Surface water concentrations of CO₂, CH₄, and N₂O have rarely been measured simultaneously in river systems modified by human activities, contributing to large uncertainties in estimating global riverine emissions of greenhouse gases (GHGs). Basin-wide surveys of the three GHGs were combined with a small number of measurements of C isotope ratios in dissolved organic matter (DOM), CO₂, and CH₄ in the Han River basin, South Korea, to examine how longitudinal patterns of the three gases and DOM are affected by four cascade dams along a middle section of the North Han River (hereafter termed middle reach) and treated wastewater discharged to the lower Han River (lower reach) traversing the Seoul metropolitan area. Monthly monitoring and two-season comparison were conducted at 6 and 15 sites, respectively, to measure surface water gas concentrations and ancillary water quality parameters including concentrations of dissolved organic carbon (DOC) and optical properties of DOM. The basin-wide surveys were complemented with a sampling cruise along the lower reach and synoptic samplings along an urban tributary delivering effluents from a large wastewater treatment plant (WWTP) to the lower reach. The levels of pCO₂ were relatively low in the middle reach (51–2465 µatm), particularly at the four dam sites (51–761 µatm), compared with those found in the largely forested upper basin with scattered patches of croplands (163–2539 µatm), the lower reach (78–11298 µatm), and three urban tributaries (2120–11970 µatm). The upper and middle reaches displayed generally low concentration ranges of CH₄ and N₂O, with some local peaks influenced by agricultural runoff and impoundments. By comparison, the lower reach exhibited exceptionally high concentrations of CH₄ (1.2–15766 nmol L⁻¹) and N₂O (7.5–1396 nmol L⁻¹), which were significantly correlated with different sets of variables such as DO and PO₄³⁻ for CH₄ and NH₄⁺ and NO₃⁻ for N₂O. Downriver increases in the levels of DOC and optical properties such as fluorescence index (FI) and protein-like fluorescence indicated an increasing DOM fraction of anthropogenic and microbial origin. The concentrations of the three GHGs and DOC were similar in magnitude and temporal variation at a WWTP discharge and the receiving tributary, indicating a disproportionate contribution of the WWTP effluents to the tributary gas and DOC exports to the lower reach. The values of δ¹³C in surface water CO₂ and CH₄ measured during the sampling cruise along the lower reach, combined with δ¹³C and Δ¹⁴C in DOM sampled across the basin, implied a strong influence of the wastewater-derived gases and aged DOM delivered by the urban tributaries. The downstream enrichment of ¹³C in CO₂ and CH₄ suggested that the spatial distribution of these gases across the eutrophic lower reach may also be constrained by multiple concomitant processes including outgassing, photosynthesis, and CH₄ oxidation. The overall results suggest that dams and urban wastewater may create longitudinal discontinuities in riverine metabolic processes leading to large spatial variations in the three GHGs correlating with different combinations of DOM properties and nutrients. Further research is required to evaluate the relative contributions of anthropogenic and in-stream sources of the three gases and DOM in eutrophic urbanized river systems and constrain key factors for the contrasting impoundment effects such as autotrophy-driven decreases in pCO₂ and in-lake production of CH₄ and N₂O.