Morphology and Facies Models of Channel Confluences

Channel confluences represent points of significant change within river networks that are of importance to geomorphologists, sedimentologists and engineers. At scales varying over four orders of magnitude confluences are characterized by distinct areas of scour and deposition: (i) tributary mouth-bars, (ii) a deep confluence scour, (iii) bars within areas of flow separation and (iv) post-confluence mid-channel bars. The morphology of these elements and their depositional facies are controlled predominantly by confluence angle, the discharge ratio between the two channels and modifications which occur at low stage. A review of confluence morphology from flume and field studies is presented alongside examples from the Brahmaputra River; these are then used to present tentative facies models which suggest that junctions may be represented by a unique assemblage of bedforms and sedimentary structures. Sedimentation at channel junctions is particularly important in multichannel braided or anastomosed rivers where channel confluences are most abundant and their likelihood of preservation is at its highest. Complete preservation of confluence sediments may be achieved by abandonment of one channel and domination of the confluence by the other channel or, more rarely, complete abandonment of the entire junction. Partial preservation of the deeper portions of confluence scour and fill at the base of channel sandstones is more likely.