Study region: The Athabasca River Basin (ARB), Canada. Study focus: This study investigates the dynamic interactions between surface water and groundwater systems in the Athabasca River Basin. The integrated surface–subsurface model of the ARB was first calibrated under monthly normal transient flow conditions against observed surface water and groundwater data. The model was then forced with monthly average transient data to assess performance. Based on these simulations, groundwater contributions to the Athabasca River system were estimated using surface water–groundwater exchange fluxes and compared with isotope-based balance estimates. New hydrological insights for the region: Results suggest that the groundwater system interacts strongly with surface water across the basin. Groundwater contributions to the Athabasca River range from 34% during high-flow season to 63% during low-flow season, with an average annual contribution of about 45%. These findings highlight the critical role of groundwater in sustaining river flows and emphasize the need to account for groundwater when assessing potential climate change impacts on future water availability and extreme hydroclimatic events.