Anderson Ranch Dam

Anderson Ranch Dam is 456 feet tall and is on the South Fork of the Boise River, 28 miles northeast of Mountain Home, Idaho.

The U.S. Bureau of Reclamation on July 31 plans to release the draft Environmental Impact Statement for a proposal to raise Anderson Ranch Dam by 6 feet to help increase the water supply for growing southwest Idaho.

The draft EIS analyzes alternatives including taking no action, proceeding with Reclamation’s preferred 6-foot raise, and a recently added approach that would raise the dam by 3 feet. Release of the draft EIS triggers a 45-day comment period.

Public meetings are planned in August in Boise and Mountain Home. Reclamation is working to finalize dates, times and venues given COVID-19 considerations, said Bryan Horsburgh, the agency’s acting area manager based in Boise.

The draft EIS will be followed by about a week by release of a draft feasibility report analyzing project aspects including the economic impacts of the various alternatives, and potential contracting mechanisms for the additional stored water.

The 456-foot-tall Anderson Ranch Dam on the South Fork Boise River has a storage capacity of about 413,000 acre-feet. Raising it by 6 feet would add about 29,000 acre-feet.

Release of the draft EIS was delayed by several months as officials analyzed the order by which refill rights for the additional water would be granted, per the state’s water-rights accounting system, and how that would impact federally required flow augmentation for salmon and steelhead, Horsburgh said. Refill rights come into play during high-water years that bring flood-control releases from dams.

Officials also spent time developing a second action alternative. Raising the dam by 3 feet was selected after approaches that would not build additional capacity — like lining and automating canals, or using the state’s Treasure Valley Managed Recharge Study of the aquifer — were deemed much less viable.

“Now that we have resolved those two issues, we are back on schedule to meet the Jan. 1, 2021, WIIN Act deadline,” Horsburgh said, for potential authorization under the Water Infrastructure Improvements for the Nation Act.

Reclamation officials reported progress toward meeting the deadline to the Idaho Water Resource Board May 28.

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