Walla Walla District

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 will provide 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, authorizing a feasibility study for flood control on the Boise River; and Section 4038, WRDA 2007, modifying the 1999 authority to include ecosystem restoration and water supply as project purposes.

The feasibility study is currently focusing 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.  You can read the most current fact sheet for an update on study status.

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 purpose of the analysis was to

• Identify the most appropriate surface water storage concept for the Arrowrock site ( i.e., raise the existing structure or construct a new facility downstream),
• Determine whether there were major engineering or geological constraints that would make a dam raise or a new downstream dam technically unfeasible or cost prohibitive, and
• Identify any significant issues that should be considered for future analyses.

The analysis relied on available data, including a review of historical documents related to the planning and construction of Arrowrock Dam, and geological records and maps.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 Corps coordinated with Bureau of Reclamation engineers and geologists. (Arrowrock Dam was constructed by and is operated by Reclamation.) The Preliminary Evaluation of the Arrowrock Site report provides more information about the preliminary analysis.

The Corps and IWRB, in coordination with Reclamation, are currently identifying study scope, schedule and budget to complete the feasibility study.