 million acre-feet of water, which is divvied out through the year for farming and drinking water needs across great stretches of California. The dam was built by Brown’s father, Gov. Pat Brown, and was paid for by regional water agencies, such as the Metropolitan Water District of Southern California, which are members of the State Water Project.
The cost-sharing for repairs between the water agencies and the state has yet to be determined, Carlson said. Croyle, though, raised the prospect of getting federal dollars to help with fixes.
When all is said and done, Croyle said, he anticipates the lower half of the main spillway will be ruined as the forceful releases, required for flood control at the dam, continue to gnaw at the concrete and underlying soil and rock. “In my opinion, it’s toast all the way to the bottom,” he said.
He said he would consider halting releases from the main spillway if an extended dry spell is forecast, and make temporary repairs.
Ironically, the water began flowing over the emergency spillway as Oroville enjoyed its first sunny morning in days. The sky was completely blue, and officials said they hoped a spell of dry weather would continue to diminish the volume of water flowing into Lake Oroville from the vast Sierra watershed that feeds the Feather River and its tributaries.
At various vista points in Oroville, a community of 15,000 just downstream of the dam, residents gathered under clear skies to gape in awe at the Feather River, so infused with mud and debris it resembled liquid clay as it tore through its channel.
Longtime resident Tom Oxford, 55, took in the view from a residential ridge on Mira Loma Drive, where the river surged beneath a railroad trestle. He took note that the trestle and levees on both sides were holding, with heavy moss rock that had been there for decades undisturbed by the fast-moving water.
Still, the sight was piercing. “We’ve never seen the river that brown before,” Oxford said, “and I’ve been here 50 years.”
“Then again,” he added, “we’ve never seen it go over the (emergency) outflow.”
Early Friday, DWR officials had expressed confidence that they could avoid the emergency spillway option. Despite the crater in the main concrete spillway, engineers ramped up water releases to 65,000 cubic feet per second. With rains letting up, and inflows into the lake slowing, they believed they could keep the water level behind the dam to below 901 feet.
But Friday, at about 8 p.m., they had to throttle back the releases to 55,000 cfs to prevent erosion along the side of the main spillway from compromising the transmission tower that links to the dam’s power plant.
Normally, the power plant is the main vehicle for pushing water out of the lake. But DWR has temporarily shut the plant because all the concrete and other debris streaming into the river from the broken spillway has raised water levels to the point that the turbines can’t run. DWR needs the transmission tower to remain operational to be able to reopen the power plant.
DWR officials have said it isn’t yet clear what caused the spillway to splinter. Inspectors at Oroville Dam found “minor” cracks in the main spillway in 2009, according to state inspection reports. The reports indicate the damage was repaired the following year.
Kevin Dossey, a DWR engineer, said Friday that additional cracks had appeared in the main spillway as recently as 2013 but also were repaired. With water hitting the concrete chute at an estimated 50 mph, “spillway erosion is a natural thing,” he said.