Linking process and depositional architecture in large rivers: an integrated
field and modelling approach
THE RIO PARANA RESEARCH TEAM
J.L. BEST1, P.J. ASHWORTH2, M.L. AMSLER3, R.J. HARDY4, S.N. LANE4, A.P.
NICHOLAS5, O. ORFEO6, D.R. PARSONS7, A.J.H. REESINK2,8, G.H. SAMBROOK
SMITH8, S.D. SANDBACH4,5 AND R.N. SZUPIANY 3
Universities of Birmingham (UK), Brighton (UK), Centro de Ecologia Aplicada del Litoral
(Argentina), Durham (UK), Exeter (UK), Illinois (USA), Leeds (UK) and Nacional del
Litoral (Argentina)
1Departments
of Geology and Geography and Ven Te Chow Hydrosystems Laboratory,
University of Illinois, Urbana, USA
2School of Environment and Technology, University of Brighton, UK,
3Universidad Nacional del Litoral, Facultad de Ingeniería y Ciencias Hídricas, Santa Fe,
Argentina
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 Earth and Environment, University of Leeds, Leeds, UK
8School of Geography, Earth and Environmental Sciences, University of Birmingham,
Birmingham, UK
Little is known about the processes, dynamics and deposits of the World’s largest multithread rivers and whether they differ from smaller (less than 1 km wide) channels. This
paper reports on an integrated field and numerical modelling campaign on the sandy Rio
Paraná, Argentina – the World’s 6th largest river.
Bathymetric and 3D flow data were taken in a 38 km long, 4 km wide reach using singlebeam echo sounders, acoustic Doppler current profilers and dGPS. Results demonstrate
that the main channels of the Rio Paraná, at low flow, are dominated by dunes up to 3.5 m
high and less common unit bars 2-5 m high, with some up to 10 m high. Analysis of
satellite images from 1973-present shows that mid-channel bars develop from an
amalgamation of unit bars that stall at barheads and wrap around bartails. However, in the
Rio Paraná, and unusually for such a large and dynamic river, new mid-channel bar
formation is relatively infrequent and is often associated with flow divergence at, or near,
established vegetated islands.
The deposits of seven, km-scale, mid-channel bars were characterised by ~43 km of
Ground Penetrating Radar (GPR), and ground truthed by suction cores. The internal
structure of mid-channel bars is dominated by: (i) decimetre to sub-m high, stacked dune
sets, (ii) up to 7 m thick, high-angle, bar margin sets, and (iii) decimetre-thick ripple sets,
most commonly, but not exclusively found at the bartop. Re-activation surfaces on bar
margins are common. The input of a significant finer, suspended sediment load, from the
Rio Paraguay, leads to a change in the depositional architecture of the bars immediately
downstream, with larger scale cross-stratified sets being less frequent.
The fieldwork has been complemented by numerical modelling of flow, sediment transport
and morphological change in the 38 km study reach using a combination of 3D
Computational Fluid Dynamic (CFD), Reduced Complexity (RC) and Delft3D depthaveraged models. The models show a good match between field and modelled flow
distributions. Simulated channel change is characterised by talweg shifting, but does not
indicate the significant influence of migrating unit bars. Ongoing modelling is quantifying
the impact of unsteady discharge on bar formation and simulating bar evolution in a 10 km
sub-reach over ~30 years, in order to test if the process of bar evolution matches the rate
and form of change observed in the satellite record.