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.