MODSIM is a comprehensive, generalized river basin management decision support system (DSS) under continuous development and enhancement since 1979 at Colorado State University. MODSIM is designed to be flexible and robust, allowing a wide range of analyses from short-term daily operations requiring hydrologic flow routing to long-term monthly river basin planning and management. MODSIM is capable of modeling large-scale river basin systems by employing an efficient minimum cost network flow optimization algorithm for simulating allocation of water according to water rights, storage ownership contracts, interstate compacts, and economic valuation. An important objective in MODSIM development has been to provide an easy-to-use modeling platform for decision makers and managers by incorporating a powerful graphical user interface for network creation, data import and editing, and georeferenced graphical output results. MODSIM is capable of simulating stream-aquifer interactions for conjunctive use of surface water and groundwater. MODSIM software development under the MS .NET Framework enables extensive customization capabilities, allowing MODSIM to be integrated with water quality models such as QUAL2E and numerical groundwater flow models such as MODFLOW, as well as incorporation of complex operating rules and regulations.