Morphodynamics of neck cutoffs on elongate meander loops, White River, Arkansas, USA

Abstract:

Meander cutoff and oxbow lake formation are essential components of alluvial architecture and riverine habitat of meandering river floodplains. Yet, despite their ubiquitous presence within active floodplains, the detailed processes involved in the initiation of cutoffs and oxbow lakes remain incompletely understood, primarily due to the intermittent nature of such events. Furthermore, conceptual models of meander cutoff and oxbow lake formation have been primarily developed for chute cutoffs and relatively simple planform configurations. Less attention has been given to neck cutoff dynamics occurring on highly sinuous meandering rivers with complex planform morphology. During the formation of a neck cutoff on a compound elongate loop, the upstream and downstream limbs can become oriented roughly subparallel with flow in opposite directions separated by a narrow meander neck. Immediately following cutoff of this thin neck, flow from the upstream limb is sharply redirected into the downstream limb over a short distance. These conditions of tight bend flow should become more pronounced as the ratio of radius of curvature to channel width become smaller, leading to complex patterns of three-dimensional velocities that have implications for the evolution of the cutoff channel and the transformation of the abandoned bend into an oxbow lake. This paper investigates the process dynamics of neck cutoff and oxbow lake formation using detailed field measurements of three-dimensional flow velocities, channel bed topography and geotechnical analysis of the banks and floodplains from three neck cutoffs along the White River, Arkansas (USA), each representing a different stage in the morphologic evolution from cutoff to oxbow lake. Results from this study suggests that the planform geometry of neck cutoff on an elongate meander loop can influence the spatial pattern of sediment deposition within the abandoned loop leading to increased hydrologic connectivity to the main channel and the overall morphodynamics of highly sinuous meandering rivers.