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River channel confluences are widely acknowledged as important geomorphological nodes which control the downstream routing of water and sediment, and which are locations for the preservation of thick fluvial deposits overlying a basal scour. Despite their importance there has been little study of their stratigraphic characteristics, or the role of confluence morphodynamics in influencing stratigraphic character and preservation potential. As a result, although it is known that confluences can migrate through time, models of confluence geomorphology and sedimentology are almost always presented from the perspective that the confluence remains at a fixed location. This is problematic for a number of reasons, not least of which is the continuing debate over whether it is possible to discriminate between scour that has been generated by autocyclic processes (such as confluence scour) and that driven by allocyclic controls (such as sea level change). This paper addresses the questions of spatial mobility of river confluences by using the 40-year record of Landsat Imagery to elucidate the styles, rates of change and areal extent over which large river confluence scours may migrate. On the basis of these observations, a new classification of confluence scour types is proposed and applied to the Amazon and Ganges-Brahmaputra-Meghna (GBM) basins. This analysis concludes that the drivers of confluence mobility are broadly the same as those which drive channel change more generally. Thus the high sediment supply, large monsoonal driven discharge variability and easily erodible bank materials of the GBM basin results in a catchment where over 80 % of large confluences are mobile over this 40-year window; conversely this figure is less than 40 % for the Amazon basin. These results highlight that; i) the potential areal extent of confluence scours is much greater than previously thought, with the location of some confluences on the Jamuna River migrating over a distance of 20 times the tributary channel width, ii) extensive migration of confluence location is more common than currently assumed and iii) confluence mobility may be tied to the lithological and hydrological characteristics of the drainage basins that determine sediment yield.
|Journal||Earth Science Reviews|
|Early online date||28 Sept 2017|
|Publication status||Published - 1 Jan 2018|
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- 1 Finished
The sedimentology of fluvial megascours
Natural Environment Research Council
1/04/12 → 1/04/17
Project: Research Councils