Morphological resilience at the confluence of a very low discharge creek and a large river (upper Parana, Brazil)

Confluences are important environments within the drainage system for exchanges in energy, sediments, water, and the control of local ecology. Despite the significant advances in the understanding of confluences, the study of the junction of small rivers with very large rivers is still incipient. We present a case study of a confluence bar with an extremely low discharge ratio (Qr = 0.0002) and a high obtuse confluence angle (145°) just downstream of the Porto Primavera Dam, in the Upper Paraná River, Brazil. Our analysis includes bathymetric and sedimentological data from surveys prior to dam closure (1997), and after dam closure (2006, 2018), spanning a total of 21 years. Dam closure reduced the mean water level and suspended sediment load in the Paraná River, as well as controlling the occurrence of natural floods. Although the São Pedro Creek confluence bar has reduced, in terms of spatial area and sediment input, its morphology, sedimentation zones, and hydrodynamics has been preserved during this time frame (1997–2018). The stability of the confluence bar is probably mainly due to the high confluence angle (structural control) that causes high resistance in the Paraná River flow, allowing the generation of a large stagnation flow zone (depositional area). The Upper Paraná River, which is one of the most dammed watersheds in the world, has the São Pedro Creek and other small tributaries as important sediment sources for suspended load, mainly after the Porto Primavera dam construction. The results indicate that the stability of the São Pedro confluence bar is due to the association of the systems involved in the supply of sandy sediment, but mainly due to the obtuse angle, that imposes local conditions of flow and sedimentary circulation necessary for its formation and morphological maintenance.