The accuracy and efficiency of computing floodplain elevations and mapping floodplain boundaries were measured and compared for two different approaches. One approach was traditional: floodplain cross sections derived from contour maps and field surveys were entered through the HEC-RAS user interface, and floodplain boundaries were delineated using contour maps. The other approach was GIS-based: floodplain cross sections extracted from a TIN of the watershed were transferred to HEC-RAS, and floodplain boundaries were delineated automatically on the TIN.