Engineers Assess Spillway Problem at Oroville Dam

Reservoir operators say that if current releases from the almost full reservoir can be maintained, it is less likely the lake will rise to the point where water flows into the emergency spillway at Lake Oroville over the weekend.

Earlier this week, chunks of concrete flew off the almost mile-long spillway, creating a 200-foot-long, 30-foot-deep hole.

The lake was rising at a half-foot per hour Thursday as the inflow peaked at 121,000 cubic feet per second, officials said at a news conference about a mile from the spillway. That could save them from having to use an emergency spillway for the first time in the reservoir's 48-year history. But the deluge was expected to continue easing into Saturday, low enough that officials can begin reducing the lake level to prepare for future winter storms.

The state hatchery inundated by the debris from the Oroville spillway produces nearly a third of all hatchery-raised fish in the state, supporting struggling native fish species and a $4 billion salmon-fishing industry.

Both the most recent report and the report from February 2015 note a "long-standing diagonal crack" on the left pier for one of the gates, but they say the crack had not extended to the point where it required an investigation.

Still, preparations are underway to use the emergency spillway.

Water topping the Oroville Dam may cause severe property damage, and it would be catastrophic for downstream salmon and steelhead populations.

DWR officials said the flow over the auxiliary spillway will range between 5,000 and 10,000 cubic feet per second (cfs).

The cost could approach $100 million, though department spokesman Doug Carlson said the estimate by a department engineer is an early, ballpark figure.

But faced with little choice, the state Department of Water Resources resumed ramping up the outflow from Lake Oroville over the damaged spillway to keep up with all the runoff from torrential rainfall in the Sierra Nevada foothills.

The department said it preferred not to use the emergency spillway because it would dump water onto trees and put debris into the Feather River, a source of water for parts of California.

Salmon eggs require clear, cool shallow water to hatch and the muddy torrent already plunging down the main spillway into the Feather River was enough to cause numerous eggs in the hatchery to be lost, officials said.

Officials said they expected the cavity to widen as a result - and it did.

They dumped rocks and poured cement in an attempt to create a course that will keep the emergency flow away from the existing spillway. Lake Oroville came within a foot of spilling into the emergency spillway in January 1997.

California's Oroville Dam is the tallest in the United States.