Morphology and Hydrodynamics of Fluvial Confluence on Rocky Bed: Field Study in the Basalt Plateau, Paraná

Rocky river confluences are sensitive areas of the drainage network in the landscape, as the angle at which streams enter fractured basaltic bedrock can significantly influence fluvial morphology. The current study was designed to investigate the morphology and hydrodynamics of an obtuse-angle river confluence with bedrock, located on basalts of the Serra Geral Group, in the state of Paraná. Morphological and hydraulic variables, such as width, bed slope, average depth, and bankfull flow, were measured in the field for both the main channel and the tributary. The results indicated that the final reach of the tributary is embedded in a structural lineament, and its entry into the main river, directing the flow towards the opposite bank, modifies the flow dynamics at the confluence, creating a specific distribution of erosion and sedimentation zones. Upstream of the junction, the main channel widens as it approaches the confluence, and the bed is also excavated. A (neo)tectonic control (block tilting) favors vertical incision and a reduction in width downstream of the confluence. The analyses indicated that the morphology and hydrodynamics of the river confluence with bedrock are influenced by both the angle of the river junction and (neo)tectonic control.