Riverine freshwater inputs are critical to ocean stratification and influence ocean circulation patterns and deep-water formation. The Community Earth System Model (CESM) with coarse horizontal grids cannot fully resolve the estuaries or continental shelves due to high computational costs. Consequently, the physical processes affecting riverine fresh water passing through estuaries and continental shelve are poorly described in the CESM. A shelf box model is developed to improve the CESM representation of buoyancy-driven flow and wind-driven mixing and transport. It outputs freshwater fluxes that cross the continental shelfbreak to the open ocean. The plume is treated as a collection of segments formed each time step (at least an inertial period, approximately one day) outside the estuary mouth. These segments then propagate downshelf under no-wind conditions and are mixed and advected offshore for upwelling-wind conditions. The source conditions for the plume are determined by an estuary box model. The box model system is tested with Columbia River, and their results are compared with coastal 3D numerical simulation results. The framework for integrating the box modeling system with CESM also will be discussed.