Floodplains are critical landscape features that are highly distinguished from neighboring uplands in terms of their hydrologic, geomorphologic, biogeochemical, and ecological processes. Thus an essential step in floodplain studies is the determination of the lateral floodplain extent and its variability within a basin. Recent advances in topographic mapping and GIS-based analysis provide a means for developing automated techniques to determine floodplain extent. In this study, we propose a hydrogeomorphic floodplain delineation which links a simplified inundation algorithm with the river basin properties. We utilize the method to investigate the physical controls on floodplain formation, as captured by scaling relations between floodplain width and contributing area. Our hypothesis is that an efficient delineation grounded in hydrogeomorphic theory can improve our understanding of floodplain geometry across a range of conditions. We test the method for two distinct watersheds, a humid catchment in Italy and an ephemeral basin in New Mexico, and note the existence of power law scaling of the floodplain width after averaging over stream orders. An analysis of the algorithm parameters is conducted to identify possible limitations. Further systematic tests are needed to establish the generality of the floodplain scaling and its control on floodplain processes.