GEOMORPHOLOGICAL PROBLEMS OF THE MIDDLE REACHES OF THE TSANGPO RIVER,TIBET

The middle reaches of the Tsangpo River consist of alternating section s of wide valleys and gorges. The wide valley sections have braided an d anastomosing channels, gentle hydraulic gradients, thick alluvial de posits and low terraces. In contrast, the gorge sections exhibit singl e, straight and deeply entrenched meandering channels with steep hydra ulic gradients, bare rock river beds and higher terraces. Several hypo theses have been used to explain these unusual fluvial landforms, but geological, landform and sedimentary analyses alone with dating inform ation, suggest that the key could be the active faults across the rive r valley. All gorge sections are located on the upthrown side of activ e faults, which mainly occurred in or after the Pliocene, whilst the w ide valley sections appear on the downthrown side. The faulting blocke d the river and caused the formation of palaeolakes, with thick deposi ts laid down behind the faults. Therefore, depositional wide valleys w ere formed and old terraces were buried, On these downthrown sides of the faults, braided and anastomosing channels have developed. On the u pthrown sides, strong incision of the river occurred because of the ch anges of the local base levels and river gradients. As a result, deep gorges and deeply entrenched meandering channels formed in various lit hologies. The terraces on the gorge slope indicate different stages of river incision and the related knick points appeared close to the loc al active faults. Rock resistance is only a minor influence on the alt ernation of valley forms and river gradients in this area. (C) 1998 Jo hn Wiley & Sons, Ltd.