Dynamic programming (DP) optimization and HEC-5 simulation models were developed for planning the long-term operation of the Karoon-Dez multireservoir system in Iran. The well-known dimensionality problem associated with DP was controlled using a heuristic approach to narrow the required search algorithm within the state space of DP. HEC-5 as a common and widely available computer program for studying reservoir systems regulation was also used to simulate operation of the Karoon-Dez system with water supply and hydropower generation objectives. HEC-5 is able to consider flood control, hydropower generation and conservation purposes. Both the DP and HEC-5 models were applied to the Karoon-Dez system consisting of six reservoirs; five reservoirs in series parallel to a sixth one. The results obtained by the HEC-5 and DP models were analysed and compared. The results show that, although the DP results were better than those of HEC-5 in terms of meeting system operating targets and energy generation, HEC-5 as a simulation model demands much less computer time and memory, making it attractive for practical use.