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 (Δρ) was imposed between the tributaries (Δρ = 0.5 kg/m3)—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/m3) on river confluence hydrodynamics. Plain Language Summary Where rivers join turbulent mixing of suspended sediments and pollutants affect the morphology and habitat quality of the reach downstream of the confluence. Streamwise orientated vortices (SOVs), which resemble atmospheric tornadoes rotating parallel to the confluence's bed, were first predicted in computational fluid simulations in the early 2010s. Since, SOVs have garnered much interest, yet have evaded detection with conventional field instrumentation. Consequently, clear evidence of SOVs at river confluences has not been reported—leading to much debate on their existence and the fluid motions responsible for their formation. To improve understanding of SOVs, we present and analyze aerial observations of SOVs revealed at the confluence of two rivers with a strong contrast in turbidity. Three dimensional computer simulations demonstrate the SOVs could only be accurately reproduc