GEOMORPHIC LEGACY OF MEGAFLOOD DEPOSITION IN MOUNTAIN RIVERS, EASTERN HIMALAYA

Ice and debris dam-burst megafloods are large (discharge ≥ 1 Sv), infrequent flows capable of causing profound landscape change. The great potential of these events to rapidly erode is well documented. Megafloods also transport and deposit large volumes of sediment, from silt and sand to house-sized boulders, yet few studies have considered the role of this deposition in river processes and evolution. Here, we evaluate the processes and geomorphic legacy of megaflood deposition in mountain rivers using flood hydraulic simulations, new observations of flood deposits, and a process-based numerical model of bedrock channel evolution. We focus on the Eastern Himalaya, a tectonically active region that has experienced numerous outburst floods including Holocene megafloods sourced from Tibetan proglacial lakes and large historical landslide-dam-break floods. Field observations of >100 boulder bars and hydraulic simulations of reconstructed outburst floods show the strong control of rugged valley topography on flood hydraulics and subsequent patterns of coarse sediment (>4 m diameter boulders) deposition. This result implies that 1) the spatial distribution of megaflood boulder deposits should differ from that of boulders deposited through other processes, such as meteoric flooding or typical hillslope-river interactions, and 2) repeated megafloods may deposit large boulders in the same locations. We quantified the magnitude and timing of post-flood river response to regionally synchronous boulder deposition by megaflooding using a channel profile evolution model constrained by >10,000 boulder measurements. Modeled boulder deposition from a single megaflood introduces over 100 meter-scale knickpoints and increases local channel steepness more than boulders deposited by incision-rate-dependent hillslope processes—channel disequilibrium effects that compound over multiple megafloods and may persist for >20 kyr. Lastly, we link flood hydraulic simulations to geomorphic and sedimentological observations of inundated tributary valleys to explore the legacy of slackwater sediment deposition. Our findings show the importance of intermontane sediment deposition from megafloods in the evolution of net-erosional, tectonically active landscapes, shedding new light on the role of infrequent, high-magnitude flood events in shaping planetary surfaces.