<p>Clarifying surface water (SW)–groundwater (GW) interactions and their effects on material fluxes is essential for water-resource protection and ecosystem stability. This study integrated hydrochemical data, stable isotopes (δD and δ¹⁸O), a ²²²Rn mass balance model, riverbed vertical permeability coefficients, and land use types to investigate the spatial variability and controlling factors of SW–GW interactions and material fluxes in the Xiliugou watershed, Ordos Plateau. Results showed that both SW and GW were dominated by the Ca–Na–HCO₃ hydrochemical type, while overlapping δD and δ¹⁸O values suggested similar recharge sources. Spatial variations in total dissolved solids (TDS), δ¹⁸O, and ²²²Rn concentrations revealed that the upstream reach was dominated by SW recharge, the midstream by GW discharge, and the downstream by anthropogenic regulation. These patterns were corroborated by hydraulic head and a ²²²Rn mass balance model. Combined with land use data, the results revealed a spatial decoupling between hydrological exchange and material fluxes. In the upstream reach, intensive agricultural activities maintained high groundwater TN concentrations and TN fluxes despite relatively low SW recharge fluxes. In the midstream reach, although GW discharge fluxes were relatively high, the dominance of barren land resulted in low TN concentrations and thus low TN fluxes. In the downstream reach, intensive agricultural activities also contributed to elevated groundwater TN concentrations, and although GW discharge fluxes were lower than those in the midstream, the resulting TN fluxes exceeded the midstream values. Based on these findings, SW–GW interactions and land use types should guide management.</p>