Mass wasting caused by large-magnitude earthquakes chokes mountain rivers with several cubic kilometres of sediment. The timescale and mechanisms by which rivers evacuate small to gigantic landslide deposits are poorly known, but are critical for predicting post-seismic geomorphic hazards, interpreting the signature of earthquakes in sedimentary archives and deciphering the coupling between erosion and tectonics. Here, we use a new 2D hydro-sedimentary evolution model to demonstrate that river self-organization into a narrower alluvial channel overlying the bedrock valley dramatically increases sediment transport capacity and reduces export time of gigantic landslides by orders of magnitude compared with existing theory. Predicted export times obey a universal non-linear relationship of landslide volume and pre-landslide valley transport capacity. Upscaling these results to realistic populations of landslides shows that removing half of the total coarse sediment volume introduced by large earthquakes in the fluvial network would typically take 5 to 25 years in various tectonically active mountain belts, with little impact of earthquake magnitude and climate. Dynamic alluvial channel narrowing is therefore a key, previously unrecognized mechanism by which mountain rivers rapidly digest extreme events and maintain their capacity to incise uplifted rocks.

大地震造成的物质浪费使山区河流的沉积物淤积了几立方公里。河流从小规模滑坡到巨大滑坡沉积物撤离的时间尺度和机制鲜为人知，但对于预测地震后的地貌危害，解释沉积档案中地震的特征以及破译侵蚀与构造之间的耦合至关重要。在这里，我们使用一种新的二维水成沉积演化模型来证明，与现有理论相比，河流自组织进入覆盖基岩谷的较窄冲积河道大大增加了泥沙输送能力，并减少了巨量滑坡的出口时间。预计的出口时间服从于滑坡量和滑坡前谷地运输能力的普遍非线性关系。将这些结果升级为现实的滑坡种群后，发现在各种构造活跃的山地带中，消除由大地震在河流网络中引入的总粗泥沙量的一半通常需要5至25年的时间，而对地震震级和气候的影响却很小。因此，动态冲积河道变窄是关键的，以前未被认识的机制，山区河流通过这种机制迅速消化极端事件并保持其切开隆起的岩石的能力。