OROVILLE, Butte County — A foaming mass of whitewater cascaded down the badly damaged Oroville Dam spillway Friday after state officials upped the flow in an attempt to avoid what would be an even more disastrous overflowing of California’s second-largest reservoir.

By increasing the flow, dam operators were conceding they were likely to lose a big portion of the spillway to erosion — perhaps the entire bottom half, or about 150 yards of concrete that will have to be painstakingly rebuilt during the dry months. The cost, state officials said Friday, will likely top $100 million.

As they spoke, the gaping hole in the spillway — which first cracked open Tuesday — got bigger as 65,000 cubic feet of water per second ripped into it, causing a rain-like mist to fall throughout the area just as the real rains that had caused the debacle ceased.

“We’re going to lose a lot of the spillway,” said Chris Orrock, a spokesman for the California Department of Water Resources, which manages the nation’s tallest dam, about 75 miles north of Sacramento. “The director has said we are willing to lose the bottom of that spillway to make sure we maintain flood control for the downstream communities.”

The torrent was crashing over the side of the spillway onto a bed of rocks, scouring the hillside clear of vegetation and taking with it so much dirt that the Feather River turned into a soup full of debris, endangering millions of hatchery fish downstream.

As of Friday afternoon, Orrock said, more water was still flowing into the reservoir — which can hold 3.5 million acre-feet of water and helps supply farms and million of people — than was coming out.

If the water rose about 10 more feet, he said, it would begin gushing over the dam’s emergency spillway, a dirt channel 21 feet below the brim that has never been used in the structure’s 48-year existence.

That spillway has been criticized as deficient and dangerous by environmental groups. A 2002 analysis by the Yuba County Water Agency said use of the auxiliary spillway would cause “severe erosion” and deposit so much debris in the river that downstream structures could be damaged.

Although officials didn’t expect to have to use the emergency spillway, forestry workers were clearing trees and other debris from the channel just in case.

The good news, Orrock said, is that the larger spillway, made of reinforced concrete, was peeling downward and not threatening the integrity of the 770-foot-high dam itself. “If the erosion was moving up toward the dam, they would stop the flow,” he said.

Nearby Oroville residents were still worried. Dan Rogers and several of his friends were so afraid of flooding Thursday that they left town and spent the night miles away in Chico.

“It’s pretty crazy,” said Rogers, who returned after the rain stopped.

Although officials said the area wouldn’t flood, the frothing Feather River had picked up so much muck that it was threatening to asphyxiate millions of salmon 4½ miles downstream at the Feather River Fish Hatchery, forcing workers to frantically collect 8 million hatchlings and truck them 10 miles to a holding pond that uses well water.

The water flowing into the hatchery was measured to be 20 times as muddy as normal, said Harry Morse, a spokesman for the state Department of Fish and Game. He said 6 million fish had been moved and that hatchery staffers would work through the day and night moving 2 million more. As many as 2 million of the 2-inch-long juvenile fish, though, will have no place to go.

“You can only put so many fish in one place without them lacking oxygen,” he said. “We hope to get the silt settled. If we can’t do that, we may have to let the last 2 million loose in the Feather River floodplain.”

The Feather River hatchery produces more than half of all the salmon caught in the ocean and rivers, the most of any hatchery in California, which boasts a multimillion-dollar salmon industry.

“We’re in an emergency situation,” Morse said. “This is stuff that has never happened in the 48 years of the dam, so we are really scrambling. We’re bringing staff, engineers, trucks in. It’s a full-court press.”

The forecast called for a five-day window of clear weather, and engineers plan to use the time to assess the situation and figure out what fixes need to be made. Clearly, though, the destruction is extensive.

Kevin Dossey, a civil engineer for the Department of Water Resources office in Oroville, said repairs to the spillway would likely take four to five months and cost more than $100 million.

On the hook for payment, state officials said, are beneficiaries of the California State Water Project — 29 urban and agricultural water agencies that include the mammoth Metropolitan Water District of Southern California and the Santa Clara Valley Water District.

“It wouldn’t be surprising that state water contractors would be paying the bill,” said Jim Fiedler, the chief operating officer for the Santa Clara Valley Water District. “But we would certainly hope that they would seek emergency funding to help pay for it.”

While the cause of the break in the spillway is not yet known, officials said repairs were made on the chute in 2013 after it was used during storms in 2011 and 2012. Dossey said he wasn’t sure whether the fixes were made in the same area as the new damage.

“I don’t think anybody on the inspection team or repair team would say more should be done because there wasn’t any evidence more needed to be done,” Dossey said. “The repairs were smooth.”

An inspection of the dam and spillway in July 2015 deemed it safe, but experts did not walk the sloped surface to look for cracks and other potential problems, state records show. A “visual inspection from some distance indicated no visible signs of concrete deficiencies,” said the report compiled by the Division of Safety of Dams, a branch of the Department of Water Resources.

It wasn’t clear why inspectors didn’t walk the spillway’s discharge chute, and a spokesperson for the agency didn’t respond to a request for comment. The dam also passed an inspection earlier in 2015.

Dam operators first noticed an eroded section of the spillway Tuesday as they attempted to increase the flow down the chute during a rainstorm. They shut it down to take stock of the damage, but as the rain increased, they had no choice but to increase flows, causing the fissure to balloon outward.

The dam’s spillway — and valves in the Edward Hyatt Power Plant at the bottom of the reservoir — were releasing 79,000 cubic feet per second of water Friday, but the flow was reduced overnight. About 130,000 cubic feet per second was flowing into the dam from the surrounding mountains.

If it were not damaged, the spillway could usher out up to 200,000 cubic feet per second of water, though that flow would be too much for the Feather River, which can handle 150,000 cubic feet per second without flooding.

Oroville Dam operators previously came under scrutiny in 2009 after a wall collapsed at the Hyatt Power Plant and five employees were nearly sucked out of the building by a powerful vacuum, causing one man to suffer broken bones and other serious injuries and spend four days in a hospital.

State workplace safety regulators found the Department of Water Resources was at fault for ordering the workers to open valves that were missing a critical part and couldn’t handle the pressure.

Melody Gutierrez, Peter Fimrite and Michael Bodley are San Francisco Chronicle staff writers. Email: mgutierrez@sfchronicle.com, pfimrite@sfchronicle.com, mbodley@sfchronicle.com Twitter: @melodygutierrez @pfimrite @michael_bodley