The hydrodynamics of estuarine groundwater reservoirs is governed by both subsurface dams and river systems. However, existing research has largely overlooked how river-groundwater interactions influence groundwater discharge and residual saltwater desalination. This study combines field data from the Dagu River Basin with numerical simulations to explore how surface water-groundwater interactions promote residual saltwater desalination behind subsurface dams in coastal aquifers. The results indicate that continuous river-supplied freshwater creates a hydraulic barrier, limiting saltwater intrusion and enhancing freshwater infiltration. Following the construction of dam, Cl