Hydroelectric power plants associated with reservoirs in different river basins supplying an interconnected systems should be modeled as a whole for the efficient satisfaction of energy demand. Yet, as the number of reservoirs in the system increases, the mathematical model becomes increasingly complex, consuming too much computer time to be solved. With the aggregation-optimization-disaggregation systems approach, a system of reservoirs in different river basins can be aggregated into an equivalent reservoir model which can be optimized to determine optimal operation rules. As a final step, the optimal operation rules of the resulting system can be disaggregated into decisions for the multi-reservoir system again by the use of optimization techniques. In this study, the application of the aggregation-optimization-disaggregation systems approach to a multi-reservoir system composed of reservoirs in different river basins has been developed. As an example, optimization techniques which can be used for the optimization and the disaggregation steps of the systems approach are applied to a multi-reservoir system comprising different combinations of the Sariyar, Hirfanli, Özköy and Demirköprü reservoirs. © 1991, Taylor & Francis Group, LLC. All Rights Reserved.