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

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 (M_r>1), the confluence works as an obtuse-angle junction, whereas when the ratio is less than 1 (M_r<1), the observed behaviors correspond to a confluent meander bend.