To obtain better constraints on the control of seasonal hydrological variations on dissolved organic carbon (DOC) dynamics in headwater catchments, we combined hydrometric monitoring with high‐frequency analyses of DOC concentration and DOC chemical composition (specific UV adsorption, δ13C) in soil and stream waters during one complete hydrological cycle in a small lowland catchment of western France. We observed a succession of four hydrological periods, each corresponding to specific DOC signatures. In particular, the rise of the upland water table at the end of the rewetting period yielded to a strong increase of the specific UV absorbance (from 2.5 to 4.0 L mg C−1 m−1) and of the δ13C values (from −29 to −27‰) of the soil DOC. Another striking feature was the release of large amounts of DOC during reduction of soil Fe‐oxyhydroxides at the end of the high‐flow period. Comparison of hydrometric data with DOC composition metrics showed that soils from the upland domains were rapidly DOC depleted after the rise of the water table in these domains, whereas wetland soils acted as quasi‐infinite DOC sources. Results from this study showed that the composition and ultimate source of the DOC exported to the stream will depend on the period within the annual hydrological cycle. However, we found that the aromatic DOC component identified during the high‐flow period will likely represent the dominant DOC component in stream waters on an annual basis, because most of the annual stream DOC flux is exported during such periods. Key Points DOC composition in soil and stream waters exhibit strong seasonal variations Seasonal changes in DOC source pools in wetland soils driven by hydrology Upland soils quickly depleted during the wet season contrary to wetland soils