Columbia-Pacific Northwest Region

The dam raise will involve modifications to the current dam embankment and spillway, along with adjustments to the left and right abutments to support an increase in reservoir elevation by 6 feet. The final design will ensure that the finished roadway accommodates two-way traffic. The materials used for the modifications will be sourced from local borrow sites, or commercially available resources.

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

Increasing the storage of water at the Anderson Ranch Dam in Idaho can benefit several stakeholders, including:

1. Agricultural Producers: Farmers and ranchers in the region can benefit from increased water storage, as it helps ensure a more reliable water supply for irrigation. This can lead to improved crop yields and better management of water resources during dry periods.
2. Municipal Water Users: Communities and municipalities that rely on the reservoir for their water supply can see enhanced water availability for drinking, sanitation, and other essential services.
3. Environmental Management: Enhanced water storage can support environmental management efforts, including maintaining streamflow for fish habitats and wetlands, thus benefiting local ecosystems.
4. Recreational Users: Increased water levels can enhance recreational opportunities for fishing, boating, and other water-based activities, fostering tourism and local economies.
5. Hydropower Generation: If applicable, the increased storage could also enable better management of water for hydroelectric power generation, contributing to energy supplies in the region.
6. Emergency Management: Improved water storage can aid in flood control efforts, helping to mitigate the impact of high-water events on downstream communities and infrastructure.

Overall, increasing the water storage capacity at the Anderson Ranch Dam can provide a broad range of benefits to agricultural, municipal, environmental, recreational, and energy stakeholders.

Current estimated schedules show construction beginning in late 2027 with on-site work expected early 2028.

Current estimates are for an approximate 4-year construction window for the proposed raise of Anderson Ranch Dam.

During construction, Highway District Road 134 leading from U.S. Highway 20 to and crossing Anderson Ranch Dam may experience intermittent closures when required during the construction process.

Designs have been prepared to provide erosion control protection at the Pine and Lime Creek Bridge locations. There is no planned work on these two bridges or others around the reservoir.

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.

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.

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 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.

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.

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.

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.

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.)

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.

For more information, please visit the draft environmental impact statement page.

Currently, Reclamation is considering:

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

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; and modifications to the Fall Creek Marina.

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 for the project to proceed.

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.

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.

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.

The IWRB filed a water right permit application for the new storage. Reclamation intends to reserve 10% of the space for federal benefits, which could include fish and wildlife and other environmental purposes.

90% of the space would be used for non-federal benefits. The IWRB anticipates entering into contracts with interested water users or offering water through the Idaho Water Supply Bank’s Water District 63 rental pool, or both.