Obtuse-angle vs. confluent sharp meander bends: insights from the Paraguay-Cuiabá confluence in the tropical Pantanal wetlands, Brazil

Abstract

River confluences are notable features in the drainage system with complex hydrodynamics, bed morphology, sediment transport, and mixing processes. Junction planform plays a pivotal role in the confluence hydrodynamics. However, most investigations have concentrated on the junction of two straight channels, and few studies have focused on confluent meander bends or obtuse-angle confluences. Here we analyze a confluence with an obtuse-angle planform–the tributary enters the confluence at an angle higher than 180°–that abruptly curves to join the main river. In the Pantanal wetlands, the strong flood wave is responsible for significant variations of the discharge and momentum-flux ratios within the confluence. Consequently, the Paraguay-Cuiabá confluence is able to shift from an obtuse-angle working mode to a confluent meander bend working mode according to the hydrological conditions. When the momentum ratio is greater than 1 (Mr>1), the confluence works as an obtuse-angle junction, whereas when the ratio is less than 1 (Mr<1), the observed behaviors correspond to a confluent meander bend.