Oroville dam releases further erode hole in damaged spillway

Integrity of dam is not affected from latest cascade of water.

As storm runoff poured into fast rising Lake Oroville Thursday, the state resumed releases down the reservoir’s damaged spillway, creating dramatic scenes of muddy torrents gushing over the concrete chute.

The releases tore a larger hole in the spillway and jumped over the structure, creating an instant waterfall.

“These pictures are dramatic and they will continue to change and be dramatic,” William Croyle, acting director of the state Department of Water Resources, said at an evening news briefing.

“As we upped the flows … that’s further eroding around the lower part of the spillway and the spillway itself. That’s not a surprise to us,” he added.

The spillway is separate from the dam, Croyle emphasized, adding that “none of this is affecting the integrity of the dam itself.”

State officials stopped the spillway releases Tuesday after workers discovered a huge hole in the lower portion of the long concrete chute.

But with this week’s storms dumping more runoff in the reservoir and pushing the lake closer to the level that would trigger releases down a separate emergency spillway, officials decided they had no choice but to use the eroded structure.

Thursday afternoon the reservoir level was 884 feet, 17 feet below the emergency release point of 901 feet. Croyle said managers hoped that if they could get more water out of the reservoir in coming hours they could keep the lake below the critical level.

But if that doesn’t work, Croyle said it was possible water would start flowing over the unpaved emergency spillway sometime Saturday.

“It will be pretty amazing to see what happens,” he said. “It will be a thin veneer of water flowing over the crest into ravines and into the river.”

Crews were clearing trees and brush from the emergency pathway Thursday. The state also moved a couple of million fish out of the downstream Feather River Hatchery because water from the eroded spillway is too muddy for hatchery use.

Officials do not know what caused the collapse of a section of the spillway, gouging a hole that was 45 feet deep and 250 feet long. The cavity has since doubled in size.

The state’s second largest reservoir, Oroville provides the main storage for the State Water Project that sends Northern California supplies to the Southland.