For the first time in history, officials used an emergency spillway.
Evacuations were first ordered after officials warned of "imminent failure" within 60 minutes at Oroville Dam's emergency spillway at 4:45 p.m. PST Sunday. They were unable to open the dam because of a 200-foot-long hole in the spillway, which they feared would get bigger. On Sunday, Butte County Sheriff's department said that the auxiliary spillway also suffered "severe" damage through its use.
Authorities this evening have issued evacuation orders in communities near Lake Oroville due to worries that a spillway could fail, sending a surge of uncontrolled water down the Feather River.
Moreover, the state's National Guard said it would provide eight helicopters to assist in reconstructing the emergency spillway.
The California Department of Water Resources said earlier on Sunday afternoon that it was releasing as much as 100,000 cubic feet (2,830 cubic metres) of water per second from the main spillway to try to drain the lake.
About 150 miles northeast of San Francisco, Lake Oroville is one of California's largest man-made lakes, and the 770-foot-tall Oroville Dam is the nation's tallest.
What nearly happened this year is a stark reminder of how much we depend on dams, levees, bypass channels, seawalls and other human efforts to keep destructive tendencies of nature at bay. Rather than risk thousands of lives, Honea said, a decision was made to order the evacuations.
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Late Sunday night, officials said the threat has diminished somewhat as the erosion was stopped.
Still, evacuation orders remained in place.
After so much rain and snow in recent weeks, water began flowing from Lake Oroville into a spillway created for the very goal of relieving pressure on the dam. "Officials are aggressively attempting to lower Lake Oroville's water levels, as another atmospheric river storm system is scheduled to arrive within 48 hours", Brown wrote.
The Oroville dam is almost full following winter storms that brought relief to the state after four years of drought.
The dam is about 110km north of Sacramento, and just upstream and to the east of Oroville, a city of more than 16,260 people. But that's an ambitious endeavor since it was dropping at a rate of about 8 inches a day and rain is expected to arrive Thursday.
As part of the relicensing application, state water resources department officials wrote in a final environmental impact report dated June 2008 that no "significant concerns" about the hillside's stability had been raised in any government or independent review.