Regular flows and infrequent outburst floods shape many mountain landscapes, but the relative contributions from these gradual processes and infrequent high-magnitude events have been widely debated, in part due to a paucity of reliable data from historical outburst floods. Here we quantify erosion and deposition from a catastrophic outburst flood in June 2000, caused by a landslide-dam failure on the Yigong River in a rapidly exhumed region of the Eastern Himalaya. The flood, with a peak discharge of 105 m3/s, lasted for only ~ 10 hours, but its geomorphic effects were equivalent to the cumulative effect of 1–2×103 years of long-term fluvial processes in the region. Ubiquitous boulder bars deposited in the channel by the flood promoted extensive lateral erosion through increased bed roughness. As a result, the valley floor widened threefold, triggering many additional secondary landslides, which contributed boulders to the riverbed. The resultant widespread bank erosion and concurrent landslides will continue to influence fluvial dynamics until the next catastrophic flood remobilizes the boulders. Our quantitative findings highlight the importance of recurrent outburst floods for gorge development and landscape evolution in southern Asia and in rapidly uplifting mountain belts worldwide. Teaser Infrequent outburst floods from short-lived natural dams strongly impact fluvial and hillslope processes in rapidly uplifting mountain belts, and they are a major process of incision and gorge formation in river valleys.

No abstract