Study of Anderson Ranch Dam expansion progressing

Aerial view of Anderson Ranch Dam & Reservoir. The reservoir was at 84percent capacity.

The U.S. Bureau of Reclamation and Idaho Water Resource Board next spring expect to be closer to formally proposing additional storage in the Boise River Basin, namely at Anderson Ranch dam and reservoir north of Mountain Home.

Project Manager Selena Moore, the bureau’s Snake River Office planning and project management supervisor, said Reclamation and IWRB are working on geotechnical analysis, feasibility-level design and other analysis for Anderson Ranch that is expected to be completed in March.

“Then we anticipate we will be performing environmental compliance, which will start with a public scoping meeting anticipated in May 2019,” she said.

Reclamation and IWRB — which last summer agreed to focus on expanding Anderson Ranch initially rather than Arrowrock or Lucky Peak dams — aim to complete the feasibility report and environmental impact statement in May 2020, and then go through the approval process with the U.S. Secretary of the Interior, Moore said.

Reclamation and IWRB are sharing the study’s $6 million cost equally.

“Reclamation has placed a high priority on enhancing our infrastructure, and we’re delighted to partner with the state to meet critical water supply needs,” Reclamation Snake River Area Manager Roland Springer said in a news release.

Additional capacity could enable storage of more runoff in high water years such as 2017, enhancing long-term water supply for critical irrigation, domestic, industrial and municipal needs while continuing to meet endangered species and power generation needs, Reclamation said in the release. Adding capacity also may provide more protection from flood risk.

IWRB manages available water supply, and addresses aquifer declines statewide including in the Treasure Valley and Mountain Home aquifers.

“Given the growth anticipated in the Treasure Valley, additional surface water storage may be required to offset aquifer declines and meet future water needs,” Board Chairman Roger Chase said in the release.

Reclamation-owned Arrowrock and Anderson Ranch prioritize irrigation. Lucky Peak, closest to Boise, is a U.S. Army Corps of Engineers dam prioritizing flood control.

Moore said Anderson Ranch is the least complex of the three to raise, enabling compliance with a major federal deadline, and is projected to provide the most water per dollar invested. It’s possible Reclamation and IWRB would evaluate raising the other dams later, she said.

The 456-foot Anderson Ranch Dam can store 474,900 acre-feet of water. The feasibility study contemplates raising it by six feet, which would provide an additional 6.1 percent or 29,000 acre-feet of storage.

Reclamation received partial funding for the study under the December 2016 federal Water Infrastructure Improvements for the Nation Act, which may also authorize funding for construction of projects that by Jan. 1, 2021, are determined to be feasible.


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