High-magnitude outburst floods in mountainous terrains can exert significant impacts on Earth’s surface due to their immense hydraulic force. However, the mechanisms involved in bedrock erosion during extreme floods remain incompletely understood, primarily due to the scarcity of such events and challenges in conducting high-resolution measurements during flood events. Using a 2D HEC-RAS dam-breach hydrodynamic model, we investigate two outburst floods of varying magnitudes - the Gega Megaflood and the Yigong Superflood - along the Tsangpo Gorge in the eastern Himalayas. Our study reveals distinct flow patterns and erosion mechanisms associated with each flood event. The Gega Megaflood (~106 m3/s) exhibits a high potential for focused erosion, characterized by elevated shear stress levels (10-20 kPa) and flood power (~105 kW/m2), resulting in the formation of a persistent vortex for up to two days. In contrast, the Yigong Superflood (~105 m3/s) displays intense spindle-shaped flow dynamics lasting several hours. Changes in flood magnitudes yield variations in inundation extent, flow structures, and erosion mechanisms, with the Superflood erosion primarily driven by abrasion and lateral scour, leading to slope failures and valley widening. While the erosion process of the Megaflood involves a dynamic vortex with effective “plucking” sustained by alternating rotational forces, high shear stress, and significant water depths. Our findings underscore the critical role of hydraulic thresholds, defined by water depth and velocity, in shaping distinct flow structures and erosion mechanisms observed in outburst floods of varying magnitudes in the rugged mountains of the eastern Himalayas.

River aggradation or incision at different spatial-temporal scales are governed by tectonics, climate change and surface processes which all adjust the ratio of sediment load to transport capacity of a channel. But how the river responds to differential tectonic and extreme climate events in a catchment is still poorly understood. Here, we address this issue by reconstructing the distribution, ages and sedimentary process of fluvial terraces in a tectonically active area and monsoonal environment in the headwaters of the Yangtze River in the eastern Tibet Plateau. Field observations, topographic analyses and optically stimulated luminescence (OSL) dating reveal a remarkable fluvial aggradation, followed by terraces formations at elevations of 62-55 m (T7), 42-46 m (T6), 38 m (T5), 22-36 m (T4), 18 m (T3), 11 m (T2), 2-6 m (T1) above the present floodplain. Gravelly fluvial accumulation more than 62 m thick has been dated prior to 24-19 ka. It is regarded as a response to cold climate during the Last Glacial Maximum. Subsequently, the strong monsoon precipitation contributed to cycles of rapid incision and lateral erosion, expressed as cut-in-fill terraces. The correlation of terraces suggests that specific tectonic activity controls the spatial scale and geomorphic characteristics of the terraces, while climate fluctuations determine the valley filling, river incision and terrace formation. Debris and colluvial sediments are frequently interbedded in fluvial sediment sequences, illustrating the episodic short-time blocking of the channel around 20 ka. This indicates the potential impact of extreme events on the geomorphic evolution in the rugged terrain.