Scales and causes of heterogeneity in bars in a large multi-channel river: Río Paraná, Argentina

To date, published studies of alluvial bar architecture in large rivers have been mostly restricted to individual bar case studies and single locations. Relatively little is known on how the composition of km-scale bars varies within a multi-km reach or over several 100s km downstream. This study presents ground-penetrating radar(GPR) and core data from 11, km-scale bars from the Río Paraná, Argentina. The investigated bars are located between 30 km upstream and 540 km downstream ofthe Paraná-Paraguay confluence, where a significant volume of fine-grained suspended sediment is introduced into the system. Bar-scale cross-stratified sets with lengths and widths up to 600 m and thicknesses up to 12 m, which are diagnostic of large river deposits, are only present in half the surface area of the bars. The majority of these bar-scale sets (~90%) are found on top of laterally-extensive, fine-grained layers of ripple sets that have been deposited in the troughs downstream of bars. Bar-scale sets make up as much as 58% of the volume of the deposits in small, incipient mid-channel bars, but this proportion decreases significantly with increasing age and size of the bars. Contrary to what might be expected, a significant proportion of individual sedimentary structures found in the Río Paraná is similar in scale to the structures found in much smaller rivers. In other words, large river deposits are not always characterised by big structures that allow a simple interpretation of river scale. However, the large scale of the depositional units in big rivers causes small-scale structures such as ripple sets to be grouped in thicker co-sets, which indicate river scale even when no obvious large scale sets are present.The results also show that the composition of bars differs between the studied reaches upstream and downstream from the confluence with the Rio Paraguay. Relative to other controls on downstream fining, the tributary input of fine-grained suspended material from the Paraguay causes a marked change in the composition of the bar deposits. Compared to the upstream reaches, the sedimentary architecture of the downstream reaches in the top ~5 m of mid-channel bars shows (i) an increase in the abundance and thickness (up to m-scale) of laterally extensive (100s of metres) fine-grained layers; (ii) an increase in the percentage of deposits comprised of ripple sets (to >40% in the upper bar deposits); and (iii) an increase in bar-trough deposits and a corresponding decrease in bar-scale cross strata (<10%).In contrast to the bar-top deposits, even just downstream from the Paraguay input, the thalweg of the Río Paraná is covered with m-scale dunes and its deposits are composed of dune sets even in areas where bar-top deposits are dominantly fine-grained. Thus, changes in bar composition due to a tributary point-source of fine grained sediment are expressed primarily in the composition of the bar-top deposits