OROVILLE, Butte County — Nearly 200,000 people downriver from Lake Oroville were ordered to evacuate Sunday night after an emergency spillway next to the reservoir’s dam appeared in danger of collapse.

While the integrity of Oroville Dam is not at risk, officials said a catastrophic amount of water was in danger of bursting through the wall of the auxiliary spillway alongside the 770-foot-tall dam. Pressure from the water cascading over the concrete lip of the spillway had caused rapid erosion below it, threatening to burrow a hole beneath and through the auxiliary structure.

Should such a rupture occur, water would inundate the Feather River and surge toward cities on the eastern side of the Sacramento Valley.

Back to Gallery Nearly 200,000 ordered evacuated as Oroville Dam danger... 63 1 of 63 2 of 63 Photo: JOSH EDELSON/AFP/Getty Images 3 of 63 Photo: Michael Macor, The Chronicle 4 of 63 Photo: Elijah Nouvelage/Getty Images 5 of 63 Photo: Michael Macor, The Chronicle 6 of 63 Photo: Michael Macor, The Chronicle 7 of 63 Photo: JOSH EDELSON/AFP/Getty Images 8 of 63 Photo: Brian van der Brug/LA Times via Getty Images 9 of 63 Photo: JOSH EDELSON, AFP/Getty Images 10 of 63 Photo: JOSH EDELSON, AFP/Getty Images 11 of 63 Photo: JOSH EDELSON, AFP/Getty Images 12 of 63 Photo: JOSH EDELSON, AFP/Getty Images 13 of 63 Photo: JOSH EDELSON, AFP/Getty Images 14 of 63 Photo: Santiago Mejia, The Chronicle 15 of 63 Photo: Stephen Lam, For The Washington Post 16 of 63 Photo: Marcio Jose Sanchez, Associated Press 17 of 63 Photo: Marcio Jose Sanchez, Associated Press 18 of 63 Photo: Michael Macor, The Chronicle 19 of 63 Photo: JOSH EDELSON, AFP/Getty Images 20 of 63 Photo: JOSH EDELSON, AFP/Getty Images 21 of 63 Photo: JOSH EDELSON, AFP/Getty Images 22 of 63 Photo: JOSH EDELSON, AFP/Getty Images 23 of 63 Photo: Santiago Mejia, The Chronicle 24 of 63 Photo: Santiago Mejia, The Chronicle 25 of 63 Photo: Handout, Getty Images 26 of 63 Photo: Michael Macor, The Chronicle 27 of 63 Photo: Santiago Mejia, The Chronicle 28 of 63 Photo: Santiago Mejia, The Chronicle 29 of 63 Photo: Santiago Mejia, The Chronicle 30 of 63 Photo: Michael Macor, The Chronicle 31 of 63 Photo: Michael Macor, The Chronicle 32 of 63 Photo: Carlos Avila Gonzalez, The Chronicle 33 of 63 Photo: Carlos Avila Gonzalez, The Chronicle 34 of 63 Photo: Max Whittaker/Prime, Special to The Chronicle 35 of 63 Photo: Carlos Avila Gonzalez, The Chronicle 36 of 63 Photo: Carlos Avila Gonzalez, The Chronicle 37 of 63 Photo: Max Whittaker/Prime, Special to The Chronicle 38 of 63 Photo: Max Whittaker/Prime, Special to The Chronicle 39 of 63 Photo: Max Whittaker/Prime, Special to The Chronicle 40 of 63 Photo: Max Whittaker/Prime, Special to The Chronicle 41 of 63 Photo: Carlos Avila Gonzalez, The Chronicle 42 of 63 Photo: Carlos Avila Gonzalez, The Chronicle 43 of 63 Photo: Max Whittaker/Prime, Special to The Chronicle 44 of 63 Photo: Max Whittaker/Prime, Special to The Chronicle 45 of 63 Photo: KCRA 46 of 63 Photo: Rich Pedroncelli, Associated Press 47 of 63 Photo: Kelly M. Grow, California Department of Water Resources 48 of 63 Photo: Kelly M. Grow, California Department of Water Resources 49 of 63 Photo: Max Whittaker/Prime, Special to The Chronicle 50 of 63 Photo: Carlos Avila Gonzalez, The Chronicle 51 of 63 Photo: Carlos Avila Gonzalez, The Chronicle 52 of 63 Photo: Carlos Avila Gonzalez, The Chronicle 53 of 63 Photo: Rich Pedroncelli, Associated Press 54 of 63 Photo: Kelly M. Grow/California Department of Water Resources 55 of 63 Photo: DWR 56 of 63 Photo: Kelly M. Grow, California Department of Water Resources 57 of 63 Photo: Rich Pedroncelli, Associated Press 58 of 63 Photo: Carlos Avila Gonzalez, The Chronicle 59 of 63 Photo: Rich Pedroncelli, Associated Press 60 of 63 Photo: Michael Macor, The Chronicle 61 of 63 Photo: Michael Macor, The Chronicle 62 of 63 Photo: Michael Macor, The Chronicle 63 of 63 Photo: Rich Pedroncelli, Associated Press





























































































































