SAN FRANCISCO (AP) — Starting last Tuesday, the California Department of Water Resources issued a dozen press releases about damage to a spillway attached to the nation’s tallest dam, in Oroville, California.
Until Sunday, water officials maintained public safety was not at risk. Then, on Sunday afternoon, nearly 200,000 people living downstream were ordered to evacuate after the department determined an auxiliary spillway was in danger of imminent failure.
Here’s what officials said and when:
TUESDAY: The department announces it halted flows from the Oroville dam spillway around noon “to investigate concrete erosion” after engineers noticed odd flow patterns. “There is no anticipated threat to the dam or the public.”
WEDNESDAY: The department says it plans to release water over the damaged spillway for two hours to determine how much flow it can handle. “The dam is sound,” the Department of Water Resources states, “and no imminent threat to the public exists.”
THURSDAY: The department says the test release of water further damaged the spillway as expected. Nonetheless, officials say they plan a bigger release of water through the spillway to make room in the reservoir for expected heavy rains. The department prepares for water to flow over an “emergency spillway” for the first time in the dam’s 48-year history.
“There is no imminent or expected threat to public safety or the integrity of Oroville Dam,” the agency states.
THURSDAY, 9:34 p.m.: The department says water will likely start flowing over the “emergency spillway” as early as Saturday. If the vegetated hillside near the dam is used, “water would wash away large amounts of soil.” Crews work to clear trees and brush from the hillside “to minimize any flow of debris into the river.”
Workers also evacuate a downstream fish hatchery to save fish and eggs.
FRIDAY: The department says it’s increasing the flow rate of water from the “nearly full” reservoir to handle runoff from heavier-than-expected rains and avoid using the emergency spillway. “Oroville Dam itself is sound and there is no imminent threat to the public,” the department says.
SATURDAY, 6:45 a.m.: The department says it expects water to flow over the emergency spillway by noon, as water pours into the reservoir faster than it leaves. It repeats that the Feather River is not expected to flood and there is no danger to public safety.
SATURDAY, 5:25 p.m.: A “relatively light flow of water” continued washing over an “auxiliary spillway” — which the agency had previously referred to as an “emergency spillway,” after initially starting around 8 a.m., the department says. Erosion in the dam’s main spillway “has slowed considerably,” and the department says a power plant at the dam was shut down.
SUNDAY, noon: At a news conference, Department of Water Resources spokesman Eric See characterizes “overall conditions as stable with this event.” He says the auxiliary spillway appeared to be holding up with no visible erosion.
SUNDAY, 4:45 p.m.: The department tweets an evacuation warning: “Auxiliary spillway at Oroville Dam predicted to fail within the next hour. Oroville residents evacuate northward.” Butte County Sheriff Kory Honea shortly afterward orders downstream communities evacuated. “This is not a drill,” he posts on the sheriff’s Facebook page.
SUNDAY, 5 p.m.: Officials double the flow of water through the damaged dam to “avert more erosion at the top of the auxiliary spillway.” Erosion “threatens to undermine the concrete weir and allow large, uncontrolled releases of water from Lake Oroville. Those potential flows could exceed the capacity of downstream channels.”
MONDAY, 12:30 p.m.: Officials at a news conference in Oroville offer no timeline for evacuated residents to return home.