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 = 1231.8[Formula: see text] [Formula: see text]) and a drought scenario constrained by an environmental-flow threshold (4.1[Formula: see text] [Formula: see text]). Results reveal the emergence of hydrodynamic thresholds driven by cumulative reach-scale losses ([Formula: see text]), producing nonlinear downstream discharge decay under low-flow conditions and requiring a minimum upstream inflow of [Formula: see text] 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.