Estuarine hydrology and seasonal monsoon forcing are major controls on the transport and transformation of dissolved organic carbon (DOC). This study examined DOC concentration, radiocarbon (Δ¹⁴C) content, and fluorescent dissolved organic matter (FDOM) in two contrasting estuaries: the barrage-controlled Geum River estuary and the naturally open Seomjin River estuary. FDOM exhibited contrasting spatial distributions between the two estuaries, reflecting differences in estuarine mixing and terrestrial organic matter transport under contrasting estuarine settings. The Geum River estuary showed a pronounced spatial transition in DOM characteristics, whereas the Seomjin River estuary exhibited a more gradual downstream decrease. These differences became more evident during the monsoon season, indicating that increased freshwater discharge amplified the influence of estuarine setting on DOC dynamics. Radiocarbon (Δ¹⁴C) results from bulk DOC and DOC fractions obtained by stepwise UV oxidation will be presented and discussed to constrain DOC sources, estuarine mixing, decomposition processes, and the influence of seasonal freshwater inputs in contrasting estuarine systems.