Sluice gates open to lower Shoshone River for study

The Willwood Irrigation District took a gander at its dam last week.

Two of three sluice gates are partially open on Willwood Dam to lower the Shoshone River on the upstream side so district employees can determine what repairs are needed for the sluice gates and examine sediment that’s been piling up there for years. (See related story below)

From the downstream side of the dam, the water jets from gates like a horizontal fire hydrant gushing full blast.

The dam is releasing between 420 and 430 cubic feet per second (cfs). That water volume is enough to support the fishery. Many winters, they only release 200 cfs, Willwood Irrigation District Manager Tom Walker said.

Now visible, perhaps 6 inches above the swirling surface, are timbers. The timbers are a plug to keep the water out of an unused penstock, Walker said.

District employees spotted a BMW motorcycle in the receding water that they fished out of the river. It has Park County plates, but Walker could not tell what year the plates were issued.

Other than looking a little muddy, the bike is in remarkably good shape.

Walker suspects the bike was stolen, then ditched in the river.

The dam was completed in 1909. Years of sediment is heaped just upstream of it. The dam contains three sluice gates. The north and middle gates are in operation, but the south gate is buried in silt — tons of it.

The south gate has been covered in silt since 1973. The district suspected there was around 350,000 cubic yards of sediment upstream of the dam. At one time, the cost estimate to remove the sediment was $5 million, Walker said.

On the south bank the scouring river has undercut the bank, forming a jagged small-scale cliff hanging precariously over the water.

“All this has accumulated over decades,” Walker said.

He wants to get some silt out of there, but not necessarily all of it.

Tom Walker, Willwood Irrigation District’s manager, wants to gradually ease silt through Willwood Dam’s gates so the sediment does not build up and jam the gates shut. If enough silt can pass through the gates, a costly silt-dredging operation will not be necessary, he said.

That was part of Walker’s update Feb. 11 at the annual meeting at district headquarters on Road 9.

There are three sluice gates on the dam. The north gate was the only gate operating. The other two gates were blocked by silt.

However, district employees were able to open the middle gate at the end of the irrigation season last year, Walker said. For years, he believed the gate could not be opened but they were able to open it.

“We got really, really lucky,” said Keith Murray, district chairman.

Over the summer when water volume was higher, silt flowed through the gates, without muddying the water or hurting fish, Walker said.

“We still have to maintain the river,” Walker said. “We’re still responsible for not damaging the fishery.”

For years, Walker has been planning a massive dredging operation because it was believed that was the only course the district could take to keep the gates open.

His hope is this summer more silt can be removed. Not all the silt requires removal. If enough of the stuff can be passed through the gates, they won’t be forced to dredge now or in the future, Walker said.

Still, the 92-year-old dam needs some fixing.

The Wyoming Department of Environment Quality OK’d a variance to allow the district to lower the water level on the dam.

Once the water goes down, they can ascertain the amount of silt and whether a dredging project is necessary. And, the district can determine the degree of dam rehabilitation needed this fall, Walker said.

At the end of the 2015 irrigation season, the district will again lower the river just enough to place a temporary cofferdam for dam rehabilitation.

Work will be done on the north and middle gates. Gate stems and the brackets that hold them will be replaced along with dam equipment on top. He hopes they won’t have to replace the sluice gates. Necessary repairs also will be made to the canal gate, Walker said.

Walker cannot estimate the actual cost of the project without seeing what is waiting under water. But $1.6 million has been appropriated. Two-thirds of the money is from the Wyoming Water Development Commission (WWDC) and the remaining third is the district’s responsibility. Walker said he is hoping the project will cost less than the above figure.

This WWDC grant will cover two-thirds of materials, engineering and construction costs, Walker said.

He estimates dam repairs will take two to three months at most.

The $1.6 million was secured when Walker believed costly dredging was required. “I’ve been working on it since 2007,” he said.