THE ALLUVIAL ARCHITECTURE OF A FINE-GRAINED MEANDERING RIVER: THE RIO BERMEJO, ARGENTINA

Although the sedimentary facies and heterogeneity of modern sand and gravel bed meandering rivers are reasonably well understood, finer-grained alluvial channels (fine sand, silt, clay) are far less well known. This represents a significant knowledge gap as it is unlikely that models developed for coarser-grained rivers can be simply transferred to fine-grained channels. The primary aim of this paper is to use a new technique, the parametric echosounder (PES), to provide the first large-scale, quantitative visualisation of the alluvial architecture of a fine-grained meandering river, the Rio Bermejo, Argentina.

The study took place on the Rio Bermejo, Argentina, a subtropical river with a seasonal discharge that is greatest during January - April, when channel width is c. 200 m and thalweg depth ranges between 4 - 10 m within the study reach. The mean grain size of bed sediments is sandy silt, with very high suspended sediment concentrations of up to 10,000 mgL^-1. The river is highly active, migrating through an alluvial floodplain characterized by scroll bar topography and many oxbow lake fills. In this study, we use PES, which can uniquely provide high-resolution, decimetric, subsurface imaging in fine-grained sediments, to collect subsurface data from the channel bed at high flow in February 2012. To provide ground truthing of the PES data, several cores were taken at low flow in November 2012.

Analysis of the data reveals that the deposits of the Rio Bermejo are characterized by a lower unit comprising long, long-angle surfaces associated with active point bar evolution, and large-scale scour surfaces resulting from channel migration. These sediments are truncated and overlain by vertical accretion deposits, with sets associated with small bars, dunes and climbing ripples, and cut and fill structures resulting from cross-bar channels. This overall style of alluvial architecture is very different from other modern silt-bed-rivers in the literature that tend to emphasise the presence of oblique accretion. The Rio Bermejo differs from these other rivers because it is much more active, with very high rates of bank erosion and channel migration. Our study demonstrates the considerable potential of PES to transform understanding of fine-grained rivers in as significant a way as GPR has for coarser-grained channels.