Abstract
River channel confluences are widely acknowledged as important geomorphological nodes that 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 the stratigraphic characteristics of river junctions, 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 usually 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 investigates the 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 the types of confluence scour is proposed and applied to the Amazon and Ganges-Brahmaputra-Meghna (GBM) basins. This analysis demonstrates that the drivers of confluence mobility are broadly the same as those that drive channel change more generally. Thus in the GBM basin, a high sediment supply, large variability in monsoonal driven discharge and easily erodible bank materials result in a catchment where over 80% of large confluences are mobile over this 40-year window; conversely this figure is <40% for the Amazon basin. These results highlight that: i) the potential areal extent of confluence scours is much greater than previously assumed, with the location of some confluences on the Jamuna (Brahmaputra) River migrating over a distance of 20 times the tributary channel width; ii) extensive migration in the confluence location is more common than currently assumed, and iii) confluence mobility is often tied to the lithological and hydrological characteristics of the drainage basins that determine sediment yield.
Highlights
River confluences are important nodal points in alluvial networks, often representing abrupt downstream changes in discharge, grain size and channel geometry, which in turn may exert a significant control on channel morphology, migration and avulsion (Mosley, 1976; Richards, 1980; Ashmore, 1991; Bridge, 1993; Ashmore and Gardner, 2008; Best and Rhoads, 2008)
We focus on exploring the planform morphodynamics of large confluences and linking this to the subsurface sedimentology
Since the depth of junction scour and mobility of the confluence are determined by flow processes in the confluence hydrodynamic zone (Best and Rhoads, 2008), it can be argued that differing junction dynamics may produce a range of characteristic confluence zone sedimentology from sandy bar development to mudfilled scours
Summary
River confluences are important nodal points in alluvial networks, often representing abrupt downstream changes in discharge, grain size and channel geometry, which in turn may exert a significant control on channel morphology, migration and avulsion (Mosley, 1976; Richards, 1980; Ashmore, 1991; Bridge, 1993; Ashmore and Gardner, 2008; Best and Rhoads, 2008). Understanding the planform mobility of confluences, and the potential spatial extent of basal scour surfaces, in large rivers, is key to interpreting alluvial stratigraphy and discriminating between autocyclic and allocyclic scour surfaces (Best and Ashworth, 1997; Fielding, 2008), reconstructing palaeohydraulics and channel sedimentary architecture (Bristow et al, 1993; Siegenthaler and Huggenberger, 1993; Miall and Jones, 2003; Davies and Gibling, 2011), as well as identifying potential sites for hydrocarbon exploration (Ardies et al, 2002)
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