Numerical modeling to assess the hydrodynamic behavior of the Gallinas River in San Luis Potosí under atypical hydrometeorological conditions

River systems characterized by strong groundwater–surface water interaction exhibit complex hydrodynamic responses under hydroclimatic extremes. This study investigates how infiltration-dominated river reaches modulate flow persistence during drought and floodplain activation during extreme rainfall. A two-dimensional Environmental Fluid Dynamics Code (EFDC) model was implemented for a 20.35 km reach of the Gallinas River (Mexico) using high-resolution UAV-derived bathymetry and field-based discharge measurements. The model was calibrated and independently validated prior to simulating a 25-year return period flood (peak discharge = 1231.8m³ s⁻¹) and a drought scenario constrained by an environmental-flow threshold (4.1m³ s⁻¹). Results reveal the emergence of hydrodynamic thresholds driven by cumulative reach-scale losses (~ 3.1m³s⁻¹), producing nonlinear downstream discharge decay under low-flow conditions and requiring a minimum upstream inflow of 7.2m³s⁻¹ to maintain ecological continuity. Under flood forcing, inundation patterns are primarily controlled by channel geometry and longitudinal slope reduction rather than discharge magnitude alone. These findings demonstrate that infiltration-influenced rivers exhibit dual hydrodynamic controls under contrasting extremes and highlight the importance of explicitly representing cumulative exchange processes in two-dimensional modeling frameworks. The study provides transferable insights for assessing drought resilience and flood risk in permeable or groundwater-connected river systems facing increasing hydroclimatic variability.