Landscape change in response to multiperiod glacial debris flows in Peilong catchment, southeastern Tibet

High-magnitude glacial debris flows in small basins in Himalayas have a significant impact on landscape. The Peilong catchment, a tributary of the Parlung Zangbo River in southeastern Tibet, was chosen as a case study of topographic response to multi-period glacial debris flows. There were no recorded debris flows in the catchment before 1983, but four large-scale glacial debris flow events with peak discharge up to 8195 m3/s blocked the river during 1983–1985 and in 2015. A combination of field survey, examination of historical records and interpretation of multi-period remote sensing images was used to assess triggering factors and geomorphic impact of the events. The results show that the debris flows during 1983 and 1985 may be attributed to seismic events in 1981 and 1982, while the event in 2015 resulted from large amount of landslide deposits caused by glacier retreat during 1993–2013 and high precipitation in 2015. In the upper-midstream broad valley, erosion and accumulation of the flows changed the channel morphology, resulting in course diversion. In the lower-midstream narrow valley, lateral erosion of debris flows induced a large number of landslides but had little impact on the channel longitudinal profile. The ability of massive glacial debris flows to change valley topography is more than ten times that of regular water flows. The landscape of the accumulation fan at the outlet of the valley is controlled by the interaction between the sediment transportation capacity of debris flows and erosional capacity of the main river. The flow transport capacity of the Peilong is greater than the delivery capacity of the Parlung Zangbo, resulting in continuous aggradation of the confluence zone.