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Dirty dancing with two left feet: the Rio Bermejo-Paraguay confluence tango!

Channel confluences form key nodes within all alluvial networks and extensive research over the past 25 years has shown how the dynamics of mixing at these sites, and nature of the bed morphology, are controlled by the junction angle, the momentum ratio between the two confluent flows, the junction planform shape and the nature of any bed height discordance between the two channels. Although remote sensing of many open channel confluences, particularly of larger rivers, often reveals a difference in turbidity between the two flows, there has been little work conducted on how the density differences that such turbidity contrasts may generate may influence the dynamics of mixing at confluences.

This paper presents results from a field study of flow and bed morphology at the confluence of the Río Bermejo and Río Paraguay, Argentina, where the sediment load, and thus bulk density, of the Río Bermejo flow is always greater than that of the Río Paraguay.The Río Bermejo is an important river in the Paraná basin as, although it may only contribute ~5% of the fluid discharge to the Río Paraná, it may contribute up to ~80% of the total wash load. Surveys of the confluence were undertaken in May 2005 and April 2007, when the discharge ratio between the two rivers (Bermejo: Paraguay) was 0.17 (combined discharge ~ 2800 m3s-1) and 0.23 (combined discharge ~ 5050 m3s-1) respectively. However, in 2007 the density excess of the Río Bermejo over the Río Paraguay was 5.1 kg m-3, whilst in the study period in 2005 it was 2.2 kg m-3. Fluid mixing at this site is dominated by a density underflow from the Río Bermejo that penetrates underneath the fluid of the Río Paraguay and then upwells at the outer bank of the Río Paraguay downstream from the confluence. Mixing occurs through instabilities along the shear layer between the two flows, fluid instabilities along the top of the underflow current, topographic forcing over bedforms and interactions with the channel banks. This paper will illustrate these mechanisms of mixing and the possible significance of density contrasts between confluent flows in determining the morphodynamics at such channel junctions.