An assessment of Kootenai River channel migration and riparian habitat encroachment

Abstract

The lower reach of the Kootenai River in northern Idaho is dominated by large and actively migrating meander bends. Flow conditions, altered by both upstream flow reductions and by downstream backwater effects, are believed to be important geomorphological drivers throughout the reach. For example, Trout Creek Peninsula, located in an area of active meander migration, is undergoing continued bank erosion that has the potential to capture South Fork Trout Creek and result in an alteration of the Kootenai River active channel. Although chute-cutoffs are natural and common in meandering channels and can increase habitat diversity, avulsion events on regulated rivers with heavily used floodplains can also lead to loss of land and habitat. A chute-cutoff at Trout Creek Peninsula would create an island and potentially result in large volumes of erosion and deposition on private and tribal land. This could result in decreased riparian habitat and private land access along Trout Creek. To address landowner risk we refine estimates of bank erosion rates and the potential timeline for the capture of Trout Creek to inform stakeholder decisions about monitoring or managing bank erosion processes. Historical imagery, airborne and boat-mounted LiDAR, and repeat bathymetric surveys suggest that localized areas of high short-term bank erosion rates have increased to about 15 ft/yr near South Fork Trout Creek and the possibility of a chute cutoff event by 2043. Changes in bank erosion rates over time were compared to flow records to better understand how the complex flow regulation in this reach might be impacting geomorphic processes.