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Decision support systems (DSS) that have been applied in the water engineering area were reviewed from a historical standpoint, and the DSS were described in terms of definition, structures and components. The L-THIA web application that is a DSS based on an integration of WWWbased programs, GIS, and databases is also introduced. The L-THIA web DSS was analyzed from the point of view of DSS structure that is comprised of a modeling system, database system and interface. As a DSS, L-THIA web has been constructed to support decision makers and to give them information about the hydrological impacts of water quantity and quality due to urban sprawl. To achieve these purposes, L-THIA web has a runoff and NPS estimation model, internet-GIS and ORACLE based weather database. The L-THIA web has its own special features that were prepared for novice users who are targeted as the potential users of the L-THIA web and would have limited knowledge about hydrology. Therefore, DSS enhancement features are introduced that provide useful functions like impact evaluation, treatment suggestions and urban best management practices (BMP) cost estimation. The enhancement features can help and guide users in decision making, and increase users comprehension about the effects of urban sprawl on water quantity and quality.