At least 1,88,000 people have been asked to evacuate over concerns California's Oroville Dam's emergency spillway could fail.
Officials say Oroville Lake levels are decreasing as they let water flow from its heavily damaged, main spillway but point out water is still spilling over the dam.
Water started flowing over an emergency spillway at the nation's tallest dam, on Lake Oroville, for the first time on Saturday after erosion damaged the Northern California dam's main spillway.
Officials hoped to avoid using Oroville Dam’s emergency spillway, fearing it could cause trees to fall and leave debris cascading into water that rushes through the Feather River, into the Sacramento River and on to the San Francisco Bay. Crews prepared for several days, clearing trees and brush.
Water began running over the emergency spillway around 8 am, according to California's Department of Water Resources. It was the first time the emergency spillway has been used in the reservoir's nearly 50-year history.
Water was expected to continue flowing over the emergency spillway for 38 to 56 hours, agency spokesman Eric See said at a news conference on Saturday afternoon. In addition to the emergency spillway, water is also flowing through the main spillway that was significantly damaged from erosion, he said.
"This is a very unusual event for us here in Oroville," See said.
Unexpected erosion chewed through the main spillway earlier this week, sending chunks of concrete flying and creating a 200-foot-long, 30-foot-deep hole that continues growing. Engineers don’t know what caused the cave-in that is expected to keep getting bigger until it reaches bedrock.
The city of Oroville is inhabited by 16,000 people and has historically come up due to the construction of the dam in 1971.
Bill Croyle, the Department of Water Resources' acting director said officials are continuously monitoring the erosion both on site and through cameras. "This is mother nature kind of kicking us a few times here," he said.
Croyle said the main spillway will need a "complete replacement" from the damage. Officials noted earlier this week that the cost of repairing the dam could approach $100 million, but they noted the estimate was an early, ballpark figure.
Officials have stressed Oroville Dam is sound and there is no imminent threat to the public.
State officials also had been attempting to rescue millions of hatchery-raised fish imperilled by muddy water flowing downstream alongside the damaged spillway after sections of its concrete walls collapsed earlier this week
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. The lake is a central piece of California’s government-run water delivery network, supplying water for agriculture in the Central Valley and residents and businesses in Southern California.
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