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It is indispensable to explore simulation techniques that not only represent complexdynamic systems in a realistic way but also allow the involvement of end users in modeldevelopment to increase their confidence in the modeling process. System dynamics as a feedbackbasedand object-oriented simulation approach is presented for reservoir operation modeling. Thequick modeling process, the trust developed in the model due to user contribution, group modelsdevelopment possibility and the effective relations of model results are the most significant strongpoints of this approach. The simple modification of model in response to changes in system andcapability to accomplish sensitivity analysis make this approach more attractive and useful ratherthan traditional reservoir operation models. In this paper system dynamics is applied to simulateoperation of a free reservoir with an Ogee spillway, a reservoir with a gated spillway and finally amulti-reservoir system with simple and gated spillways. The multi-reservoir system on Karun riverin south of Iran is modeled under flood condition as a case study in order to demonstrate thecapabilities of the developed model.