Lobe-Switching Avulsions: Delta Geometry Could Determine Locations

Avulsions on deltas often occur at distances from the river mouth that scale with the backwater length (channel depth divided by river slope). This backwater scaling could arise from hydrodynamic variability between flood and inter-flood periods that promotes net in-channel sediment deposition, or from geometric differences between river and floodplain profiles. In this ‘geometric mechanism’, a high-curvature zone in floodplain profiles leads to local elevation of the river above the floodplains. Here we explore whether the model-motivated geometric mechanism is relevant on natural deltas. A new framework to analyze profile curvatures from digital elevation datasets, applied to the Mississippi Delta, reveals curvature patterns that support the geometric mechanism. An analysis of the timescales for creating and burying high-curvature zones suggests that the geometric mechanism is relevant on deltas built by large, low-slope rivers for which floodplain aggradation is weakly coupled to channel-belt aggradation, producing relatively narrow alluvial ridges.