Sub-Reach 1 Design P2, Lower Lemhi Rehabilitation

Sub-Reach 1 (SR1) is one of 4 sub-reaches within a 2.5 mile contiguous Lower Lemhi Rehabilitation segment that Idaho Department of Fish and Game has been developing since 2007. The overall goal of SR1 is to increase habitat capacity for juvenile parr and winter pre-smolt life stages in the lower Lemhi River through aggressive habitat treatment. The Sub-reach 1 Design P2 project developed an engineered design concept that will support these actions. Habitat treatments spanning approximately 1.25 miles of the lower Lemhi River will focus on restoring floodplain function within 135 acres of existing pasture. Treatments include construction of additional mainstem river and side channels, raising surface water and reducing floodplain elevations, removal of levees and dikes to allow for floodplain inundation, and installation of engineered log jams and other woody debris structures throughout the existing and newly created stream channels. An assembled team of engineers, geomorphologists, and biologists proposed and reviewed all of the design iterations. Survey data was collected, LiDar data was evaluated, and design concepts were developed at the 15% and 30% completion levels. A cost estimate was also generated that enabled IDFG to pursue funding in 2019.

During the contract and design development, the USBR joined the project as design and implementation partners. At this point, USBR started paying contracting fees for design, which allowed IDFG to recognize substantial cost savings they were able to return to Idaho OSC to put back on the ground for other projects.

The purpose of this project is complete 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.