1. Project detail

Work on raising Anderson Ranch Dam will start slightly later than anticipated based on new designs, according to U.S. Bureau of Reclamation officials.

Construction is now expected to start in early to mid-2026 and take about 3½ years, said Chris Keith, the project manager.

A late 2025 construction start was envisioned last year, when the project reached the 30% design milestone. Design progress recently reached 60%.

“We’ve done a lot of work between 30% and 60%, and it’s the next iteration of our design,” Keith said.

Each time a milestone is reached — the next will be at 90% — “we better understand the design requirements, we better refine those design requirements and we know a lot more about constructability of the project. And it helps us lead to more assurance with our cost estimates.”

The project cost estimate has increased by $1 million, to $125.8 million, as a result of the analysis completed at the 60% design stage.

“We were thinking in certain parts of the project we would be able to decrease cost estimates, where we better understand project requirements,” Keith said. “Some things we thought would go down, and there were some parts of the design we were not sure on. We needed to do the work and figure out the methods and the requirements for raising the facility.”

Estimated costs decreased for mitigation work around the reservoir rim such as moving a campsite to higher ground or adding riprap to control erosion, he said.

The 0.8% increase in the total project’s estimated cost can be attributed to higher material costs, “with the mechanical and electrical components of the dam raise construction design identified between the 30% and 60% design phases,” said Marc Ayalin, a Reclamation public affairs specialist.

Reaching the 60% design milestone also helps the analysis that is part of the Supplemental Draft Environmental Impact Statement, Keith said.

The Supplemental Draft EIS is expected to be released in 2025, Ayalin said.

An exact schedule is yet to be determined. A Final EIS and record of decision would follow.

Reclamation in late 2022 started the supplemental process to address concerns raised during the comment period.

Concerns included the potential need for reservoir operational restrictions during construction, and water-supply mitigation.

A design change was made to the spillway, in part to reduce construction time and the potential need for reservoir restriction.

The team continued to refine the spillway design, Keith said.

Part of the existing spillway will be replaced with an overlay, which will allow for the added reservoir elevation.

Anderson Ranch Dam, northeast of Mountain Home on the South Fork Boise River, is the largest and farthest upstream of the three major dams on the Boise River.

The reservoir can hold 413,000 acre-feet of water.

Plans call for increasing capacity by about 29,000 acre-feet by raising the 456-foot dam 6 feet.

Reclamation and the Idaho Water Resource Board are partnering on the project.

Proponents see the expansion as benefiting a range of water uses as the region’s population grows, and increasing management options.