California officials worked frantically into the night Sunday to evacuate thousands of residents downstream from the Oroville Dam after a hole on an emergency spillway raised fears of flash floods.
"There was significant concern that it would compromise the integrity of the spillway, resulting in a substantial release of water," Kory Honea, the Butte County Sheriff, told reporters at a press conference Sunday evening. "I couldn't risk the lives of thousands of people so we took this rather significant step."
The emergency spillway at the nation's tallest dam was activated Saturday for the first time ever in the dam's 48-year history after the swollen dam reached above its capacity following a deluge of rain in the Northern California region.
However, state engineers on Sunday discovered significant erosion had occurred back towards the face of the emergency spillway at Oroville Dam after huge water outflows, meaning the structural integrity of the dam's auxiliary spillway was at risk. The emergency spillway is an unlined channel that is essentially a natural hillside with rocks, brush and other debris.
Normally the dam would use its primary spillway but that too was found to have significant concrete erosion earlier in the week.
"The concern is that erosion at the head of the auxiliary spillway threatens to undermine the concrete weir and allow large, uncontrolled releases of water from Lake Oroville," the California Department of Water Resources said Sunday. "Those potential flows could exceed the capacity of downstream channels."