Organic carbon exports from land to rivers and oceans play a vital role in regional and global carbon cycling and are subject to the impacts of both natural and anthropogenic disturbances. Most Earth system models, however, do not have a process-based representation of leaching from soil to rivers and transport and transformation of dissolved organic carbon (DOC) in rivers. Here we introduce a new riverine DOC module based on the Model for Scale Adaptive River Transport (MOSART) as part of the Energy Exascale Earth System Model (E3SM), denoted as MOSART-DOC. MOSART-DOC takes the DOC leaching from the E3SM Land Model (ELM), and simulates riverine DOC transport and transformation in both natural and managed river systems. We apply the coupled ELM-MOSART-DOC over the Mississippi River basin between 2001 to 2010 at a 1/8^th-degree resolution. The model can capture the spatiotemporal variation of DOC load satisfactorily when compared against the observed DOC data from the USGS stations. More importantly, the coupled model can predict the impacts of water management and stream temperature on the spatiotemporal variability of riverine DOC explicitly. The modeling results have important implications for aquatic ecosystems in a changing environment.