Abstract: The Gyaca-Mainling section of the Yarlung Zangbo River Valley is located in the middle and lower reaches of the Yarlung Zangbo River, and there are 89 dunes here. In this paper, the formation mechanism of sand dunes is studied by means of field investigation, remote sensing satellite image interpretation, DEM and particle size analysis in the Gyaca-Mainling section. The results show that the dunes can be divided into three types: floodplain type dunes, terrace type dunes and valley slope type dunes according to the dunes distribution height. The formation of floodplain type dunes is closely related to the water level change of Yarlung Zangbo River. During flood season, floods submerge the floodplain and deposit a large amount of sediment. In dry season, the water level decreases. The sand material deposited in the floodplain is exposed and blown up to the bank slope by the wind and accumulated and then formed to floodplain type dunes. The formation of terrace type dunes and valley-slope dunes are related to the destruction of the overlying loess layer. With cut down of the Yarlung Zangbo River valley, the fossil alluvial sediments would be left over at different heights of terraces and valley slope. The fossil alluvial sediments are eroded by wind and formed fossil dunes. Later, due to environmental evolution, the surface of fossil dunes is covered by a layer of loess, which becomes the protective layer of fossil dunes. Once the protective layer was damaged by wind erosion, water erosion and human activities, the underlying fossil dunes would expose and formed mobile dunes.

Key words: Yarlung Zangbo River Valley, Gyaca-Mainling section, reactivation of fossil dunes, formation