“There has been severe erosion of the emergency spillway and a possible structural breach that could send uncontrolled water down the stream,” said Chris Orrock, a spokesman with the California Department of Water Resources.

Residents downstream from Lake Oroville to the Sutter County line were under mandatory evacuation order. Counties around the reservoir, the second largest in the state, down to Sacramento were warned about the possibility of flooding.

Department of Water Resources officials issued a statement just before 4:45 p.m. that the “auxiliary spillway at the dam was predicted to fail within the hour.”

Officials increased water releases out of the primary spillway to 100,000 cubic feet per second to relieve pressure on the emergency spillway.

An evacuation shelter was set up at the Silver Dollar Fairground in Chico at 2357 Fair St.

Traffic was bumper to bumper as residents of Oroville, Biggs and Gridley headed slowly out of the possible flood zone eastbound on Highway 162.

“This is very serious,” said Scott McClean, a spokesman with the California Department of Forestry and Fire Protection who was in the evacuation traffic. “From what I understand it’s the auxiliary spillway at a point of possible collapse. I’m just trying to get through traffic.”

The amount of water gushing over Lake Oroville’s emergency spillway had begun to decrease before the emergency Sunday as state officials pushed ahead with urgent measures to make room in the state’s second-largest reservoir.

The emergency spillway — an open hillside that drains to the Feather River below — had never been used since the Oroville Dam was completed in 1968. The discharge that began early Saturday raised concerns over how the backup channel would hold up, and whether debris would threaten fish and levees downstream.

The primary concrete spillway just south of the overflow area, meanwhile, was thought to have stabilized after a gigantic hole emerged in its 3,000-foot-long channel last week. The gash forced operators to reduce the outflow, which set the stage for this weekend’s unprecedented situation.

Another wet-weather system, in what has been a soaking winter, is on deck to hit Northern California on Wednesday, requiring water managers to make still more room in Lake Oroville for another surge.

The series of Pacific storms is expected to bring up to 4 inches of rain to parts of the Central Valley, said Idamis Del Valle, a meteorologist with the National Weather Service’s Sacramento office.

“We need to do everything we can to maximize our ability to move water our of this reservoir — not just for the coming storm but for the coming storms,” said Bill Croyle, acting director of department of water resources. “Our planning is both short term and long term.”

Teams were dredging an area composed of silt, rock and hunks of concrete debris that has formed under the primary spillway following recent erosion.

The blockage forced water in the diversion pool to back up toward the dam and has threatened the Edward Hyatt Power plant at the base of the concrete behemoth.

The power station, which was closed late Friday due to the threat, serves as a third release point for the reservoir when it is operational. The facility expels up to 14,000 cubic feet of water per second downriver.

Officials had stressed earlier Sunday that the structural integrity of the 770-foot Oroville Dam — the tallest in the country — had not been compromised by the damaged spillway.

Once the dredging is completed, crews must rewire overhead power lines that were taken out of service after support towers were threatened by erosion.

Operators first detected the hole in the primary spillway Tuesday. Subsequent releases down the slide made the hole grow dramatically, exposing bedrock even beyond the 180-foot-wide channel.

Officials stabilized the hole and stopped further erosion by slowing the outflow to 55,000 cubic feet per second. But 90,000 cubic feet of water per second continued to dump into Lake Oroville and pushed the reservoir over capacity on Saturday.

“The problem is we don’t have that flexibility right now,” Croyle said. “Our bowl is full. So, what comes in comes out.”

By Sunday, the rate of inflow had fallen to 41,400 cubic feet of water per second, officials said.

The estimated cost of repairing the concrete spillway has soared to as much as $200 million, and officials are debating whether it can be patched when the rains end, or if a new chute will be needed altogether.

Under clear skies, many residents from nearby communities headed to Oroville on Sunday in hopes of seeing the raging Feather River or the damaged spillway. They parked and walked along streets overlooking the murky water, but public access to the spillways next to the dam was blocked.

The reservoir is the second largest in the state behind Lake Shasta and supplies water to Central Valley along with districts in the Bay Area and Southern California.

On Sunday, the cause of the hole in the main spillway was still being investigated. But experts noted repairs had been made to the spillway in 2013 near where the hole emerged.

Robert Bea, a U.C. Berkeley engineering professor, reviewed 2015 inspection reports that made note of the earlier repairs to the concrete slabs.

“From what I can tell from the photographs, it appears that the water pressures from the recent releases were great enough to cause failure of the repairs made to the base slabs,” he told The Chronicle.

Downstream from the dam at the Feather River Fish Hatchery in Oroville, officials with the California Department of Fish and Wildlife had moved nearly all their salmon hatchlings into nearby holding ponds due to brown and silty water that threatened the fish.

Chronicle staff writer Peter Fimrite contributed to this report.

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