Eolian and fluvial modification of Missouri River sandbars deposited by the 2011 flood, USA

Abstract

The 2011 flood, an approximately 500-yr flood on the Missouri River, built many large sandbars that have since been modified by eolian and fluvial processes. The ‘59-mile’ segment of the Missouri National Recreational River, located between Yankton, South Dakota and Ponca, Nebraska, USA is a relatively free-flowing stretch of the river downstream of a major dam which has drastically reduced sediment loads since 1955. Critical sandbar habitat has gradually declined on this stretch of the river, presenting challenges to river management. Satellite and LiDAR imagery was used to measure changes in area, volume, and surface characteristics of sandbars over three years since the 2011 flood. The Portable in situ Wind Erosion Lab (PI-SWERL) was used to measure the potential for bare sandbars to emit dust. Our results suggest that eolian processes are important in the early evolution of sandbars, as the wind modifies the surface into dunes and gravel lags. Furthermore, sandbars are important local sources of dust until gravel lags stabilize the surface, limiting further eolian deflation. The lowering of sandbar surfaces by wind erosion makes them more susceptible to being overtopped by high water events. Fluvial erosion along the margins of the sandbars ultimately reduces their area over time. Vegetation growth further stabilizes sandbar surfaces. The rate of eolian and fluvial erosion ultimately depends on the position of the sandbars relative to the thalweg as well as wind direction. New sandbars are rarely built by fluvial processes on a regulated river like the Missouri, and are in a constant state of erosion and modification, ultimately limiting their longevity as quality habitat.