Construction of dams is essential for an arid country like Iran so water can be stored to assure the availability of a reliable supply to satisfy domestic, industrial and agricultural requirements, control floods and generate hydroelectricity. Dams are thus essential for the country's socio-economic development. While dams have contributed to very significant benefits, their construction has also resulted in many negative environmental impacts. Only these negatives impacts are analysed in this paper. Assessments and understanding of these negative impacts will contribute to more efficient water management practices in the future.