[en] A technique is described for incorporating a drainage recognition capability into a graphical information system (GIS) database. This capability is then utilized to export digital topographic profiles of stream-channel cross-sectional geometries to the Hydrologic Engineering Center's Water Surface Profile (HEC-2) model. This model is typically used in conjugation with the Flood Hydrograph (HEC-1) package to define floodplain boundaries in complex watersheds. Once these floodplain boundaries are imported back into the GIS framework, they can be uniquely referenced to the New Mexico state plane coordinate system. A combined GIS-HEC application in ungaged watersheds at Los Alamos National Laboratory is demonstrated. This floodplain mapping procedure uses topographic data from the Laboratory's MOSS database. Targeted stream channel segments are initially specified in the MOSS system, and topographic profiles along stream-channel cross sections are extracted automatically. This procedure is initiated at a convenient downstream location within each watershed, and proceeds upstream to a selected termination point. HEC-2 utilizes these MOSS channel data and HEC-1 generated storm hydrographs to uniquely define the floodplain. The computed water surface elevations at each channel section are then read back into the MOSS system. In this particular application, 13 separate elongated watersheds traverse Laboratory lands, with individual channels ranging up to 11 miles in length. The 50, 100, and 500-year floods, and the Probable Maximum Flood (PMF) are quantified in HEC-1. Individual floodplains are then defined for each channel segment in HEC-2 at 250 foot intervals, and detailed 1:4800 scale maps are generated. Over 100 channel miles were mapped using this combined GIS-HEC procedure