Effects of Discharge Ratio and Temperature Difference on Three-Dimensional Flow and Thermal Mixing at a 90° Confluence

River confluences are highly heterogeneous hydraulic environments where flows with different discharges and temperatures merge, generating complex three-dimensional flow structures and thermal mixing that directly influence downstream mass transport, water quality, and aquatic ecosystems. In this study, a three-dimensional numerical model was used to investigate flow structures and thermal mixing at a 90° open-channel confluence. The study was designed to systematically examine how changes in discharge ratio modify three-dimensional flow and thermal mixing under the same channel geometry and total discharge, and how these responses differ between isothermal and non-isothermal conditions. Four main-to-tributary discharge ratios were combined with two thermal conditions. In the isothermal condition, the main channel and tributary had equal inflow temperatures. In the non-isothermal condition, the main channel inflow was 10 °C warmer than the tributary inflow. Eight cases were analyzed quantitatively in terms of the spatial trajectories of the maximum-velocity and thermal cores, momentum ratio, densimetric Froude number, mixing index, and gradient Richardson number. The results showed that the initial position of the maximum-velocity core was governed primarily by the relative momentum of the two incoming flows, whereas density and buoyancy effects induced by the temperature difference developed progressively downstream, indicating distinct spatial ranges over which momentum and buoyancy influenced the flow. When tributary momentum was substantially dominant, rapid initial mixing occurred together with a spatially extensive density-inversion structure. In contrast, when main-channel momentum was dominant, stable density stratification developed and delayed thermal homogenization. These results suggest that thermal mixing at river confluences with temperature differences is governed not by buoyancy alone, but by the interaction between the relative momentum of the main-channel and tributary flows and buoyancy. In particular, even under the same temperature difference, variations in discharge ratio altered the relative importance of inertia-driven three-dimensional rearrangement of the incoming water masses and density stratification, thereby producing distinct thermal-mixing characteristics. This study provides a quantitative basis for interpreting thermal mixing at river confluences in terms of the coupled effects of flow structure, momentum, and density-driven processes.