To take the next step in trying to reduce the amount of sediment entering Bighorn Reservoir, the Bureau of Reclamation needs to find about $150,000 for a study.
“It’s more of an appraisal to see if there’s enough of a project there to take it to the feasibility study,” said Lenny Duberstein, head planner at the BuRec Montana Area Office.
“If it looks like it will pan out, then we go to a feasibility study, which takes congressional authorization,” he added. “That wouldn’t necessarily happen quickly.”
Bob Croft, a board member of the Lovell, Wyo.-based Friends of Bighorn Lake, sees great possibilities in the development of settling ponds above the reservoir as a way to capture the sediment before it enters the reservoir, although he noted that there are many unanswered questions.
“We’ve eliminated most of the proposals that were made two years ago,” Croft said. “We’ve done several on-site tours. I think it’s going to happen fairly rapidly because there is so much hingeing on it.”
Bighorn Reservoir cuts a sinuous, 71-mile course through a deep desert-like canyon while crossing the Montana-Wyoming border. At full pool, the reservoir's Yellowtail Dam impounds about 1.32 million acre-feet of water, covering roughly 17,200 acres.
The problem of sediment filling the southern end of the reservoir has been a concern for at least 20 years. Extensive drought earlier this decade heightened reservoir users’ anxiety when boat ramps close to Lovell, Wyo., were left high and dry. The problem was especially evident at Horseshoe Bend Marina in the Bighorn Canyon National Recreation Area.
The area provides the closest and most developed access to the reservoir on its southern end. But because of its location at a bend in the historic river channel, it is a natural sediment trap. As sediment has built up at Horseshoe Bend, higher reservoir levels are needed to ensure it is usable.
In 2010, the Bureau of Reclamation released a study of options to reduce sedimentation, looking at everything from dredging Horseshoe Bend to raising lake levels. None of the options is without extensive costs or consequences.
Croft sees the creation of two settling ponds along the Bighorn River above the reservoir as the most practical option remaining to extend the life of the water body. One pond would be used for about five years or until filled, then it would be left to dry while the other pond would be used. Once the sediment is dried, it could be removed and possibly used for topsoil for remediation at local bentonite mining sites. Or maybe the sediment could be sold as a marketable product, Croft said, helping to defray the costs of the operation.
Not without problems
In the 2010 BuRec study, the cost of building and maintaining a sediment trap was estimated at $34 million. The study calculated that a trap that captured 70 percent of the Bighorn River’s sediment load could keep 6 to 8 feet of sediment out of Horseshoe Bend over 20 years. An estimated 6,900 to 11,500 acre-feet of material could be removed from the system.
The problems foreseen with the sediment trap included blowing sand from the drying basin, an alteration to the habitat of the Yellowtail Habitat Area at the southern end of the reservoir affecting the species that use it, the cost of maintaining the trap and what to do with the material that is captured.
Duberstein said the reservoir is doing “reasonably well as far as sedimentation goes.” He said the trend for sediment deposition is even slightly under what was predicted in the 2010 study. More than 50 feet of sediment has settled into some portions of Horseshoe Bend since the dam was closed in the 1960s. The study predicted that by 2017 or 2027, depending on river flows, another 12 feet of sediment could be added to the area.
Still, Duberstein said, “This reservoir isn’t even on the top of our list for sediment problems.”
He noted that Fresno Reservoir on the Milk River in northern Montana has lost about one-third of its water storage capacity since it was built in the 1930s. Bighorn Reservoir has lost only 4 to 5 percent, he said.
Croft sees the sedimentation work as the natural next step of the Bighorn River Systems Issues Group that was formed to bring reservoir advocates in Wyoming to the negotiating table with trout advocates who fish the Bighorn River below the reservoir’s Yellowtail Dam.
For years they had been at odds over the allocation of the reservoir's water – river users wanted more downstream for trout, reservoir users wanted more left in the lake. For now, that dispute has quieted as reservoir operations have become more transparent.
“We’re moving ahead, and it’s been a positive experience,” Croft said.
“Extending the life of the reservoir for another 100 to 200 years is what we’re talking about now.”