Autogenic translation and concave bank deposition in meandering rivers

While both modeling and observations suggest that the kinematics of meandering is more complicated than simple bend expansion, the classic model of largely coarse-grained deposition on convex banks / point bars still dominates our understanding of these systems. In recent decades it has been increasingly recognized that deposition along concave banks and development of counter point bars occur along many meandering rivers; however, these features are interpreted to result only under special circumstances such as downstream channel migration caused by decreased bank erodibility. Using a kinematic model of channel meandering and time-lapse satellite imagery from the Mamoré River in Bolivia, we show that downstream translation and the associated concave-bank deposition are essential, autogenic parts of the meandering process, and they are expected at shorter bends with high curvatures. We put forward a new parameter for differentiating counter point bars from point bars and to estimate the likelihood of counter point bar occurrence: the 'bar type index' is the product of dimensionless curvature and normalized migration rate and it is negative where concave bank deposition occurs. The higher the migration rate and curvature, the higher the absolute value of the bar type index. Curvature and migration rate are related; and short bends with high curvatures and high migration rates are the ones with strongly negative bar type indices. These bends are perturbations of the channel that form at locations of channel reorganization like cutoffs and channel reoccupations. Adding a depth dimension to the kinematic model also allows us to investigate the impact of autogenic counter point bars on the stratigraphic architecture of meandering river deposits and the three-dimensional distribution of the related finer-grained sediments. One of the implications of this work is that zones of concave-bank deposition with lower elevations, relatively fine-grained deposits, and stagnant water bodies are more common than previously considered.