Confluence-diffluence units are key elements within many river systems and present a complex and linked pattern of flow and sediment transport between these two nodes. Past work has seen much progress in our modelling ofriver confluences, and increasing attention is being paid to bifurcations and the important role of bifurcation asymmetry. However, most of these studies have been conducted in laboratory flumes or within small rivers (depths less than 3 m and widths < 100 m). This paper presents a study of the bed morphology and 3D flow within a very large confluence-diffluence unit in the Rio Paraná, Argentina. The paper details the central scour within the reach and reveals that the flow within the confluence-diffluence is dominated largely by the bed roughness, and that the coherent secondary flow cells, that have been identified in smaller channels and assumed to be present within large rivers, are generally absent in this reach.