The dynamics and deposits of a big river: the Rio Paraná, Argentina

ASHWORTH P,; AMSLER M,; BEST, J; HARDY R,; LANE, S,; NICHOLAS, A.; ORFEO, O.; PARSONS, D.; REESINK, A.; SANDBACH, S.; SMITH G, SAMBROOK; SZUPIANY R,

University of Aberdeen, UK

Conferencia; From River to Rock Record; 2009

University of Aberdeen

The dynamics and deposits of a big river: the Rio Paraná, Argentina Little is known about the morphology, dynamics and sedimentology of the world’s largest multi-thread rivers. There are good theoretical reasons to suggest that observations and data obtained for small rivers (less than 1 km wide) may not be transferable directly to the world’s largest rivers. For example, force-balance considerations suggest a dependence on width-depth ratio that does not scale linearly with flow discharge and this may then affect the depositional sedimentology. This paper reports on a combined ‘process’ and ‘depositional form’ study in one of the world’s largest multi-thread rivers - the sandy Rio Paraná, Argentina. Bathymetric and 3D flow data were taken in a 38 km long, 4 km wide reach from boat surveys using single and multi-beam echo sounders, acoustic Doppler current profilers (ADCP) and dGPS. Initial results demonstrate that the main channels of the Paraná at low flow are dominated by dunes up to 3.5 m high that scale with flow depth. Unit bars in the Paraná are less common than observed in smaller rivers. The deposits of recent km-scale mid-channel bars were characterised using Ground Penetrating Radar (GPR), resistivity surveys and shallow (5 m) and deep (30 m) cores. The internal structure of mid-channel bars is dominated by decimetre to sub-m high, stacked dune sets and up to 7 m thick, high-angle, bar margin accretionary sets. Re-activation surfaces on bar margins are common, whereas cross-bar channels are less prevalent as compared to smaller braided rivers. ´From River to Rock Record´ Conference, University of Aberdeen, 10-12 January 2009 ´From River to Rock Record´ Conference, University of Aberdeen, 10-12 January 2009 These preliminary field observations suggest that whilst there may be some differences in the basic flow processes in small and large rivers, the deposits of large rivers have the same characteristics as smaller rivers. Thus, with appropriate scaling, it may be possible to use geostatistical databases derived from a mix of different sized rivers in reservoir models.