This paper introduces the development of a GIS-based modeling and management support system for analyzing site-specific phosphorous loading and transport in Florida's Lake Okeechobee watershed. The system, named GIS-PLAT, links together a relational database in Oracle, a GIS database in ArcInfo, and a model-specific parameter database as ASCII files. It integrates AML programmed menus, Avenue customized ArcView GIS projects, and UNIX shell scripts with FORTRAN coded numerical models. Computing platforms involved are a VAX minicomputer (Oracle database), a UNIX server (GIS database), UNIX workstations (modeling), and Windows NT personal computers (data query and display). The system is completely menu-driven and seamless across platforms.