An Assessment of Kootenai River Channel Migration and Riparian Habitat Encroachment

The lower reach of the Kootenai River in northern Idaho is dominated by large and active meander bends. Flow conditions, altered by both upstream regulation and by downstream regulation that produces a backwater effect, are important geomorphological influences throughout the reach. For example, Trout Creek Peninsula, located in an area of active and ongoing meander migration, is undergoing continued bank erosion that has the potential to intersect a small tributary (South Fork Trout Creek) and result in a significant avulsion of the Kootenai River. Although avulsion is common on meandering channels and can increase habitat diversity, avulsion events on regulated rivers with heavily used floodplains can also lead to land loss and loss of limited, critical habitat. This is a possibility at Trout Creek Peninsula where avulsion would turn the peninsula into an island and potentially create large volumes of sediment erosion and deposition on private and tribal land, resulting in decreased riparian habitat and private land access to Trout Creek. Therefore, it is necessary to refine estimates of bank erosion rates here and the potential timeline for avulsion to inform stakeholder decisions about monitoring or managing the avulsion process. Previous studies suggested an average bank erosion rate of 2.5-ft/year with the potential for avulsion by 2034. This study uses historical imagery, airborne and boat-mounted LiDAR, and repeat bathymetric surveys to provide an updated average bank erosion rate and channel migration rates at Trout Creek Peninsula. Changes in bank erosion rates over time are compared to flow records to better understand how the complex flow regulation in this reach might be impacting this process. Further, the high resolution and repeat bathymetric surveys provide an opportunity to investigate how the presence of more resistant sediment layers on the channel bed may be influencing lateral channel migration.