Frequently Asked Qustions

Anderson Ranch Dam Raise

How will the dam be raised?

The dam raise is described as a downstream zoned earth raise, parapet wall, and as necessary spillway modifications to allow for additional fill to be placed against the spillway walls at the dam crest structure. The feasibility design of the dam raise modification includes crest and abutment excavation, foundation treatment, zoned earth fill, parapet wall, downstream soil cement slope, and the road across the dam to allow for two-way traffic. Materials for the embankment raise will be placed above the existing embankment material. Materials will be produced from the excavation phase of work or borrowed from nearby sources. Other materials are proposed to be commercially sourced.

Anderson Ranch Dam raise will be constructed in accordance with Reclamation's Safety of Dam's guidelines and standards for embankment dams.

When is construction planned to start?

Current feasibility level estimated schedules show construction beginning in 2025.

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How long would it take to build the raise of Anderson Ranch Dam?

Initial feasibility-level designs estimate a 3.5 to 4-year construction window for the proposed raise of Anderson Ranch Dam.

What does further design on the project mean?

Currently, designs and cost estimates for the proposed plan are developed to a feasibility level as verified through the design, estimating, and construction review process. Beginning in fall 2021 through winter 2023, Reclamation plans to complete the final design phase for the proposed dam raise plan. During this time, Reclamation will take the current design from feasibility level to 30%, 60%, 90% design levels and then to a complete 100% final design specification and cost estimate.

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Will there be road closures during construction?

During construction, Highway District Road 134 leading from U.S. Highway 20 to and crossing Anderson Ranch Dam would be closed when required during the construction process. An alternate detour route is identified to enable travel between U.S. Highway 20 and the west side of Anderson Ranch Dam and reservoir.

This proposed route would be on Highway District Road 131 crossing the South Fork Boise River at Cow Creek Bridge, roughly eight miles downstream from the dam. Prior to closing the dam road, a section of this detour route would be widened and straightened to improve driver visibility and facilitate safer passage of larger vehicles and trailers. This detour route would be kept open and maintained year-round throughout the project.

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Are there potential impacts to the Pine Bridge?

The Pine Bridge over the South Fork Boise River was identified as being potentially impacted by the proposed 6-foot surface water elevation increase. The proposed increase in reservoir water surface elevation could reduce the bridge’s existing designed freeboard below the 2-foot minimum required by Idaho Transportation Department during a 50-year flood event if the reservoir level is high when a flood event occurs. Reclamation acknowledges a low probability of the conditions occurring that would result in a reduction of freeboard and therefore is pursuing a variance.

If granted, the variance would negate the need for the proposed abutment modifications, including a modified design that would increase the superstructure elevation by 1 foot to satisfy the minimum ITD freeboard requirement.

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What are the potential impacts to boat docks and campgrounds?

Although there will be no permanent loss of access to recreation sites or reduction in the number of sites available around Anderson Ranch Reservoir, there will be temporary effects. The dam raise would cause many existing recreation facilities around the reservoir to be seasonally flooded in years when the reservoir refills completely. Several sites would need to be shifted above newly increased water levels. These recreation facility modifications are incorporated into the proposed project.

During the project, many campgrounds around the reservoir would be closed temporarily while they are modified and reconstructed. Reclamation would minimize this disruption by scheduling construction at campground and boat ramp facilities outside of peak use season. Boat ramp access would be maintained throughout construction.

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Will there be any changes to flows in the South Fork Boise River below the dam?

Overall, water operations at Anderson Ranch Dam would not significantly change. Following construction, refill of the additional reservoir storage space would occur in high-runoff years. During these years, peak flows in the South Fork Boise River that have historically ranged between 4,400 cubic feet per second and 5,900 cfs would be reduced by up to 710 cfs, for one to seven days. Summer, fall, and winter flows would remain virtually unchanged.

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Boise River Basin Feasibility Study

What is the purpose of a feasibility study?

The purpose of a Reclamation feasibility study is to determine the viability of a proposed plan or project based on the following:

  • How well the planning objectives are met.
  • The economic justification.
  • The validity of the scientific, technical, and design assumptions.
  • The ability to construct a project, implement a non-structural plan, or both, according to Reclamation standards and practices, within the estimated cost and schedule.
  • The reliability of the estimated costs and benefits.
  • The reliability of the proposed construction schedule.
  • The capability and willingness of project partner(s) to financially support the project.

