An application integrating the Hydrologic Engineering Center's ͑HEC͒-Hydrologic Modeling System hydrologic simulation model and the HEC-River Analysis System hydraulic simulation model into a seamless floodplain mapping application is presented. The application is implemented with an ArcGIS 9 workflow model called Map to Map, which converts a map of rainfall data to a flood inundation map. The simulation models are integrated into the application by establishing information exchange points at which time series of information are passed to a model or returned from a model. Communication between simulation models and the Geographic Information System ͑GIS͒ is made possible by interface data models, which provide a one-to-one mapping between data structures within the simulation model and the GIS. A case study is presented for Rosillo Creek in Texas, in which the Map-to-Map model computes flood inundation polygons from rainfall data. Map to Map gives the user a powerful floodplain mapping and real-time flood forecasting tool.