Aerial observations and numerical simulations confirm density‐driven streamwise vortices at a river confluence

When rivers collide, complex three-dimensional coherent flow structures are generated along the confluence's mixing interface. These structures mix streamborne pollutants and suspended sediment and have considerable bearing on the morphology and habitat quality of the postconfluent reach. A particular structure of interest-streamwise orientated vortices (SOVs)-were first detected in numerical simulations to form in pairs, one on each side of the mixing interface rotating in the opposite sense of the other. Since, it has proven difficult to detect SOVs in situ with conventional pointwise velocimetry instrumentation. Despite the lack of clear evidence to confirm their existence, SOVs are nevertheless considered important drivers of mixing and sediment transport processes at confluences. Additionally, their causal mechanisms are not fully known which hinders a complete conceptual understanding of these processes. To address these gaps, we analyze observations of strongly coherent SOVs filmed in aerial drone video of a mesoscale confluence with a stark turbidity contrast between its tributaries. Eddy-resolved modeling demonstrates the SOVs' dynamics could only be accurately reproduced when a density difference (Delta rho) was imposed between the tributaries (Delta rho = 0.5 kg/m(3))-providing compelling evidence the observed SOVs are indeed a novel density-driven class of SOV. This work confirms that SOVs exist, expands understanding of their generative processes and highlights the important role of small density gradients (e.g., <= 0.5 kg/m(3)) on river confluence hydrodynamics.