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Why is Reclamation studying storage?

Additional surface water storage can be an important element for addressing water supply needs in the Treasure Valley and surrounding areas.

Water users in the Boise River basin rely heavily on the existing reservoir system as well as the snowpack to store and manage surface water supplies. Despite the reservoirs and additional storage provided by the higher elevation snowpack, an average of 1.1 million acre-feet of water leaves the Boise River basin annually. Ground water rights are limited in some areas and the interconnectivity between ground and surface water has resulted in restrictions on new ground water development in parts of the Treasure Valley.

In addition, predictions of changes in precipitation and runoff patterns due to climate variability may require additional surface water storage capacity to capture rainfall previously held as snowpack, as well as excess water generated in wet years to offset dry years.

In general, additional water storage would enhance and sustain long-term water supply for critical irrigation, domestic, municipal, and industrial needs, while continuing to meet endangered species and power generation needs. It would also have the potential to provide additional flood risk protection and store additional runoff in high water years similar to 2017.

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Why is Reclamation only looking at storage?

Public Law 111-11 authorized Reclamation to perform feasibility studies on storage projects identified in Reclamation’s 2006 Boise/Payette Water Storage Assessment Report. Reclamation received additional authority and funding under Public Law 114-322 – the Water Infrastructure Improvements for the Nation Act. This authority is limited to storage projects.

The Idaho Water Resource Board, Reclamation’s feasibility study partner, has authority to participate in the project under House Joint Memorial 8 which focuses on storage projects. The 2019 Idaho State Legislature passed House Joint Memorial 4 and House Bill 285 affirming support for the feasibility study and identified a raise of Anderson Ranch as a high priority for the State of Idaho.

The Boise River Basin Feasibility Study, at the request of the Idaho Water Resource Board, was initiated in 2017, published in November 2020, and its proposed plan was found feasible in December 2020.

Why is Reclamation only focusing on Anderson Ranch Dam at this time?

Based on results from initial data collection of Anderson Ranch, Arrowrock, and Lucky Peak dams, Reclamation and IWRB focused the feasibility study on a raise of Anderson Ranch Dam due to its relatively low complexity and projections that the reservoir will provide the most water for the investment. Potential raises at the other two dams may be evaluated in future years.

In December 2020, the Secretary of the Interior determined the recommended plan, to raise the dam crest of Anderson Ranch Dam 6 feet for an additional 29,000 acre-feet of storage capacity, met the requirements of Section 4007 of the Water Infrastructure Improvements for the Nation Act.

Where does the projected future water demand number come from?

A water demand projection study completed for the Idaho Water Resource Board in 2016 predicted that population in the Treasure Valley would increase to 1.57 million people by 2065. The report concluded there would be a water demand increase for domestic, commercial, municipal, and industrial uses of 109,000-to-188,000-acre feet by 2065. (Treasure Valley DCMI-Water Demand Projections 2015-2065, SPF Water Engineering, LLC, Aug. 8, 2016.)

For more information, please visit the IWRB project website at

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Environmental Compliance (NEPA)

Why is the EIS on pause and what does that mean?

The EIS is on pause to allow Reclamation’s Technical Service Center to complete further design of the dam raise and associated project components. With a further developed design, the EIS resource specialists can better identify impacts and corresponding mitigation in the final EIS.

“On pause” means the EIS is delayed until Reclamation can incorporate information from the design process and impact analyses from Reclamation's resource specialists. Work on the EIS is expected to resume late 2022.

For more information on the draft environmental impact statement, please visit

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What does further design on the project mean?

Currently, designs and cost estimates for the proposed plan are developed to a feasibility level as verified through the design, estimating, and construction review process. Beginning in fall 2021 through summer 2024, Reclamation plans to complete the final design phase for the proposed dam raise plan. During this time, Reclamation will take the current design from feasibility level to 30%, 60%, 90% design levels and then to a complete 100% final design specification and cost estimate.

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How was the environmental compliance process incorporated into the feasibility study?

