AbstractThe lateral transport and degassing of carbon in riverine ecosystems is difficult to quantify on large spatial and long temporal scales due to the relatively poor representation of carbon processes in many models. Here, we coupled a scale‐adaptive hydrological model with the Dynamic Land Ecosystem Model to simulate key riverine carbon processes across the Chesapeake and Delaware Bay Watersheds from 1900 to 2015. Our results suggest that throughout this time period riverine CO2 degassing and lateral dissolved inorganic carbon fluxes to the coastal ocean contribute nearly equally to the total riverine carbon outputs (mean ± standard deviation: 886 ± 177 Gg C·yr−1 and 883 ± 268 Gg C·yr−1, respectively). Following in order of decreasing importance are the lateral dissolved organic carbon flux to the coastal ocean (293 ± 81 Gg C·yr−1), carbon burial (118 ± 32 Gg C·yr−1), and lateral particulate organic carbon flux (105 ± 35 Gg C·yr−1). In the early 2000s, carbon export to the coastal ocean from both the Chesapeake and Delaware Bay watersheds was only 15%–20% higher than it was in the early 1900s (decade), but it showed a twofold increase in standard deviation. Climate variability (changes in temperature and precipitation) explains most (225 Gg C·yr−1) of the increase since 1900, followed by changes in atmospheric CO2 (82 Gg C·yr−1), atmospheric nitrogen deposition (44 Gg C·yr−1), and applications of nitrogen fertilizer and manure (27 Gg C·yr−1); in contrast, land conversion has resulted in a 188 Gg C·yr−1 decrease in carbon export.