Extreme floods in mountain rivers are usually regarded as agents of erosion and sediment transport, whereas their deposits are commonly used as records of past flood stages. Here we show that a megaflood deposit can also reorganize tributary drainage. At the Heishui–Jinsha confluence in the lower Jinsha River, southwestern China, the present Hulukou Canyon cuts through bedrock beside an abandoned low valley-floor route, indicating that the lower Heishui River was redirected after the former outlet was obstructed (Kochel & Baker, 1982).
Field mapping, grain-size analysis, borehole stratigraphy, optically stimulated luminescence dating, and three-dimensional hydraulic modelling were used to investigate this process. A thick, high-elevation gravel body at Wuxing Village, together with a paired gravel body at Datong Town, is located across the tributary-mouth zone. These deposits are clast-supported, fines-poor, poorly sorted, and much coarser than ordinary fluvial terrace deposits, suggesting rapid high-energy emplacement. Basal fine-grained horizons beneath the gravel body constrain the event to late Marine Isotope Stage 3, approximately 40–30 ka.
Hydraulic modelling indicates that a high-stage Jinsha River megaflood could have produced backwater intrusion into the Heishui River valley and reduced transport capacity near the tributary mouth. This promoted rapid trapping of coarse gravel, obstruction of the former outlet, and diversion of subsequent flow toward the Datong side. Continued focused incision along this new pathway formed the present Hulukou Canyon.
This case suggests that megaflood deposits can act not only as archives of extreme floods, but also as active geomorphic agents that block tributary mouths, redirect flow, and initiate secondary bedrock canyon incision.

megaflood,tributary-mouth blockage,backwater deposition,canyon incision,Jinsha River