The planform mobility of river channel confluences .The planform mobility of river channel...
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The planform mobility of river channel confluences:Dixon, Simon; Sambrook-Smith, Greg; Best, James L.; Nicholas, Andrew; Bull, Jonathan;Vardy, Mark E.; Sarker, Maminul H.; Goodbred, StevenDOI:10.1016/j.earscirev.2017.09.009
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Citation for published version (Harvard):Dixon, S, Sambrook-Smith, G, Best, JL, Nicholas, A, Bull, J, Vardy, ME, Sarker, MH & Goodbred, S 2018, 'Theplanform mobility of river channel confluences:: Insights from analysis of remotely sensed imagery' EarthScience Reviews, vol 176, pp. 1-18. DOI: 10.1016/j.earscirev.2017.09.009
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The planform mobility of river channel confluences: Insights from analysisof remotely sensed imagery
Dixon Simon J.a,, Sambrook Smith Gregory H.a, Best James L.b, Nicholas Andrew P.c,Bull Jon M.d, Vardy Mark E.d, Sarker Maminul H.e, Goodbred Stevenf
a School of Geography, Earth and Environmental Science, University of Birmingham, Birmingham B15 2TT, UKb Departments of Geology, Geography and Geographic Information Science, Mechanical Science and Engineering and Ven Te Chow Hydrosystems Laboratory, University ofIllinois at Urbana-Champaign, IL, USAc Geography, College of Life and Environmental Sciences, University of Exeter, Exeter EX4 4RJ, UKd National Oceanography Centre, University of Southampton, Southampton SO14 3ZH, UKe Centre for Environmental and Geographic Information Services, House 6, Road 23/C, Gulshan-1, Dhaka 1212, Bangladeshf Department of Earth and Environmental Sciences, Vanderbilt University, Nashville, TN, USA
A B S T R A C T
River channel confluences are widely acknowledged as important geomorphological nodes that control thedownstream routing of water and sediment, and which are locations for the preservation of thick fluvial depositsoverlying a basal scour. Despite their importance, there has been little study of the stratigraphic characteristicsof river junctions, or the role of confluence morphodynamics in influencing stratigraphic character and pre-servation potential. As a result, although it is known that confluences can migrate through time, models ofconfluence geomorphology and sedimentology are usually presented from the perspective that the confluenceremains at a fixed location. This is problematic for a number of reasons, not least of which is the continuingdebate 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 in-vestigates the spatial mobility of river confluences by using the 40-year record of Landsat Imagery to elucidatethe styles, rates of change and areal extent over which large river confluence scours may migrate. On the basis ofthese observations, a new classification of the types of confluence scour is proposed and applied to the Amazonand Ganges-Brahmaputra-Meghna (GBM) basins. This analysis demonstrates that the drivers of confluencemobility are broadly the same as those that drive channel change more generally. Thus in the GBM basin, a highsediment supply, large variability in monsoonal driven discharge and easily erodible bank materials result in acatchment where over 80% of large confluences are mobile over this 40-year window; conversely this figureis< 40% for the Amazon basin. These results highlight that: i) the potential areal extent of confluence scours ismuch 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 confluencelocation is more common than currently assumed, and iii) confluence mobility is often tied to the lithologicaland hydrological characteristics of the drainage basins that determine sediment yield.
River confluences are important nodal points in alluvial networks,often representing abrupt downstream changes in discharge, grain sizeand channel geometry, which in turn may exert a significant control onchannel morphology, migration and avulsion (Mosley, 1976; Richards,1980; Ashmore, 1991; Bridge, 1993; Ashmore and Gardner, 2008; Bestand Rhoads, 2008). The morphology of the confluence zone also hasmany ramifications for understanding and managing aspects of river
behaviour, such as the fact that the dynamic morphological adjustmentsat these sites may make managing land use and infrastructure difficult(Ettema, 2008). Meanwhile, the morphological and geochemical het-erogeneity often present at confluence sites has led ecologists to con-clude that they are hotspots of high biodiversity (e.g. Benda et al.,2004), and/or may form sites of appreciable biological change (e.g.Rice et al., 2008). Even at confluences that possess a relatively stableplanform location, the hydraulic processes at junctions are still highlycomplex, which makes understanding of pollutant pathways, for
http://dx.doi.org/10.1016/j.earscirev.2017.09.009Received 6 October 2016; Received in revised form 27 August 2017; Accepted 12 September 2017
Corresponding author.E-mail address: email@example.com (S.J. Dixon).
Earth-Science Reviews 176 (2018) 118
Available online 28 September 20170012-8252/ 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/).
example, problematic (Biron and Lane, 2008). In the present paper, wefocus on exploring the planform morphodynamics of large confluencesand linking this to the subsurface sedimentology.
River confluences have the potential to create some of the points ofdeepest incision into underlying sediments (Mosley, 1976; Best, 1988;Bristow et al., 1993; Salter, 1993; Siegenthaler and Huggenberger,1993; Best and Ashworth, 1997; Miall and Jones, 2003; Ullah et al.,2015) and hence their subsequent fill has been argued to possess thehighest preservation potential of fluvial channels (Huber andHuggenberger, 2015). Since the depth of junction scour and mobility ofthe confluence are determined by flow processes in the confluencehydrodynamic zone (Best and Rhoads, 2008), it can be argued thatdiffering junction dynamics may produce a range of characteristicconfluence zone sedimentology from sandy bar development to mud-filled scours. Furthermore, understanding the planform mobility ofconfluences, and thus the potential spatial extent of basal scour sur-faces, particularly in large rivers, is key to interpreting alluvial strati-graphy and discriminating between autocyclic and allocyclic scoursurfaces (Best and Ashworth, 1997; Fielding, 2008), reconstructingpalaeohydraulics 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 forhydrocarbon exploration (Ardies et al., 2002).
Despite the fact that the sedimentary fill of confluences may bepreferentially preserved and that their large scale may lead to confusionin discriminating between autocyclic and allocyclic scour, to date therehas been no comprehensive analysis of confluence mobility to resolvequestions concerning the extent and ubiquity of migrating confluencelocations. For example, Holbrook and Bhattacharya (2012) questionwhether confluences can migrate sufficiently to produce a scour largeenough to resemble that of an incised valley, and hence be mistaken fora product of allocylic-driven erosion. However, some case studies, suchas the confluence of the Ganges and Jamuna rivers, Banglade