The environmental compliance process evaluated alternatives from the feasibility study to determine any associated environmental impacts and appropriate mitigation. Environmental impacts and mitigation costs were incorporated into the feasibility study cost estimates for construction of the dam raise.

What alternatives are you considering in the Draft EIS?

Currently, Reclamation is considering:

  • No-action alternative
  • 6-ft raise of Anderson Ranch Dam
  • 3-ft raise of Anderson Ranch Dam

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Are there other federal agencies participating in the NEPA process?

There are two additional federal agencies acting as decision-makers in this process: The USDA Forest Service, and the U.S. Army Corps of Engineers.

The Forest Service manages the recreation sites which would be affected by a dam raise. Forest Service involvement in this project includes participation in the modification of recreation sites along the reservoir perimeter; amending several special use permits; modifications to the Fall Creek Marina; and modifying an easement currently issued to the Mountain Home Highway District for partial road reconstruction and relocation for the proposed detour route necessary during dam construction.

The U.S. Army Corps of Engineers is responsible for the regulating of discharges of dredged or fill material into waters of the United States and structures or work in navigable waters of the United States. The Corps would participate by issuing a Section 404 permit, which would be required under section 404 of the Clean Water Act and section 10 of the Rivers and Harbors Act of 1899 in order for the project to proceed.

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Have you considered the effects on fish?

Reclamation looked at fish and aquatic habitat in and around Anderson Ranch Reservoir, extending downstream along the South Fork Boise River, and including Arrowrock Reservoir. Reclamation's fishery biologists analyzed effects to fish, including bull trout, in waters within the project area that provide potential fish habitat and areas that may be affected as a result of either ongoing water management or by future construction activities and water operations.

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How is the Boise River system operated?

The Boise River System includes Anderson Ranch, Arrowrock, and Lucky Peak reservoirs. The current total active storage of the system is 949,700 acre-feet. The reservoir system is operated for multiple purposes including irrigation and municipal water supply, flood risk management, power, recreation, and fish and wildlife.

In general, water is balanced between the Upper System (Anderson Ranch) and the Lower System (Arrowrock and Lucky Peak) to meet flood risk management space requirements, irrigation demand, and other purposes. During the flood risk management season (typically January through July), water supply forecasts are used to determine flood risk management space requirements and guide releases from the reservoir system.

During this period, the reservoir system is jointly operated by USACE and Reclamation. Outside of the flood risk management season, Reclamation works closely with the Water District 63 watermaster to operate the reservoirs to meet downstream demands.

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Why is water supply a concern in the Treasure Valley?

Management of the Boise River system is highly dependent upon storage of water in the snowpack. Climate studies project more winter rain and less winter snow in the future, reducing water stored in the snowpack. Existing water storage facilities may not be capable of offsetting the loss of water storage previously provided by the snowpack. Therefore, additional water storage may be necessary to support existing and future water supply needs.

In addition, significant growth is anticipated in the Treasure Valley and surrounding areas; projections indicate Treasure Valley population increasing to 1.57 million people by 2065. As a result, future demand for water supply is expected to increase.

As a result, this water is not available for year-around use or for use in subsequent years when water supply is limited (e.g., drought years). In addition, parts of the aquifer system have limited supply, thereby restricting potential growth and development in portions of the Treasure Valley.

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Who gets the water?

An application for a water right permit for the new storage has been filed with the Idaho Department of Water Resources. Reclamation would issue one or more contracts the Idaho Water Resource Board and potentially with water user organizations for space in the enlarged reservoir with. The Idaho Water Resource Board could in turn contract with water users for portions of the space.

The Idaho Water Resource Board may also offer some of the additional water supply through the Water District 63 rental pool. Reclamation intends to reserve 10% of the space for federal benefits, which could include fish and wildlife benefits and other environmental purposes. Reclamation would coordinate with local, state, and federal entities to determine use of this water in alignment with existing environmental requirements.

For more information, please visit the IWRB project website at

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Environmental Impact Statement

Jim Taylor

Bureau of Reclamation
Snake River Area Office
230 Collins Road
Boise, Idaho 83702

Chris Keith
Project Manager
(208) 378-5360

Bureau of Recamation
Regional Office
1150 N. Curtis Road
Boise, Idaho 83706

Last Updated: 2/9/22