A Decision Support System (DSS) for optimal operation of multireservoir systems employs the powerful graphical user interface for the MODSIM river basin network flow model in creating applications for the CSUDP dynamic programming package. The DSS provides capability for graphical construction of multireservoir system networks and object oriented database development for fully dynamic optimization. The interface automatically constructs the required system state dynamics and objective function routines required for the CSUDP model. The DSS provides a powerful adjunct to the MODSIM river basin network model by performing fully dynamic optimization for development of optimal reservoir operational rules. These operational guide-curves can then be input into MODSIM for realistic evaluation of river basin management alternatives. An efficient dynamic programming successive approximations algorithm is applied for systems with up to 30 reservoirs and other system state variables. The DSS is applied to the Mahaweli system in Sri Lanka as a case study.