The U.S. Army Corps of Engineers (Corps) and Idaho Water Resource Board (IWRB) are partnering on a feasibility study of the Boise River. The feasibility study will evaluate alternatives to reduce flood risk, and meet current and future water supply needs in the lower Boise River watershed, from Lucky Peak Dam downstream to the confluence with the Snake River. The study will also seek to provide ancillary ecosystem-restoration benefits, while minimizing socioeconomic effects and impacts to sensitive species. A preliminary set of alternatives has been developed for public review. The Corps and IWRB are requesting public input regarding the scope of study alternatives and analyses to consider throughout the feasibility study process and preparation of an Environmental Impact Statement (EIS).   

The study team reviewed public input and previous related studies to identify potential measures that could be combined to develop a preliminary set of alternatives that address identified flood-risk and water supply problems. The table below summarizes these alternatives. An “X” in the row beside a measure indicates that this measure is included within the alternative.



The following is a general description of the measures comprising each preliminary alternative:

• Arrowrock Dam Raise: Raising Arrowrock Dam was previously identified as the top-ranked storage option for both flood risk management and water supply. A range of potential dam raise heights is possible, with a currently estimated maximum raise of 74 feet. The maximum raise would provide an estimated 320,000 acre feet of storage for flood risk management, and current and future water demand.
• Managed Aquifer Recharge: This measure involves a deliberate strategy of recharge existing groundwater aquifers. The potential for gaining additional water supply through managed aquifer recharge and additional flood-risk-management benefits associated with this measure will be based on previous groundwater studies.
• Upgrade Existing Irrigation Headgate Structures: Headgates control the diversion of water into irrigation canals and could be improved to reduce the risk of localized flooding. It is likely some headgates have greater potential flood-risk-management benefits than others.
• Replace Push Up Dams with Inflatable Weirs: Push up dams assist the diversion of water into irrigation canals. Replacing specific dams with inflatable weirs that can be lowered during high water events is expected to have localized flood-risk-management benefits.
• Replace or Upgrade Undersized Bridges: Some bridges in the project area have potential to cause localized flooding during high-flow events, and could be raised or replaced to reduce the risk of localized flooding.
• Controlled Flooding of Gravel Pits and Ponds: This measure involves designing a controlled method of flooding ponds and pits with high potential for pit capture.  
• Temporary Conveyance of Water in Floodplain: In some areas, there are opportunities to re-grade parks or develop perched side-channels to reduce localized flooding. These measures also have the potential to provide some ancillary environmental benefits. 
• Flow-Split Structure at Eagle Island: Under some high-water scenarios, a controlled split of flows into the north and south river channels around Eagle Island could be beneficial.  
• Non-Structural Measures: There may be cost-effective opportunities to provide “non-structural” flood protection (e.g., flood-proofing buildings, ring levees around critical infrastructure, or raising structures in place) in certain areas to reduce the frequency of flooding.  
• Measures Common to All Alternatives: Certain measures will be included in all alternatives, such as water-conservation measures with potential to reduce future water demand, floodplain-management plans to help limit future floodplain development and altered-system operations.

In order to evaluate the benefits, costs and environmental effects of possible solutions to flood risk and water supply in the project area, the study will first establish an anticipated future condition for the project area.  This will include projected population growth, land-use changes, measures that may reduce flood risk without the project, and measures that may decrease future water-supply demand (e.g., conservation). The study “alternatives” will be compared against this future project condition, and economic costs will be compared against projected benefits of preventing future flood damages and providing additional water supply. These factors, along with consideration of the environmental and socioeconomic impacts of various alternatives, will be used to select a proposed alternative that best meets study objectives. The preferred alternative and supporting analysis will be circulated for public review as part of the draft feasibility study and EIS. After all comments have been addressed, a decision will be made regarding which alternative to move forward into a more advanced design phase. Once the design is complete for the selected alternative, a final feasibility study and environmental impact statement will be circulated for public review. When comments from this final review have been received and addressed, a Record of Decision (ROD) will be signed, indicating the selection of an alternative and completion of the feasibility study.

The National Environmental Policy Act of 1969, as amended (NEPA), is a law requiring federal agencies to objectively evaluate a range of alternatives for any potential federal action. Alternatives must include a “No Action Alternative,” which addresses the consequences of completing no federal action at the location. The NEPA process requires the agency to consider the environmental and social impacts of alternatives so that informed decisions are made with knowledge of potential environmental consequences. Federal agencies are required to prepare an EIS when a proposed major federal action could significantly affect the quality of the human environment. During the Boise River Feasibility Study, the Corps will prepare an integrated feasibility study and EIS report.  Through previous coordination and the collection of background information, the Corps is aware that impacts to the following must be addressed in the EIS, along with other issues:
• Bull Trout, a fish species listed under the Endangered Species Act 
• Yellow Billed Cuckoo, a bird species proposed for listing under the Endangered Species Act 
• Other sensitive species, including Bald Eagle and Blue Heron
• Sensitive habitats, including wetlands and big game winter range
• Fish habitat in the South Fork-Boise River
• Recreation, including fishing and rafting on the South Fork-Boise River
• Hydropower generation facilities at Arrowrock Dam
• Effects to cultural and historic resources

Initial efforts for the feasibility study focused on water storage as one potential option for reducing downstream flood risk while meeting water-supply needs. The study has evaluated surface water storage sites upstream of Lucky Peak Dam and updated flood risk information. In August 2010, the study team completed a surface water storage screening analysis that scored sites for six criteria, including future water demand, flood risk reduction, hydropower potential, a relative-cost index, and social and environmental effects. Raising the existing Arrowrock Dam was the top-ranked flood risk reduction and water supply site in the analysis. The 2010 Water Storage Screening Analysis document describes the screening criteria, the process used to score the surface water storage sites and the analysis results. In October 2011, the Corps completed a preliminary analysis of the Arrowrock site. The analysis relied on available data, including a review of historical documents related to the planning and construction of Arrowrock Dam, and geological records and maps. A field reconnaissance was conducted from Arrowrock Dam downstream. The Preliminary Evaluation of the Arrowrock Site report provides more information about the analysis.

In May 2009, the Corps and the IWRB entered into an agreement to initiate the feasibility study. The IWRB has agreed to cost share the study. The Idaho Department of Water Resources provides project coordination and other staffing assistance on behalf of the IWRB.

The Corps’ study authorization is provided by Section 414, Water Resources Development Act (WRDA) of 1999, and Section 4038, WRDA 2007, which authorizes a feasibility study for a multiple-purpose project in the Boise River watershed for flood risk reduction, water supply and ecosystem restoration purposes. Based on IWRB’s interests in the study area and the authorities associated with their funding, the decision was made to focus alternatives development on the primary purposes of flood risk reduction and water supply while seeking ancillary environmental benefits where feasible.