Condit Dam reservoir’s behavior murky after breach
Officials still surprised at how fast lake drained
By Kathie Durbin
Published: October 28, 2011, 12:00am
Share:
Northwestern Lake emptied after Condit Dam was breached on Wednesday.
One day after the breaching of Condit Dam, the former reservoir behind the dam continued to offer a fascinating case study in how quickly an altered landscape can revert to its original form.
Since Wednesday, when explosives opened a tunnel in the 98-year-old dam through which the reservoir drained in little more than an hour, sediment has continued to slough off the sides of the reservoir and also has built back up in places.
“We have a couple of logs that have placed themselves upstream of the tunnel,” said Todd Olson, decommissioning project manager for the dam’s owner, Portland-based PacifiCorp. “That is starting to slow the sediment down a little. But water and sediment is still moving through the tunnel — a very large amount of sediment.”
In the event of a log jam in the tunnel itself, general contractor JR Merit Inc. of Vancouver has a plan, Olson said: “They are prepared to use the cables extending across the river. They would be rigged with a grapple to pull out any problem logs.”
A time lapse video of Northwestern Lake draining like a bathtub after the Condit Dam breach on Wednesday, Oct. 26, by the University of Montana Geomorphology Lab.
Video
Much more sediment will be unleashed next May, when two coffer dams built to reroute the White Salmon River during the dam’s construction are removed. Large quantities of sediment have built up behind those dams.
PacifiCorp officials and their contractors are still trying to figure out why the reservoir emptied so rapidly — in just over an hour, instead of in the six hours the company had predicted.
“There was a lot of uncertainty in that estimate,” said PacifiCorp construction manager Tom Hickey. Among the unknowns were how much woody debris had piled up behind the dam and the river’s flow at the time of breaching.
“The tunnel sides didn’t change, as best we could see,” Hickey said. “There wasn’t any excessive fracturing. It was beneficial to the river to get it moving as fast as we could.”
Though they don’t have a precise measurement, officials think water flowed from the dam to the river’s mouth at a rate of 10,000 to 11,000 cubic feet per second — about what they had predicted.
Among the next tasks for the utility will be assessing the bed of the Columbia River downstream from the mouth of the White Salmon for navigational purposes, required by the U.S. Army Corps of Engineers, and studying the current profile of the former reservoir.
Those studies will compare planners’ conceptual visions with the post-dam reality and also help PacifiCorp come up with a final grading plan for the restored White Salmon.
Olson said he was pleased with the way the day unfolded according to plan, from morning safety briefings to the explosion that opened the tunnel at the base of the dam shortly after noon to the surge of water and sediment that flowed downstream to the Columbia.
“We were going to do this not on a time schedule but on a progress of action schedule,” he said.
Rod Engle, the U.S. Fish and Wildlife Service biologist who led the effort to move 679 tule fall chinook salmon from the lower river to above the dam before the breaching, was still reflecting Thursday on the speed and power of the river’s unleashing.
“I have been looking at footage all day,” he said from his Vancouver office. “I’m still trying to process it.”
Engle said he was most surprised by how many cut logs emerged from the reservoir sediment and by how fast the reservoir drained.
“I wish I could be out there every day to see it all,” he said wistfully. “This first year, there are going to be so many changes.”
As for the progeny of the rescued salmon, “Those fish are in the bank,” Engle said. “They are not impacted by this. It makes me very proud of our working group and the decision they made” to transport the threatened salmon upstream to their native waters to spawn rather than breed them in a hatchery.
“Hopefully, all of them spawned,” he said. “The progeny of those fish are up there.”
The salmon fry won’t emerge from their eggs until mid-March. In the meantime, Engle said, the heaviest winter rains in the area typically occur between December and February. “That’s when the bulk of sediment should move. After yesterday, I have to think that most of that sediment has moved out already. I really hope that those fish will have safe passage.”
