The influence of fine-grained sediment on the architecture of mid-channel bars in one of the world’s largest braided rivers: Río Paraná River, Argentina

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

University of Liverpool

Jornada; Annual British Sedimentological Research Group Meeting; 2008

University of Liverpool - British Sedimentological Research Group

The Rio Paraná, Argentina, is the 6th largest braided river in the world. Nine mid-channel bars of varying size and age in the Río Paraná, both upstream and downstream from the confluence with the Río Paraguay, have been investigated using Ground Penetrating Radar (GPR), Electrical Resistivity Ground Imaging (ERGI), and coring. The Río Paraguay transports large amounts of suspended silt and clay (∼102 – 103 mg/L) compared to the Río Paraná (∼10 – 102 mg/L ), and this study sought to investigate the influence of such a sediment input on the resultant alluvial architecture. Mid-channel bars in the Río Paraná upstream from the confluence are composed of medium sand with little silt and the depth of penetration of the radar signal is typically 12-15 m. GPR and cores indicate the deposits are composed of dune- and ripple-sets with occasional large-scale sets (1-8 m thick) of angle-of-repose cross strata associated with bar margins and adjoining unit bars. Fine-grained layers associated with flow deceleration in the lee of bars are rare. A bar ∼5 km downstream from the confluence, but dominated by the influence of the Río Paraná and which receives only periodic sediment from the Río Paraguay, is composed of medium-sand but contains a layer of silt and clay which extends laterally for several hundreds of metres. Two other bars ∼8 km downstream from the junction, but always within the plume of finer sediment supplied by the Río Paraguay, are composed predominantly of fine-grained sediment, and here the depth of GPR penetration is <1 m. ERGI surveys and cores indicate coarser-grained layers (∼0.5-1 m thick) in the barhead regions and an overall downstream fining in these fine-grained bars. At bars ∼50 km downstream from the confluence, evidence from both cores and attenuation of the GPR signal at depths ranging from 2 to 7 m indicates that the deposits become composed increasingly of both sand and silt as the two rivers mix downstream. The finer-grained sediment is commonly associated with the leeside regions of unit bars and with bartails. This paper will outline the first results concerning the impact of a fine-grained sediment load upon the alluvial architecture of a sandy braided river. These field observations will feed into ongoing Computational Fluid Dynamics and Reduced-Complexity modelling of large river dynamics, sediment transport and preservation. th largest braided river in the world. Nine mid-channel bars of varying size and age in the Río Paraná, both upstream and downstream from the confluence with the Río Paraguay, have been investigated using Ground Penetrating Radar (GPR), Electrical Resistivity Ground Imaging (ERGI), and coring. The Río Paraguay transports large amounts of suspended silt and clay (∼102 – 103 mg/L) compared to the Río Paraná (∼10 – 102 mg/L ), and this study sought to investigate the influence of such a sediment input on the resultant alluvial architecture. Mid-channel bars in the Río Paraná upstream from the confluence are composed of medium sand with little silt and the depth of penetration of the radar signal is typically 12-15 m. GPR and cores indicate the deposits are composed of dune- and ripple-sets with occasional large-scale sets (1-8 m thick) of angle-of-repose cross strata associated with bar margins and adjoining unit bars. Fine-grained layers associated with flow deceleration in the lee of bars are rare. A bar ∼5 km downstream from the confluence, but dominated by the influence of the Río Paraná and which receives only periodic sediment from the Río Paraguay, is composed of medium-sand but contains a layer of silt and clay which extends laterally for several hundreds of metres. Two other bars ∼8 km downstream from the junction, but always within the plume of finer sediment supplied by the Río Paraguay, are composed predominantly of fine-grained sediment, and here the depth of GPR penetration is <1 m. ERGI surveys and cores indicate coarser-grained layers (∼0.5-1 m thick) in the barhead regions and an overall downstream fining in these fine-grained bars. At bars ∼50 km downstream from the confluence, evidence from both cores and attenuation of the GPR signal at depths ranging from 2 to 7 m indicates that the deposits become composed increasingly of both sand and silt as the two rivers mix downstream. The finer-grained sediment is commonly associated with the leeside regions of unit bars and with bartails. This paper will outline the first results concerning the impact of a fine-grained sediment load upon the alluvial architecture of a sandy braided river. These field observations will feed into ongoing Computational Fluid Dynamics and Reduced-Complexity modelling of large river dynamics, sediment transport and preservation.