This paper presents a general description and the state of progress of the GIBSI system, a watershed-based software system for integrated management of surface water quality. This spatial support-decision system is designed to assist decision makers as well as water resources professionals. An integrated structure composed of mathematical models, a Geographical Information System (GIS) and a DataBase Management System (DBMS) characterizes the system. This means interactions between users and system components are made through a single user-friendly interface. A simple scenario approach is used to examine the impact of alternative urban, industrial, and agricultural management practices on surface water quality. Interpretation of results is based on comparing different management scenarios with a reference state or other previously defined scenarios.