Evacuation orders remained in place on Monday morning for nearly 200,000 people living in northern California beneath the United States’ tallest dam, even though the threat of uncontrolled floodwaters rushing into the rural communities below is lessening.
Authorities weren’t taking any chances that an already damaged spillway in the Lake Oroville Dam could give way. The dam – more than 200 feet taller than the 550-foot-tall Washington Monument – controls the water for the state’s second largest reservoir, north of the capitol of Sacramento.
While the evacuation orders stood on Monday, it appears authorities averted a disaster after water from record-breaking rainfall began flowing from the dam’s emergency spillway for the first time in 50 years. That's when officials noticed pieces of concrete missing from this auxiliary spillway.
The erosion at the head of the spillway threatens to undermine the concrete dam and allow large, uncontrolled releases of water from Lake Oroville, according to the California Department of Water Resources. That flow could overwhelm the Feather River and other downstream waterways, channels, and levees, potentially sending a wall of water into surrounding communities.
Authorities ordered at least 188,000 people living below the lake to evacuate on Sunday afternoon, shortly after engineers spotted a hole in the spillway that was eroding back toward the top.
“This is not a drill,” a National Weather Service bulletin blared Sunday afternoon, according to The Mercury News in San Jose. “Repeat this is not a drill.... Move to higher ground now. Act quickly to protect your life.”
During the evacuation orders, the California Department of Water Resources said on Twitter the emergency spillway was “predicted to fail within the next hour.”
Several hours later the worst case hadn’t happened, with the damaged spillway still standing. State and local emergency officials then said the immediate danger had passed, with water no longer flowing over the eroded spillway.
More rain is expected on Wednesday, however, and the state's department of water resources continued to lower the water level of the dam to make room for the inflow, according to The Mercury News.
Residents in the cities of Oroville, Gridley, Live Oak, Marysville, Wheatland, Yuba City, Plumas Lake, and Olivehurst were all under evacuation orders. They fled everywhere to hotels and other evacuation sites except north of the dam.
Emergency services urged evacuees to travel east, south or west via social media. "DO NOT TRAVEL NORTH TOWARD OROVILLE," the department warned on Twitter.
Gov. Jerry Brown had already asked the Federal Emergency Management Agency on Friday to declare the area a major disaster because of the flooding and mudslides brought on by this season’s historic storms.
Opened in 1968, Lake Oroville, 70 miles north of Sacramento, supplies water for farmland in the state’s Central Valley and residents in southern California.
The auxiliary spillway was opened for the first time on Saturday after unexpected erosion chewed through the main spillway during heavy rains earlier last week, creating a 200-foot-long, 300-foot-deep hole. But acting water resources director Bill Croyle said at a news conference on Sunday “the integrity of the dam is not impacted” by the damage.
This report includes material from the Associated Press and Reuters.