Hyperconcentrated floods cause extreme gravel transport through the sandy rivers of the Gangetic Plains

Abstract The Gangetic Plains comprise steep gravelly river channels that transition to low gradient sandy 10-40 km downstream of the mountain front. This “gravel-sand transition" is characterized by an abrupt greater-than-one-order-of-magnitude drop in both and sediment grain size, suggesting a degree long-term stability. However, stratigraphic record gravel-sand Miocene Siwalik Group demonstrates intermittent transport coarse gravels tens kilometres transition; such events contemporary would drive channel avulsion(s) increase flood risk, devastating communities across plains. We combine sedimentological analysis deposits with entrainment calculations which demonstrate hyperconcentration required bedload over low-gradient Transport conditions are attainable when intense monsoon precipitation (a 200- 1000-year event) combined increased suspended concentrations channels. Predicted climate change ongoing seismicity likelihood extreme within this century.