ArcView GIS with ArcView 3D Analyst and a new extension were used to extract floodplain cross sections and reach lengths from a TIN of a watershed being restudied for the National Flood Insurance Program. Cross sections were located to coincide with field-surveyed cross sections of the channels. The extracted data was imported into HEC-RAS and combined with the field-surveyed data to construct full floodplain cross sections. HEC-RAS computed water surface elevations along the channels, and the results were transferred to ArcView GIS where the floodplain limits were mapped onto the TIN.