Mass flows and river response in rapid uplifting regions——A case of lower Yarlung Tsangpo basin,southeast Tibet,China

The fluvial geomorphology in tectonically active(particularly rapid uplift) regions often undergoes continuous change.The rapid uplift is coincident with high erosion rates;consequently,incised valleys are formed.Mass flows(for example,avalanches,landslides,and debris flows) in incised valleys can markedly influence fluvial processes and even reshape valley geomorphology.However,these processes and long-term evolution corresponding to mass flows require further clarification.Field campaigns were carried out in the region near the Yigong Tsangpo and Palong Tsangpo Rivers(hereafter the Yigong and Palong Rivers),the two largest tributaries of the lower Yarlung Tsangpo River Tibet,China,to examine the feedback between fluvial processes and mass flows.Remote sensing images from recent decades were used to compare the channel morphology before and after typical mass flows(particularly catastrophic ones).The morphology of the lower Yigong River has evidently been impacted by landslides,while that of the Palong River has mainly been shaped by glacial processes and debris flows.At present,the morphology of the latter consists of alternating sections of gorges and wide valleys,with a staircase-like longitudinal profile.The gorge sections exhibit single and deeply incised channels with a high-gradient channel bed and terraces.In contrast,the wide valley sections consist of lakes,braided or anabranching channels,gentle bed gradients,and thick alluvial deposits.Debris flows occur more frequently in gullies in the reaches of the gorge sections and rarely in gullies along the wide valley sections.The occurrence of mass flow events has resulted in an imbalance of the previous(quasi-)equilibrium in the river morphology;however,this has triggered negative feedback that is driving the transient river morphology to a new state of(quasi-)equilibrium.