The Paraná River in the argentine plain: A review of its evolution and contemporary characteristics

Abstract

This paper discusses the geological evolution of the Paraná River in Corrientes Province, located inside the Mesopotamic Argentine region, and reviews the principal geomorphological and hydro-sedimentological characteristics of this outstanding fluvial system. Active lateral migration of the Paraná River inside its large alluvial fan, which covers the north-western portion of Corrientes Province, reveals a structural control related to the separation and breakup of Gondwana. The lowlands now present in this region receive only pluvial input and constitute one of the largest wetlands in South America, the Esteros del Iberá. The best exposed geological units in Corrientes Province are the Ituzaingó Formation (Plio-Pleistocene) that is a fluvial deposit associated with evolution of the Paraná River, and the Toropí-Yupoí Formation (Pleistocene), a younger palustrine fill generated by deposition of aeolian sediment over the underlying fluvial sands. The present planform pattern of the Paraná River in Corrientes Province is anabranching with several types of bar that occupy more than 30% of the channel surface. The bed sediment is predominantly medium to fine sand with the dominant bedform being dunes, which thus also dominate its subsurface sedimentary architecture. The suspended sediment concentration of the Paraná River downstream from its confluence with the Paraguay River (its main tributary) is asymmetric across the channel width, with higher concentrations towards the west due to the much higher input of sediment from the Paraguay River, which in turn receives most of its suspended sediment from the Bermejo River. At Corrientes City, suspended sediment concentration shows extremes ranging from 1 to 616 mg L−1 on the eastern margin and from 8 to 1271 mg L−1 on the western margin. Mixing between the Paraná and Paraguay rivers flows may take more than 600 km downstream before full mixing is achieved, with the mixing processes at the junction being determined by the discharge (momentum) ratio between the two confluent rivers, topographically-driven flows and the possible influence of density driven underflows at their interface. The confluence area between the Paraná and Paraguay rivers is characterized by a large scour that is ∼30 m deep, with its deepest location being at the mouth of the Paraguay River where the channel narrows due to local bedrock control. Due to the different elevation of the Paraná River margins in the region downstream of the confluence, the contemporary floodplain is developed extensively only on the western margin. This floodplain can also be considered as proximal (flooded yearly) or distal (flooded only during extraordinary events) in its geomorphic character. Most of the headwaters of small tributaries in this part of the Paraná River basin are located in wetlands. In these wetlands, the channel beds are characterized by fine sediment consisting of silt and cohesive clays, and where the suspended sediment has a high proportion of organic matter.