A plan to raise the crest of Anderson Ranch Dam in Idaho by 6 feet was deemed feasible by the U.S. Bureau of Reclamation, which sent a final report with the finding to Congress on Dec. 9 for authorization.
Initial feasibility-level designs indicate the proposed dam raise, which could create up to an additional 29,000 acre-feet of water storage, could take up to four years and cost $83 million.
The plan was developed in partnership with the Idaho Water Resource Board under the Water Infrastructure Improvements for the Nation Act signed into law Dec. 15, 2016, by then-President Barack Obama. The act limits dam improvements to those increasing water storage.
The report is "an important step towards addressing the water supply needs of the Treasure Valley, along with providing benefits for fish and wildlife," said BuRec's Snake River Area Manager Lanie Paquin in a statement. "We are on track to meet the next important milestone—finalizing the companion environmental impact statement."
The comment period on the draft EIS closed Sept. 14.
The report describes how additional water storage capacity could help offset changes in precipitation patterns, increase runoff storage in high water years, enhance long-term water supply and increase power generation.
The earthfilled dam was built between 1941 and 1950 on the south fork of Idaho's Boise River near Mountain Home. Its main purpose was to provide irrigation supply and flood control, and it includes a 40 MW hydro project that entered service in 1950 and is part of the Federal Columbia River Power System.
The final report estimates flow from the added storage could generate an additional 3,850 MWh annually, and that the 6 feet of additional hydraulic head during full pool conditions could generate an additional 2,460 MWh annually.
The value of this added generation works out to $3.8 million, in 2025 dollars, over the project's 100-year planning horizon.
The dam is part of the Boise River System, which also includes Arrowrock and Lucky Peak reservoirs.
An initial analysis of the three dams indicated that raising Anderson Ranch Dam would provide the most water for the investment, and could be achieved through relatively low complexity. "Potential raises at the other two dams may be evaluated in future years," a BuRec FAQ on the project stated