A final decision on whether to raise the Anderson Ranch Dam northeast of Mountain Home, Idaho, could come next spring.
The dam is on the South Fork Boise River.
The U.S. Bureau of Reclamation aims to release the Boise River Basin Feasibility Study Final Environmental Impact Statement in early 2021, followed by a Record of Decision in the spring, said Michael Coffey, Boise-based spokeswoman for Reclamation’s Columbia-Pacific Northwest Region.
Comments on the Draft EIS, taken from July 31 through Sept. 14, will be considered.
Reclamation and the Idaho Water Resource Board have been studying options for increasing storage capacity on the Boise River system, partly in response to population growth.
Anderson Ranch was deemed more feasible to raise than Arrowrock and Lucky Peak dams. The project stands to benefit irrigation and other users.
Reclamation’s preferred alternative calls for raising the 456-foot dam by 6 feet, which would add about 29,000 acre-feet of storage capacity to the current total of 413,000. This would inundate about 146 additional acres around Anderson Ranch Reservoir and cost more than $87 million.
The other alternative, which would cost around 31% less, would raise the dam by 3 feet, add 14,400 acre-feet of capacity and inundate an additional 73 acres.
Coffey said most comments on the Draft EIS related to water rights, the water-contracting process, recreation and flows below Anderson Ranch Dam in the South Fork.
The Idaho Water Users Association and Treasure Valley Water Users Association in written comments said they support the project, with safeguards. They said any shortfalls of allowed water during the four-year construction period must be mitigated. They said Reclamation should assure existing storage entitlements are met after construction, contract directly with space-holders, own the water right associated with additional storage and evaluate whether uncontracted space in Lucky Peak Reservoir is available.
The impact of lower flows during the spring worried Trout Unlimited and the Idaho Wildlife Federation.
“Nowhere in the document is there a hard look at the impacts of the decreased flow releases in the spring months on the ecological conditions of the South Fork of the Boise River,” the groups commented. “Elements such as higher-flow effects on spawning and rearing habitat for rainbow trout, movement of sediment and larger substrate from years with higher flood-flow releases, loss of floodplain connection, impacts on riparian vegetation and reductions in side-channel connections and wetlands all deserve examination.”
Cynthia Bridge Clark, water projects section manager at the Idaho Department of Water Resources, said the board is reviewing the draft’s findings “and will be focused on options related to projects, financing and contracting. And the board will be looking for input from the public and water users with regard to interest in new storage space.”