At the southeastern margin of the Tibetan Plateau, the Yarlung-Tsangpo River plunges through the Himalaya to drop >2 km through the Tsangpo Gorge. Upstream, relict glacial dams and impounded lake terraces suggest that Quaternary lakes as large as 800 km3 catastrophically drained through the gorge as megafloods. We report on new megaflood deposits downstream of the gorge and use detrital zircon U-Pb provenance data to demonstrate that these high-magnitude events originated in Tibet, and more effectively focused erosion in the gorge than both the extremely erosive modern peak flows and one of the largest landslide-dam outburst floods ever documented. Our findings support the proposition that in this steep, narrow gorge, where hillslope angles are near the threshold angle of bedrock failure, megafloods provide a mechanism to rapidly evacuate hillslope material and focus erosion on channel-adjacent hillslopes. Although megaflood frequency remains unconstrained, we demonstrate the capability of these events to contribute substantially to rapid exhumation in this region.