Erosion in the dam's main spillway forced the state to open the emergency outlet, even though it had never been used in Lake Oroville's nearly 50-year history. The erosion threatens to undermine the concrete and allow large, uncontrolled releases of water from Lake Oroville, the California Department of Water Resources said. Those potential flows could overwhelm the capacity of downstream channels and levees.
However, Butte County Sheriff Koney Honea said engineers with the California Department of Water Resources informed him shortly after 9 p.m. that the erosion on the emergency spillway is not advancing as fast as they thought.
Honea says two inches of water is still coming over the dam, but that is significantly down from earlier flows.
Honea says there is a plan to plug the hole by using helicopters to drop rocks into the crevasse.
He says the evacuation order went out after engineers spotted a hole that was eroding back toward the top of the spillway.
Honea added that authorities wanted to get people moving quickly to save lives in case "the worst-case scenario came into fruition."
California officials said the cities of Gridley, Live Oak, Nicolaus, Yuba City and communities near Feathers River have been added to the evacuation order.
Hundreds of cars in wall-to-wall traffic were traveling on Highway 99 as people streamed out of Oroville away from the dam.
Heavy rains this week swelled Lake Oroville and keeping water moving over the dam takes pressure off the structure and reduces flooding risks, KGO reported.