Flood Variability Determines the Location of Lobe‐Scale Avulsions on Deltas: Madagascar

Abstract

River deltas grow through repeated lobe‐scale avulsions, which often occur at a location that correlates with the backwater lengthscale. Competing hypotheses attribute the avulsion node origin to either the morphodynamic feedbacks caused by natural flood discharge variability (backwater hypothesis) or to the prograding delta lobe geometry (geometric hypothesis). Here, using theory, historical flood records, and remotely sensed elevation data, we analyzed five lobe‐scale delta avulsions in Madagascar, captured by Landsat imagery. Avulsion lengths were 5–55 km, distances significantly longer than the backwater lengthscale and inconsistent with the geometric hypothesis. We show that the steep, silt‐bedded rivers of Madagascar have flood‐induced bed scour, driven by backwater hydrodynamics, that propagates farther upstream than the backwater lengthscale. The avulsion lengths are 3.1 ± 1.5 times the predicted flood scour lengths, similar to low‐gradient deltas, and consistent with backwater hypothesis. Results demonstrate that erosion initiated by nonuniform flows in the backwater zone is a primary control on delta avulsion locations.

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