A numerical investigation of three-dimensional flow dynamics in a density-driven channel confluence

River confluences are critical nodes within fluvial networks, where hydrodynamic structures are inherently complex. Density contrasts between tributaries, stemming from differences in temperature, salinity, or sediment concentration, are increasingly recognized as a key factor regulating confluence hydrodynamics and mixing. However, a systematic understanding of how such density differences govern the three-dimensional flow structure and the critically important secondary circulation remains limited. This study employs a Reynolds stress model to numerically investigate the effects of both the magnitude and direction of density differences on the flow field, secondary flows, and turbulent kinetics in an open-channel confluence. The results demonstrate that density contrasts induce a fundamental shift from an inertia-dominated to a buoyancy-influenced regime. This shift manifests as a compression of the separation zone, a downstream extension of the recovery region, and—most critically—a buoyancy-dominated control over the secondary flow. The direction of the helical motion is dictated by the sign of the density difference: it aligns with the density-neutral rotation when the mainstream is denser but reverses when the tributary is denser. The influence on turbulence is non-monotonic; moderate contrasts enhance turbulent kinetic energy and secondary flow intensity, while strong contrasts suppress them through stable stratification. Notably, within the studied range (Richardson number Ri = 0.16–0.32), buoyancy transitions from initiating near-bed intrusion to dominating the secondary flow pattern. These findings quantify the coupling mechanism between buoyancy and inertia and provide a refined conceptual model for predicting flow structure and mixing efficiency in confluences affected by environmental gradients.