A major challenge for predicting future landscape carbon (C) balances is to understand how environmental changes affect the transfer of C from soils to surface waters. Here, we evaluated 14 yr (2006–2019) of stream dissolved inorganic carbon (DIC) concentration and export data for 14 nested boreal catchments that are subject to climatic changes, and compared long-term patterns in DIC with patterns in dissolved organic carbon (DOC). Few streams displayed significant concentration or export trends in DIC at annual time scales. However, most streams showed decreasing DIC concentrations during spring flood over this 14-yr period, and about half showed declines during summer. Although annual runoff has generally not changed during this period, an intra-annual redistribution in runoff, with increases in spring flood discharge, explained much of the seasonal changes in DIC concentration. We observed negative DIC–discharge relationships in most streams, suggesting source limitation of DIC, whereas DOC mostly showed chemostatic behavior. The different trends and patterns observed for DIC vs. DOC underpin intra-annual changes in the composition of the total C pool (i.e., the DIC/DOC ratio) and reflect fundamental differences in how these C forms are produced, stored in riparian soils, and mobilized by hydrological events. Collectively, our results highlight the sensitivity of riverine DIC to the intra-annual distribution of runoff, but also important heterogeneity across the network that suggests local processes can also modify the mobilization of DIC in boreal landscapes.