This study systematically examined environmental effects during and after Dez Dam Reservoir flushing operations. The study focused on potential upstream sources of pollution, sediment concentrations, water-quality parameters, and uses of water downstream of the dam, particularly emphasizing the presence of fish and macroinvertebrates as well as their habitats. The study was conducted from June 16-18, 2003 in the Dez Dam Reservoir, which is located in the southern part of Iran. The Dez Dam Reservoir is facing a serious sedimentation problem, and its dead volume will be quite full in the coming 10 years. Therefore, performing flushing operations through the three irrigation gates of the dam has received much more attention in recent years. The results of this study helped determine the hydraulic conditions that could decrease the negative environmental effects of the Dez Dam Reservoir flushing operations on downstream aquatic conditions. © National Association of Environmental Professionals 2015.