We develop a novel approach based on satellite radar images to quantify millimetre-scale sedimentation during monsoon floods over a 15 km stretch of four rivers, from the Himalayan mountain front to the gravel–sand transition. The results show how sediment accumulates more rapidly near the mountain front and decreases downstream, while the floodplain sinks. This method can improve river monitoring, enhance flood prediction, and benefit communities at risk of flooding in Nepal and India.