The American Falls Dam, where improvements are set to conclude by the end of September.

AMERICAN FALLS, Idaho — A multi-year concrete project at American Falls Dam in eastern Idaho is slated to be finished by the end of September.

The $12.1 million project improves the durability of concrete on the spillway and stilling basin, according to the U.S. Bureau of Reclamation. Completing it adds flexibility in managing water for irrigation, power production, fisheries and other uses.

Work started in June 2020. It included demolition, removal and replacement of damaged concrete on the spillway and dam face.

The stilling basin lies below the dam’s angled spillway, which is adjacent to an Idaho Power Co. generation plant. Outflows pass primarily through the plant and secondarily through the spillway area, which was not available during construction.

“This project addresses aging-infrastructure issues at American Falls,” said project manager Nicole Carson.

The dam was built in 1978.

Completion of the project “allows Reclamation to meet its mission of reliably delivering water and ensuring the integrity of our facilities,” said Brian Stevens, water operations manager for Reclamation’s Upper Snake Field office in Heyburn, Idaho.

The dam impounds the American Falls Reservoir, over which State Highway 39 passes.

Other Upper Snake River reservoirs were used to store water for delivery to accommodate the project.

Water users’ allocations were not affected, Carson said.

Representatives from Reclamation, Idaho Power and the state departments of Fish and Game and Environmental Quality took part in a water quality technical team that has met weekly and will stay active, Carson and Stevens said.

The reservoir can hold up to 1,672,600 acre-feet of water. The dam is 94 feet tall.

Funding came from federal appropriations, space holders and Idaho Power. Ames Construction was the general contractor.