Do Unit Bars Co-Exist With Dunes In Big Sand-Bed Braided Rivers

Do unit bars co-exist with dunes in big sand-bed braided rivers?

Ashworth, P.J.1, Amsler, M.L.2, Best, J.L.4, Hardy, R.J.4, Lane, S.N.4, Nicholas, A.P.5, Orfeo, O.6, Parker, N.O.7, Parsons, D.R.8, Reesink, A.J.H.1,7, Sambrook Smith, G.H.7, Simmons, S.4,5 and Szupiany, R.N.2

1School of Environment and Technology, University of Brighton, UK 2Universidad Nacional del Litoral, Facultad de Ingeniería y Ciencias Hídricas, Centro Internacional de Estudios de Grandes Ríos, C.C. 217 - (3000) Santa Fe, Argentina 3Departments of Geology and Geography University of Illinois Urbana-Champaign, IL, USA 4Department of Geography, University of Durham, Durham, UK 5Department of Geography, University of Exeter, Exeter, UK 6Centro de Ecologia Aplicada del Litoral, Consejo Nacional de Investigaciones Cientificas y Tecnicas, Corrientes, Argentina 7School of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham, UK 8School of Earth and Environment, University of Leeds, Leeds, UK

Research concerning small (less than 1km wide) sandy braided rivers has shown that their bed morphology is dominated by unit and compound bars and dunes. Unit bars are relatively unmodified, often lobate in planform bars that may stall, coalesce and be reworked to eventually form compound bars. Dune superimposition upon unit bars is ubiquitous and generates a hierarchy of bounding surfaces and a range of different scales of cross-stratification. Recent work in the South Saskatchewan River, Canada, has shown that dunes and unit bars in particular control the local flow and sediment routing and together govern channel evolution and the resulting sedimentary architecture.

New bathymetric and 3D flow data obtained for a 40 km reach in the 6th largest river in the world, the sandy braided Río Paraná, Argentina, suggests the main channels at low flow are dominated by dunes up to 3.5 m high that scale with flow depth and km-wide, vegetated, compound bars. However, unit bars are uncommon in the main channels, and even when present, are of low-relief (1-3 m high) and with low-angle leesides as compared with those observed in smaller rivers. Currently it is unclear why unit bars may dominate the morphology of small sandy braided rivers but not the world’s largest rivers. Possible suggestions are: (i) in large rivers, high flows are necessary to initiate significant bedform superimposition and build unit bars that occupy the full channel width and depth – but these bars are then reworked during the falling stage; and (ii) channel-scale secondary flows are suppressed or absent in large sandy braided rivers which often have a higher width:depth ratio, and therefore reduce the potential for bar aggradation.

These first observations of channel morphology and bedform dynamics in large braided rivers at low flow suggest the dominant processes and deposits of large rivers may have some differences to those in smaller rivers.