A direct-processing approach to river system floodplain delineation is developed. Floodplain zones of part of the South Nation River system, located just east of Ottawa, Ontario, are mapped in two dimensions and three dimensions by integrating the hydraulic model of the choice with geographic information systems (GIS). The first objective was to construct and validate a Hydrologic Engineering Center's River Analysis System (HEC-RAS) river network model of the system using existing HEC-2 model-generated data. Next, HEC-RAS simulations were performed to generate water surface profiles throughout the system for six different design storm events. The in-channel spatial data of HEC-RAS were then geo-referenced and mapped in the GIS domain and integrated with digital elevation model (DEM) over-bank data to build a triangular irregular network (TIN) terrain model. In the final step, floodplain zones for the six design storms were reproduced in three dimensions by overlaying the integrated terrain model for the region with the corresponding water surface TIN.Key words: river, floodplain, delineation, GIS-approach, HEC-2 model, HEC-RAS model, data query.