Work group continues studying Willwood sediment issues

Posted 6/2/20

Work continues to address the sediment issues at Willwood Dam. Two years ago, repair work on the dam released sediment into the Shoshone River, which turned the Shoshone River waters below the dam …

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Work group continues studying Willwood sediment issues


Work continues to address the sediment issues at Willwood Dam. Two years ago, repair work on the dam released sediment into the Shoshone River, which turned the Shoshone River waters below the dam into a gray sludge and impacted aquatic life.

Jason Burckhardt, fisheries biologist with the Cody office of the Wyoming Game and Fish said that analyses show that the overall trout population is up, but “we’re still not where we need to be.”

Overall estimates placed the fish population in 2016 at 442 per mile. That went to 585 in 2017 and then 763 in 2018.

Another measurement used by the Game and Fish Department is fish biomass per mile, which was at 309 in 2016. That dropped to 268 in 2017 and rose to 397 in 2018.

“The release of the sediment had an impact, whether that’s chronic or acute,” Burckhardt said.

Three groups were formed to address the sediment issue with different roles to play in studying the problem and coming up with solutions. The first group was charged with cleaning up the results of the discharge. A second group is looking at long-term management strategies above the dam and evaluating water quality standards. A third group is addressing sediment sources upstream from the dam.

At a May 15 meeting, the group discussed the ongoing data collection as they try to determine where the sediment is coming from, so they can evaluate possible methods to manage sediment releases.

It’s a daunting task, requiring various approaches. David Waterstreet, program manager with the Wyoming Department of Environmental Quality, said they have made a lot of progress but there’s still work to be done before they have a complete picture of what’s happening with sediment around Willwood Dam.

“I think we’re going to be successful in determining a path forward, but it’s definitely been a lot of work,” Waterstreet said in an interview.

On both sides of the dam, the group is measuring sediment with two different gauges: a turbidity meter and a sonar device. The turbidity meter is highly accurate but it has a limited range. The sonar is able to gather data over a greater range with higher concentrations.

Sediment flows can occur from natural sources, as well as dam operations. Understanding how runoff affects turbidity is part of what they’re looking at.

Travis Moger, Willwood Irrigation District manager, said this year’s weather has so far lacked the kind of events that provide important data.

“We haven’t had one rainstorm yet that made for any runoff,” he said.

Jason Alexander, a hydrologist with the United States Geological Survey, discussed the need for a greater number of bathymetric surveys throughout the year. He explained that, while they can make inferences from the gauges they’re using, they won’t be able to tell where the sediment is coming from and how it’s flowing before and after dam operations.

Alexander suggested doing a survey before the dam evacuated, another the following spring, and then another the following fall. This would give them a lot better understanding of the annual flow of sediment than just the gauges show.

“It would be an additional data point that would help us not only close the sediment budget, but also tell us where the sediment is coming from behind the dam,” Alexander said.

He said that would give a better idea of where and when to excavate, as well as how much progress is made evacuating sediment over the winter.

To date, the team has conducted two surveys. Adding these other surveys, Alexander proposed, would make the group more competitive for grants that would help support their work.

As it is now, Alexander said, there are once-per-year surveys and not after an evacuation event. Armed with that information, the group would be better able to apply for grants to fund such operations.

“I think that would add … an element to the planning,” he said.

Waterstreet added that they need to know how much sediment is behind the dam and how far upstream it goes. A secondary question, that they need more data to answer, is the necessary level of sediment removal to restore functional operation of the dam — without risking huge releases of sediment like was seen in the 2016 event.

“We might not have to bring out all the sediment behind the dam because we’re in a very sediment-heavy area of the state, but operationally how far do we need to go?” Waterstreet posed.

The group is looking at some methods to deal with sediment releases during high flows, but it’s challenging.

“You can’t make a lot of adjustments at the dam to force something to happen,” Waterstreet said, but there might be some opportunities to move additional sediment.

The group discussed a couple mechanical options that might work, though it’s all theoretical at this point. They might be able to use a track loader, but the length of reach by the excavator could limit its usefulness. Air sparging would force air through the sediment to dislodge it. Another idea thrown out is using a dragline. All of the options cost money, and so the group would need the data to show they’ll actually provide solutions.

They may also be able to change operations to help manage the sediment releases.