For more than thirty years, computer models have been used as essential tools for floodplain determination. Although the combined use of hydrologic and hydraulic models has significantly increased the ability to predict flooding events, the amount of fieldwork required to create and calibrate a model is still considerable. This document presents a methodology that uses aerial mapping as a basis for creating floodplain maps when using the HEC River Analysis System (HEC-RAS) hydraulic model for floodplain analyses. The approach is based on the use of triangular irregular networks (TINs) within a geographic information system (GIS) environment. This methodology was applied to the Waller Creek watershed in Austin, Texas, and involved three phases. First, flow data required by HEC-RAS were determined using the GIS-based application CRWR-PrePro and the hydrologic model HEC Hydrologic Modeling System (HEC-HMS). During the second phase, tools were developed for embedding structures, such as buildings, into the TIN. Finally, the ArcView GIS extension AVRas was used to extract the information contained in the TIN, export it into HEC-RAS, read the results of the hydraulic model and represent the flooded areas. Two and three dimensional animations were developed to show the visualization capabilities offered by GIS. Both, the comparison of the geometric data extracted from a digital representation of the terrain with field data, and the resulting model, show that TINs can be successfully used for floodplain determination and representation purposes.