Sub-Reach 3 Completion, Lower Lemhi Rehabilitation Project P2

The Lower Lemhi River Rehabilitation Sub-reach 3 (SR3) project is one of 4 sub-reaches within a 2.5 mile contiguous river segment that the Idaho Department of Fish and Game (IDFG) is implementing to restore fish habitat. This contract supported ongoing efforts by IDFG to complete final construction actions for SR3. Most of the project actions were completed under previous contracts. However, challenges with finding sufficient large trees and other woody material at a reasonable cost, a higher than expected “low bid” that was accepted and awarded to a contractor, and an underestimation of the amount of material needing to be excavated to create floodplains resulted in a significant shortage in funding.

Project Actions. The remaining project actions that needed to be completed include construction of the last lateral side channel and floodplain complex, installation of engineered log jams, shaping of the floodplain to blend with existing pasture, connection with the mainstem river, installation of an irrigation water siphon, and installation of cattle exclusion fencing. Construction for these actions commenced in early 2019. Excavation began on the upper end of the SCC 1 floodplain complex in late January (Figures 1 and 2). Prior to excavation, the top organic layer consisting of topsoil and sod was stripped and placed on the edges of the channel complexes. Channel alignments were then excavated per design. Multiple side channels that braided through a floodplain swale and inset benches were excavated, and river banks were shaped to promote flooding. IDFG secured additional woody debris for the remaining engineered log jam treatments (Figure 3). Four different types of engineered log jams were constructed within SCC 1 (Figures 4 and 5); 1) Type 1 engineered log jams promoted vertical scour to create a large pool while maintaining a channel split, 2) bleeder log jams directed flow as prescribed to maintain a primary channel while providing in channel complexity, 3) bank log jams created pools and provided large woody debris, and 4) bank roughening complexes provided bank stability and high complex lateral fish habitat. In addition, large loose trees containing rootwads were strategically placed in the channels to provide additional roughness and fish habitat (Figure 5).

Since a large portion of the project utilized existing pasture and directly affected irrigation canals and water delivery, some mitigation was necessary to allow the ranch to irrigation other areas previously irrigated. Thus, a siphon used to convey irrigation water under one of the constructed channels was installed.

Establishment of a functional riparian area was one of several objectives we established to meet the project goal. Thus, conditions sufficient to successfully grow and support riparian vegetation were important for completing this objective. We completed final floodplain grading in the associated disturbed areas, and topsoil containing sod clumps previously salvaged was placed throughout floodplain areas, inset benches, and along channel banks. Willow trenches were installed, were 20 – 30 willow cuttings were placed in a 5 ft deep trench and backfilled with topsoil. Additional willow stakes were randomly planted throughout the floodplain complex. A native seed mixture was applied to all disturbed areas, and IDFG irrigated these areas in the summer to promote germination and growth (Figure 6). As stated previously, additional wood debris and other organic material was also placed to further promote vegetation growth (Figure 5). These actions have ultimately expanded the active floodplain significantly over previous conditions.

All floodplain complexes were activated once channel construction was completed.

The purpose of this project is 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. This reach does not effectively support most life stages of salmon and steelhead. It lacks proper stream channel structure and function, instream structural complexity, and proper bed and channel form. Sediment quantity is poor, in that, large homogenous substrate unsuitable for salmonid spawning dominates the reach. Water velocities are very high, and there are no slow water or lateral habitats suitable for fish rearing and growth. There is a substantial loss of riparian vegetation, and the existing condition is poor. Stream temperatures are elevated during summer periods, in part due to loss of riparian vegetation within and upstream of the proposed reach. Project actions will improve all of these identified issues.