Avulsing rivers create new pathways on the floodplain and the associated flooding can profoundly affect society^1,2,3,4. River avulsions are thought to occur when the water column becomes perched above the floodplain⁵ or when the slope down the flanks of the channel provides a steeper descent than the existing river channel^6,7. We test these classical ideas by quantifying the topography around avulsing rivers and show that these mechanisms, historically invoked separately, 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 fans, the alternative paths are similarly steep to the downstream path, so rivers avulse when they are perched above the surrounding landscape. We reconcile these findings and present a new theoretical framework that identifies which rivers are vulnerable to avulsion and predicts the path of an avulsing river. These first-order rules of avulsion suggest that avulsion risks are underestimated in many coastal environments⁸ and that probabilistic predictions of avulsion pathfinding can efficiently map hazards with minimal information. Applying these principles for risk assessment could particularly benefit the Global South, which is disproportionately affected by avulsions.

撕扯的河流在洪泛区开辟了新的通道，相关的洪水可能会对社会产生深远的影响^1,2,3,4 。当水柱停留在洪泛区上方⁵或当河道两侧的坡度比现有河道^6,7更陡时，就会发生河流撕扯。我们通过量化撕扯河流周围的地形来测试这些经典的想法，并表明这些机制在历史上是单独调用的，但它们可以一起发挥作用。在海岸附近，当远离河道的坡度较陡时，河流就会发生撕裂，而不是因为它们栖息。靠近山前的情况则相反。在扇形上，替代路径与下游路径同样陡峭，因此当河流栖息在周围景观之上时，它们会发生撕裂。我们协调了这些发现，并提出了一个新的理论框架，该框架可以识别哪些河流容易发生撕脱，并预测撕脱河流的路径。这些撕脱的一阶规则表明，在许多沿海环境中，撕脱风险被低估了⁸ ，并且撕脱寻路的概率预测可以用最少的信息有效地绘制危险图。将这些原则应用于风险评估可能特别有利于南半球，因为该地区受撕脱伤的影响尤为严重。