<p>Groundwater and surface water are key components of the natural hydrological cycle, but the quantitative understanding of how their interactions dynamically respond to extreme climatic events is still limited. This study investigated the riparian wetlands downstream of the Xiaolangdi Reservoir (Henan, China) to elucidate how extreme precipitation (specifically the “7·20” extreme rainfall event) disrupted normal hydrogeochemical evolution and groundwater and river water exchange mechanisms. Integrating hydrochemistry, stable isotopes (δD, δ¹⁸O), and a Two-Endmember Mixing Model, we quantitatively evaluated the interaction between the groundwater and river water. The results indicated that while rock weathering primarily governed the HCO₃-Na·Ca hydrochemical type, the extreme precipitation event triggered a dramatic dilution effect, sharply decreasing major ion concentrations in the aquifer. Isotopic analysis revealed that the δD and δ¹⁸O values of both river water and groundwater were relatively enriched during the flood season, indicating that the floodwater underwent intense pre-recharge evaporation before infiltrating the aquifer. Furthermore, the results demonstrated that extreme precipitation significantly impacted the interaction between groundwater and river water by reversing the exchange direction. Prior to the annual flood season (before extreme precipitation occurred), river water primarily recharged groundwater, whereas groundwater became the dominant source discharging into the river after extreme rainfall. This altered recharge-discharge pattern persisted for a period after the precipitation events had ended. These findings improve understanding of hydrological connectivity in reservoir-regulated riparian wetlands and provide acritical insights for regional water resource management and ecological protection under a changing climate.</p>