How mixing and thermal stratification in tributary embayments respond to reservoir regulations: Interactions between buoyancy and shear forces

Xiangxi River tributary bay, Three Gorges Reservoir (TGR), China Stratification and mixing, dominant processes in lakes and reservoirs, are governed by a balance between buoyancy ( N ²) and shear ( S ²) forces. However, the underlying mechanisms are not yet fully understood. To narrow this gap, we established a high-resolution 3D hydrodynamic-thermal model to simulate one-year round flow patterns and thermal structures. We quantified the dynamic competition between buoyancy and shear forces using the Richardson number ( R i ) and turbulent dissipation rate ( ε ) as key metrics. Our findings represent three novel insights concerning the hydrodynamics of reservoir tributaries. First, a three-phase intrusive flow pattern (surface, mid-depth, bottom) was detected with reservoir water level regulation; this intrusion reached certain points with equivalent isotherms. Second, we identified a critical discharge ratio threshold (∼300) that acts as an “intensity valve,” determining whether intrusions are confined to the surface or penetrate deeply. Third, we quantified the impact of reservoir regulation on the buoyancy-shear balance. Elevated water levels reinforce thermal stratification by reducing flow velocity, thereby suppressing shear-induced mixing. Conversely, drawdown operations can disrupt the thermocline by enhancing turbulent mixing and reducing stratification stability. These insights may constitute a predictive, mechanistic framework linking reservoir operations to tributary physical dynamics, enabling optimized operations for ecological integrity and sustainable water management. • The spatiotemporal coupling mechanisms of intrusion circulation and thermal stratification are clarified. • A quantitative framework developed is to reveal how reservoir regulation affects thermo - hydrodynamic processes. • The vertical distribution periodicity of intrusive flows and their governing factors are determined. • The dynamic impact of reservoir scheduling on buoyancy - shear equilibrium in tributary bays is assessed.