MORPHODYNAMICS OF A POINT BAR AND CUT-BANK ON A RAPIDLY MEANDERING RIVER REVEALED FROM TWENTY DRONE-BASED LIDAR SCANS

Meandering rivers are among the most common, dynamic, and captivating sedimentary systems on Earth. Meandering is conceptualized as a pair of processes: cutbank erosion and point bar deposition. However, it remains unclear if the meander bend is pulled by the cutbank or pushed by the point bar. To complicate matters, previous work has shown that these processes are tightly coupled, which allows rivers to maintain their widths as they meander over multi-annual timescales. Exactly how rivers maintain their width, however, has not been observed in natural settings due to a lack of event-scale field data. We present the results from an ongoing data collection campaign where over a four-year period we collected 20 repeat drone-based lidar surveys of a single paired point bar and cutbank (~0.35 km² in area). Each survey can resolve cm-scale vertical changes and quantifies the actions of individual floods across a variety of peak discharges. We combine our surveys with stream gauging and annual bathymetric measurements to create a holistic dataset of paired point bar and cutbank volumetric changes that can be related to different flood intensities. Over our four-year observation period, we have observed continuous widening as point bar deposition has been insufficient to match cutbank erosion volumes. We are also able to closely monitor cutbank erosion processes, providing evidence for muddy slump-block erosion as opposed to spatially continuous planing of non-cohesive sediment. Finally, we illustrate the complex patterns of deposition and erosion that can occur within a single rotating bend that could not be resolved by measuring simple representative cross sections.