Rules of River Avulsion Change Downstream

Abstract

Avulsing rivers carve new pathways on the floodplain, and the associated flooding can profoundly impact society 1-4. River avulsions are thought to occur when the water column becomes perched above the floodplain5, or when the slope down the channel's flanks provides a steeper descent than the current path 6,7. However, neither idea has been suitably field-tested. Here we quantify the topography around avulsing rivers and reveal that these mechanisms work together. Near coasts, rivers avulse when the slope away from the channel is steeper, not because they are perched. The opposite is true near mountain fronts; on alluvial fans, alternative paths are all similarly steep, so rivers avulse when they are perched over the surrounding landscape. We reconcile these findings and present a novel theoretical framework that can predict which rivers are vulnerable to avulsion, and where avulsing rivers will go after they start. By clarifying the rules of river avulsion, it is evident that avulsion risks are underestimated in many coastal environments8 and that probabilistic predictions of avulsion pathfinding can efficiently project hazard with minimal information. Applying these principles for risk assessment may have outsized benefit in the world’s poorest nations, which we find are disproportionately impacted by avulsions.