The U.S. Bureau of Reclamation through Sept. 9 will take public comments on a $6 million environmental-impact study of raising Anderson Ranch Dam by 6 feet.
The proposal would add about 29,000 acre-feet of storage capacity to the reservoir northeast of Mountain Home, Idaho. It currently holds 474,900 acre-feet of water behind a 456-foot-tall dam.
Officials have been studying opportunities to store more water in the three-dam Boise River system to accommodate population growth and create enough storage capacity to handle a 500-year flood. The Anderson Ranch project in the past year emerged as the preferred option over modifying the Arrowrock or Lucky Peak dams downstream.
The estimated cost of raising Anderson Ranch Dam is $31 million.
The Treasure Valley Water Users Association, whose members include irrigation districts and canal companies that supply water to agricultural users as well as residential and commercial developments, supports the proposal to raise the dam.
“One of our top priorities we support is additional storage for irrigation supplies here in the Treasure Valley,” Executive Director Roger Batt said. “With the population influx, there is only going to be increasing demand on our irrigation-storage supply. So it completely makes sense to have additional storage in this valley to meet future demand.”
The Idaho Water Resource Board has applied to to the federal Bureau of Reclamation for a water right related to raising Anderson Ranch Dam. However, “there is no clear indication of who is going to receive priority to that water as of today,” Batt said.
The Boise River is subject to minimum winter flows in the Boise metro area to benefit fish and wildlife, the riverside pathway network and other interests. In spring, flood-control releases occur about 70% of the time.
Anderson Ranch Reservoir is on the South Fork Boise River. Both are popular fisheries.
Trout Unlimited Idaho Water Project Staff Counsel Peter Anderson said rivers and fish adapt to high spring flows — which move gravel, reorder structure and create spawning areas. TU has not taken a position “except: Let’s look at these issues and see if there is a problem.”