ABSTRACT
In the Upper Reach of the Paraná River Basin, the system is a plateau river that runs over a mixed rock alluvial bed, with thin alluvial sediments, two alluvial terraces and several breaks in its longitudinal profile. Owing to the numerous dams that have been constructed in the Upper Paraná River and its larger tributaries, sediments are deposited in the reservoirs and the river has a very low suspended sediment load and a bed channel tending to armoring (process in which channel bed develops a layer that is coarser than the substrate). There are more than 200 islands in the Upper Paraná, plus hundreds of central and lateral sand bars. Some sections of this part of the river present up to six secondary channels, although the east channel is usually the primary channel over the entire reach. Downstream of the basaltic plateau located at the southern margin of Brazil, the Middle reach of the Paraná River flows through an extensive Argentine plain. In the first stretch, the river generates a large alluvial fan, which covers the north-western portion of the Corrientes Province of Argentina and the southern part of eastern Paraguay. Downstream of the confluence with the Paraguay River, the Paraná shows pronounced spatial differences in the concentrations of suspended sediments, related to the hydrological cycles in both rivers. The greatest difference occurs during the flooding period of the Bermejo River, the main tributary of the Paraguay River that exports fine sediments that originate in the Andes Mountains. The present multichannel pattern of the middle Paraná River is related to the bed load discharge, which is higher than 25% of the total sediment discharge. Because of this and the regional slope, several kinds of sand bars and channels are a typical feature of the Middle Reach of the Paraná. The bed sediments grain size is typically sandy, with a decrease in the grain size from the Middle to the Lower stretch. The Paraná Delta has a surface area of around 3,500 km2. It is composed of islands with a central marsh or shallow lake, surrounding by meandering streams with sandy natural levees.
