Bankfull characteristics have long been considered to be important parameters for characterizing alluvial rivers with floodplains. Empirical evidence suggests that the bankfull discharge of such streams often corresponds to a flood discharge with a 1-year to 2-year recurrence interval. The physics behind the relation between bankfull characteristics and flow hydrology, however, remains unexplained. Here we propose a new framework for modeling the formation processes of bankfull channel geometry, so as to relate bankfull characteristics primarily with flow duration curve, and secondarily with flood frequency. We hypothesize that a river tends to evolve toward an equilibrium state of channel-floodplain geometry at which sediment removal from the floodplain due to lateral channel migration is balanced against sediment emplacement on the floodplain due to overbank deposition. The proposed model has the potential to predict the change in bankfull characteristics in response to hydrologic change, as well as change in sediment supply.