Rivers deliver terrestrial and aquatic carbon to the coastal ocean, and the particulate organic carbon (POC) fraction is the component most directly tied to catchment erosion and hydrological forcing. In East Asian monsoon rivers, however, the magnitude of POC export and its sensitivity to water flow regime remain poorly constrained, in part because many flux estimates rest on short observation periods that do not span contrasting wet and dry years. Here we quantify dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and POC fluxes in three major South Korean rivers — the Han (21,700 km²), Nakdong (20,700 km²), and Yeongsan (2,150 km²) — over 2021–2024. Annual carbon loads were calculated from paired discharge and concentration records. To characterize the origin of the particulate organic carbon, a Bayesian stable-isotope mixing model (MixSIAR) was applied to δ¹³C and C/N of river POC, using Nakdong River end-members as the quantitative anchor, together with a sensitivity analysis in which end-member values were perturbed to test their transferability between watersheds. Total carbon flux was dominated by DIC in all three rivers (60.9–76.5%). POC formed a minor but systematically varying fraction that increased from the Han (8.2%) and the Nakdong (11.7%) to the Yeongsan (14.8%), inversely to watershed area. In every river, the years of maximum and minimum POC load coincided with those of maximum and minimum annual discharge, although the wettest and driest years differed among watersheds: 2022 was the wettest year in the Han (32.0 km³ yr⁻¹) but the driest in the Nakdong (6.1 km³ yr⁻¹) and Yeongsan (1.2 km³ yr⁻¹). In the Nakdong, discharge varied 3.3-fold between 2022 and 2023 (19.8 km³ yr⁻¹), while total carbon load varied threefold (101.7 to 306.2 Gg yr⁻¹) and POC load varied 4.5-fold (9.6 to 43.4 Gg yr⁻¹), so that the particulate fraction responded more sensitively to discharge than the carbon flux as a whole, rising from 9.4% to 14.2% of total carbon. Comparable contrasts occurred in the Han (POC 27.0–42.5 Gg yr⁻¹) and Yeongsan (3.1–7.6 Gg yr⁻¹). Mean total carbon concentration nevertheless remained within a narrow range across all rivers and years (14.0–18.7 mg L⁻¹), indicating that interannual differences in load were driven primarily by water volume rather than by concentration. These results indicate that the magnitude and compositional role of POC export in Korean monsoon rivers are governed primarily by hydrological regime rather than basin size (watershed area).

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