Sediment management has been an ongoing maintenance issue since the completion of Ice Harbor Dam, the first in a series of dams on the lower Snake River. Sedimentation issues have been handled on a case-by-case basis to date, primarily with maintenance dredging activities. However, the Corps believes that this ongoing problem can best be handled by evaluating sediment management throughout the entire system, as well as on a watershed basis.

The planned PSMP/EIS will identify and evaluate ways to manage sediment within the lower Snake River reservoirs, and will examine the sources and transport of this sediment. This study will determine the most effective ways to reduce sediment build-up, manage it once it reaches the reservoirs, and identify possible changes to structures and/or operations to reduce maintenance issues while still providing for all authorized purposes.

Although the Corps does not have the authority to manage lands outside of the reservoir project boundaries, management strategies for non-Corps property will be identified and evaluated.

While site-specific locations will not be addressed in this EIS, the coordination and environmental review steps required to accomplish subsequent site-specific actions will be presented

• Sediment reduction measures that would reduce the sediment load entering the McNary reservoir and/or the lower Snake River reservoirs. These measures can be structural or non-structural, and are generally targeted at areas upstream of the reservoirs. Sediment reduction measures operate under the general premise that reducing the total sediment load entering the river will result in less sediment deposition downstream. However, it is often difficult to establish a direct link between the suspended load in a tributary and the sediment deposited in a specific location in the reservoir farther downstream. Further, many of these measures rely on voluntary cooperation from landowners, and their overall effectiveness at reducing the sediment load is unknown. Therefore, most sediment reduction measures are considered indirect management measures.

  Potential sediment reduction measures identified for evaluation:

  • Structural Sediment Reduction Measures:
    • Section 206 and 1135 Restoration Reports
    • Shoreline Vegetated Filter Strips
    • Streambank Erosion Control
    • Upstream Sediment Traps
  • Non-Structural Sediment Reduction Measures:
    • Local, State, and Federal Land Conservation Programs
    • Land Use Planning
    • Public Education
    • Watershed Planning

• Sediment management measures that would remove and/or manage unwanted sediments and sediment deposition within the lower Snake River reservoirs. Sediment management measures generally seek to either reduce the rate of sediment deposition or remove existing sediment. These measures are used to target specific locations or sediment deposits, and are considered direct management measures.

  Potential sediment management measures identified for evaluation:

  • In-Water Systems to Control Sediment Deposition:
    • Agitation to Prevent Settling
    • Bendway Weirs
    • Dikes and Dike Fields
    • Air Curtains to Prevent Settling
  • Sediment Removal and Management:
    • Agitation to Re-Suspend Sediment
    • Dredging to Remove Sediment
    • Beneficial Use of Dredged Material
    • In-Water Disposal of Dredged Material
    • Upland Disposal of Dredged Material

• System management measures that would modify navigation and/or other facilities and reservoir operations to adapt to prevailing conditions with respect to sedimentation. These measures provide a range of "non-traditional" approaches, and include both direct and indirect management measures. System measures to specifically address the flow conveyance concerns at Lewiston-Clarkston are included.

  Potential system management measures identified for evaluation:

  • Modify Navigation System Infrastructure:
    • Maintain navigation channel at less than 14-foot depth
    • Relocate affected commercial navigation, recreational boating, and water intake facilities
  • Modify Reservoir Operations:
    • Raise pool levels to increase water depth
    • Modify flows to flush sediment
    • Drawdown the Lower Granite reservoir to add flow conveyance capacity
  • Lewiston High Flow Protection:
    • Provide insurance for Lewiston facilities at risk from high flows
    • Improve Lewiston facilities at risk from high flows
    • Raise the levee at Lewiston
    • Relocate Lewiston facilities at risk from high flows

The Corps held technical workshops throughout the fall and winter of 2006/2007, with public scoping meetings in February 2007. This gave agencies, Tribes, stakeholders, and the public an opportunity to help define the scope, alternatives, and associated necessary data collection and analysis for the PSMP. The Corps has prepared a summary of the comments received during scoping.

A draft EIS is scheduled for release in fall 2010. The final EIS is scheduled for public review in fall 2011.