IDAHO FALLS — Engineers and work crews will begin blasting away a final layer of earth and rock before releasing water from a reservoir into a bypass tunnel, clearing the way for the final round of repairs to Ashton Dam.

PacifiCorp has been planning since 2004 to repair the 95-year-old hydroelectric dam that has been plagued by sinkholes.

Mark Stenberg, who oversees the dam, told the Post Register the reservoir level will drop 17 feet within two weeks of the blasting next week.

Conservationists and anglers are concerned the project will release a large amount of sediment that will clog places in the Henry's Fork bedrock where young trout hide from predators and where the trout's food sources hatch.

PacifiCorp spokesman Dave Eskelsen believes the company's plan makes the likelihood of that happening as small as possible.

Company officials believe sinkholes are developing because fine particles designed to restrain water flow were placed directly behind the dam's large-particle barrier.

Slowly, water from the reservoir has pushed the fine particles through the much more porous large-particle barrier. The displacement of fine particles creates cavities in the fill, which in turn lead to sinkholes in the dam's crest.

PacifiCorp plans to replace the existing fill with a more gradual and uniform layer behind the large-particle barrier. Plans call for replacing thousands of yards of "silty sands" now in place with several distinct layers of coarse gravel, fine gravel, sand and soil. Gravel would be spread over the soil to hold it in place.

The reservoir level will remain low through November, Eskelsen said. Next year, between March and May, workers will lower the reservoir again for the final stretch of repairs, scheduled to last into late fall.

"We have a high degree of confidence that we'll be able to finish the project on schedule and exactly as planned," Eskelsen said.