The good news late Sunday was that water levels in the reservoir were low enough that water was no longer tumbling over the auxiliary spillway. This means the eroded crevices aren’t getting any worse.

But officials were unwilling to say when the evacuation orders to some 188,000 people will be reversed, or how the eroded auxiliary spillway can be repaired.

“Now that there is no more water going over the emergency spillway, though it brings stability to the situation, there are still a lot of unknowns,” Butte County Sheriff Kory Honea said at a 10 p.m. news conference.

Honea also defended his initial decision to order the mandatory evacuations.

“It can be a chaotic situation, and I understand that. But to this point, things are going fairly well,” Honea said. “We’re not at the point yet where we can make decisions about whether or not it is safe to repopulate areas.”

The first call for mandatory evacuations came shortly after 4:30 p.m. and included all low-lying areas in Butte County from Oroville south to the border of Sutter County.

“I couldn’t risk lives of thousands of people, so we took this significant step,” Honea said at a news conference earlier in the evening. “We needed to get people moving, quickly, in order to protect the public and save lives if the worst-case scenario came to fruition.”

Not long after that, evacuation orders also went out to low-lying areas near the Feather River farther south. This included Yuba City in Sutter County, with 65,000 people, and Marysville in Yuba County, population 12,000.

But as the evening went on, the edge of the auxiliary spillway wall remained intact.

Officials relieved pressure on the auxiliary spillway by increasing the amount of water being discharged down the main spillway to nearly 100,000 cubic feet per second, nearly twice the level of earlier in the day. Meanwhile, the water entering Lake Oroville from points north and east was less than 40,000 cubic feet per second — meaning that the high water levels behind the dam and spillways were beginning to drop.

“We have contingency plans drawn up, and crews are heading into the area to be on standby,” said Dan Olson of the state Department of Forestry and Fire Protection. He also said the initial estimates of the extent of the erosion and the pace at which it was spreading weren’t “as bad as the initial assessment suggest.”

That initial assessment was alarming — shortly before 5 p.m., officials warned that the spillway could give way within an hour. Roads quickly became jammed as thousands of people left a widening circle of communities around Oroville.

“This is very serious,” said Scott McLean, a Cal Fire spokesman who was stuck in the gridlock. “I’m just trying to get through traffic.”

Evacuation shelters were set up in Chico, Paradise (Butte County), Orland (Glenn County) and Grass Valley (Nevada County). In Yuba County, the Office of Emergency Services said on Facebook that it was moving its operations from nearby the Feather River “to a secure location.”

Gov. Jerry Brown also responded to the danger of large-scale flooding, declaring a state of emergency because “conditions of extreme peril to the safety of persons and property exist in Butte, Sutter and Yuba Counties.” Among other actions, the California National Guard will be activated as needed to assist in response efforts.

Water began spilling down the Oroville Dam’s auxiliary spillway Saturday after the main concrete spillway partly disintegrated under the pressure caused by more than a month of heavy rains. The discharge raised concerns over how the earth-lined backup channel would hold up, but state water officials had been confident about the integrity of the emergency spillway before the evacuation was ordered.

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,” Bill Croyle, acting director of the California Department of Water Resources, said earlier Sunday. “Our planning is both short term and long term.”

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

The reservoir is the second-largest in the state behind Shasta Lake (Shasta County) and supplies water to Central Valley along with water 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 that repairs had been made to the spillway in 2013 near where the hole emerged.

Chronicle staff writer

Peter Fimrite contributed

to this report.

Evan Sernoffsky, Jill Tucker, John King and Melody Gutierrez are San Francisco Chronicle staff writers. Email: esernoffsky@sfchronicle.com, jtucker@sfchronicle.com, jking@sfchronicle.com, mgutierrez@sfchronicle.com Twitter: @EvanSernoffsky, @jilltucker, @JohnKingSFChron, @MelodyGutierrez