The triangulated irregular network (TIN) data structure has the ability to precisely represent linear (banks, channel bottoms, ridges) and point features (hills, sinks) important in channel and floodplain geometry definition. Both ARC/INFO and ArcView GIS tools have been developed to take advantage of TIN terrain representation to support the HEC-RAS hydraulic model for open channel flow analysis. This paper describes techniques that can be used to develop quality TINs for automated floodplain delineation with HEC-RAS. Among other issues, underwater terrain modeling, inclusion of old cross section (HEC-2) data, incorporation of DEMs into TIN creation, and automated floodplain interpretation will be discussed.