Ever since a repair operation at Willwood Dam unintentionally released a deluge of sediment down the Shoshone River — killing fish and creating sludge downstream — a group of people from state and federal governments, as well as irrigation districts, have been trying to figure out how to prevent such an event from happening again. 

In April, the Willwood Dam/Shoshone River Work Group 2 (WG2) performed an experiment at the dam to determine how much sediment would be released during a short period of time with a high rate of flow. 

A “significant amount” of sediment went downstream from the dam, said David Waterstreet, program manager with the Wyoming Department of Environmental Quality. The total estimated volume was around 10,000 tons.

Jason Alexander, supervisory hydrologist with the U.S. Geological Survey, said the dam could pass more sediment at safe concentrations, if one of its sluice gates wasn’t stuck closed.

He said the experiment allowed them to better understand the current operational capacity of the dam and identify potential impacts of various alternative sediment management strategies.

The goal was to release enough material to help minimize the amount of sediment building up behind the dam, while not releasing so much that it negatively impacted fisheries downstream. 

Based on preliminary calculations, the operation achieved those aims, Waterstreet said.

“We learned the shorter duration, higher flow and concentration release of sediment was better able to move sediment downstream,” he explained.

What the work group learned from the operation will guide planning for fall releases, when the fish are hatching, as well as giving better recommendations for spring operations. 

It’s during the spring the Shoshone and dam provide more water flow with more power, which cleans gravel in preparation for the next spawning season.

“It’s like putting it in a washing machine,” Waterstreet said. 

The instrumentation they’ve been using — which includes a number of short- and longer-range scientific sensors to measure the sediment in the water — managed to hold up through the various experiments they’ve been conducting since the group started working on the problem. So, Waterstreet said, they have a good amount of data to look at. 

“We’ve completed the majority of our experiments and data collection,” he said.

The U.S. Geological Survey will take all the work WG2 has done — which includes studies, experiments, and data — and use it in a scientific report to describe the annual sediment budget, which quantifies the movement of sediment down the Shoshone River. This report will inform future recommendations for Willwood Dam operations. 

In addition to making those recommendations, the group is also exploring options to assist with sediment mobilization. Willwood Dam diverts water from the river into the Willwood Irrigation District system. Some options explore other methods for water delivery to those canals and laterals.

Waterstreet said he expects that the work the group has done will be useful in the future for other low-level dams facing sediment challenges, in Wyoming and other states. 

WG2 includes members from the Game and Fish, Willwood Irrigation District, the Bureau of Reclamation, Wyoming Department of Environmental Quality, U.S. Geological Survey and a number of other agencies. 

As WG2 moves from the data collection phase to the reporting phase, Work Group 3 is exploring methods to keep sediment from getting into the river upstream from the dam. Their efforts include building beaver dam analogs along Sulphur Creek, which are man-made structures designed to mimic the effects of beaver dams. They hope the analogs will help stabilize stream banks by promoting the reestablishment of woody vegetation, while helping to better regulate flows.

This fall, the group initiated a study on an abandoned bentonite mine west of Cody, which they believe could be a significant source of sediment. The study will determine best practices for establishing vegetation and helping control sediment flows off the mine. 

Like WG2, WG3 is composed of members from state and federal entities. Volunteer groups, such as East Yellowstone Chapter of Trout Unlimited, are also contributing work.