Sub-Reach 1 Design, Lower Lemhi Rehabilitation Project

This contract supported the development of engineered designs at the 60% level for the Lower Lemhi River Rehabilitation Reach Sub-reach 1 (SR1) project. Approximately 1.15 miles of river will be modified, with an estimated 120 acres of existing pasture incorporated into the proposed floodplain. Designs were developed collaboratively among state, federal, and consultant engineers, and expanded upon 15% conceptual designs that were previously developed in 2014. Completed tasks under this contract include the collection of survey data to better understand topography (e.g. bank and river surface water elevations), review of current LiDAR data for the project area, and validation of the LiDAR data using the topographic data. A surface model of the project area was developed and applied to 2-dimensional modeling, which could model flow patterns at different discharge rates. In addition to coordinating the design team, funds were also used to collaborate with other technical entities to review pertinent fish data and make design recommendations. Project managers divided SR1 implementation into 3 phases that will likely span 3-4 years of construction. IDFG plans to construct Phase 1 beginning in September 2020, thus, design efforts have focused on the upper 1/3 of the project. A commitment by the U.S. Bureau of Reclamation (BOR) to provide financial resources and technical support in the development of the SR1 design resulted in a savings of $41,977.71 of the contract funds.

The purpose of this project is develop engineered designs to improve freshwater productivity of the Lemhi Spring/Summer Chinook salmon major population group and the Snake River Lemhi Steelhead distinct population segment, both of which are currently listed as threatened under the ESA. SR1 is one of four sub-reaches within the Lower Lemhi Rehabilitation Reach that are channelized with poor quality fish habitat. Research, monitoring, and evaluation studies through the Integrated Status and Effectiveness Monitoring Program (ISEMP; 2015) and the Intensively Monitored Watersheds (IMW) project have identified this as a critical reach for life stage specific survival of juvenile salmon and steelhead. ISEMP and IMW projects are implementing a study design (QCI 2005) to develop fish/habitat relationships that can, in part, identify reaches that are limiting freshwater productivity. Current conditions in SR1 limit summer rearing and overwintering survival. The proposed project will address many of the degraded conditions to provide more suitable habitat for these life stages.