More than 100 people tuned in on Zoom for a Wednesday night webinar hosted by the Office of the Chehalis Basin to update the public on future plans for the Skookumchuck Dam, which is owned and operated by TransAlta.
The dam was originally built to supply water to the Centralia coal-fired steam generation facility, which is set to close in 2026.
Both Anchor QEA Senior Managing Scientist Merri Martz and Nat Kale, principal planner for the Office of the Chehalis Basin, talked about the main goals of their research: examining flood and the effects on fish.
“The Skookumchuck River has been home to runs of migratory fish for a very long time. Steelhead, coho, spring and fall chinook all use the river,” Kale said.
Kale said the reservoir created by the dam does aid with flood prevention, though that’s not its main purpose. Martz explained that despite a fish sluice being built in the 1970s, there is still an issue with fish passage at the dam.
“The fish sluice that is built into the spillway has a bottom elevation of 464 feet, so when the reservoir is between the elevations of 464 feet and 477 feet, water can go through the fish sluice. When the reservoir drops below that point, then there is no longer water going into that fish sluice, so there would be no downstream passage,” Martz said.
Currently, Martz and her colleagues are looking into several sluice upgrade options, including making modifications to the existing sluice or changing over to a flume system like those used on other rivers.
“You could create a much longer flume that would allow fish once they entered the sluice to continue passively downstream, just on their merry way. This is used in many other dams including those on the Columbia and Snake Rivers, there are long flume passageways that come out of dams and they are highly effective, but you do have to drop down about 200 feet in elevation,” Martz said.
The flume can’t be too steep either, with a 4% grade maximum angle allowed. To get a 200-foot drop in elevation means that the proposed flume for the Skookumchuck would be somewhere between 3,100 and 3,500 feet long.
The Office of the Chehalis Basin is also looking into upgrading the dam’s discharge pipe to help with flooding control. The reservoir has capacity to hold 35,000 acre feet of water. However, the current discharge pipes are only capable of discharging 600 acre feet of water per day.
“It takes about two months to fully empty the reservoir and as we know nature does not give us two months of rain-free days after every storm,” Kale said.
The Office of the Chehalis Basin is continuing work on the second phase of its study on the Skookumchuck Dam and should have their final findings compiled by the end of this year or by January 2023.
To learn more about the study and the dam, visit https://chehalisbasinstrategy.com/skookumchuck-dam-study/.
