Upper Lemhi Rehabilitation

Project Narrative

IDFG (the project sponsor) began completing steps necessary for implementing the project in 2008. This included drafting a biological assessment for ESA consultation, completing cultural resource compliance, joint State/Federal permitting, coordinating with engineers on the project design, developing bid packets, conducting project bid meetings, and hiring contractors. Final design specifications were completed in January 2011, and a contractor was selected in April. The new river channel and floodplain was surveyed and staked in May to guide construction, and construction began on the new channel in early June (See final report, Appendix A). Best Management Practices (BMP’s) were immediately implemented to dewater much of the lower section for channel excavation and direct water disturbed during construction into an irrigation ditch.All topsoil was removed from the floodplain area and stored for later use, and the new river channel was excavated from a downstream to upstream direction as part of the BMP plan. A new bridge was installed at the downstream end of the channel to provide access across the new channel to the landowner’s home. The river banks were shaped to meet design elevations, as was the new floodplain. Once the design elevations were met, a minimum of 6 inches of topsoil was added to the entire floodplain. Twelve engineered log jams were installed on the outside bank of each pool. Live willow clumps were taken from the old river channel and transplanted to the banks of the new river channel. Over a mile of river banks were then prepped for sod planting, and wetland sod mats were installed to provide much needed bank stability. Finally, jack fencing was installed on both sides of the river to exclude livestock from grazing through the entire floodplain.The newly designed and constructed river channel was created to replace the existing channelized river reach. Thus, all flow in the Lemhi River was to be directed into the new channel when construction was completed. Upon further inspection of the relatively unstable new river banks, the decision was made utilize the existing river channel to absorb some of the energy that was expected during high water periods in 2012 and 2013. Therefore, a temporary control structure was installed in-stream where the new and existing river channels diverge so that flow could enter the old channel as water rose during spring snowmelt (see final progress report, Appendix A, Figure p). This would provide 1-2 years of grass and willow growth along the new river banks, providing additional stability.Aerial views of the new channel clearly show that the objectives of the project were met (See final report, Appendix A, Figure q). The new channel has significantly increased in sinuousity, it is 35% longer (3,100 ft versus 2,300 ft), the slope of the channel is decreased by 50%, thus improving pools and slow water habitats. Additional lateral habitat in the form of an active side channel (a 700 ft. long section of the original channel) was also created.This will significantly improve habitat conditions for Chinook salmon spawning and juvenile rearing because this section of the Lemhi River is located within the primary salmon production area. As rearing conditions improve, the number of juveniles surving in the Lemhi to presmolt and smolt stages is expected to increase. Other anadromous and stream resident-fluvial salmonids will also benefit from this project.