Background and Objectives: Aydughmush dam was built on Aydughmush River at 19km in southwest of Mianeh City. The dam is multipurpose and its main aims are the spring floods control, supplying potable water for villages and providing irrigation water. Different pollutants which probably discharge to the river finally enter to the dam reservoir so; this study focuses on evaluating the quality of the dam reservoir.
Materials and Methods: In this cross sectional study standard field parameters including dissolved oxygen, temperature, Biochemical and chemical oxygen Demand, Most Probable Number of Coliforms, Fecal Coliform, Turbidity, Total Dissolved Solids, Total Solids, pH, conductivity and others were measured at eight different stations during the spring and summer in 2010. Sampling points were selected on the basis of their importance. Water quality index was calculated using water quality index calculator given by National Sanitation Foundation (NSF) information system.
Results: The highest value of WQI of the samples was 84.89 in A3 station in July while the lowest value was 67.96 in A2 station in May. The lower value of WQI has been found mainly due to the slightly lower value of DO in the dam reservoir water. Most of the water samples were found within Good category of National Sanitation Foundation Water Quality Index (NSF-WQI). Comparison of the measured parameters based on the sampling stations and  various months by variance and t-student analysis showed a significant relationship for some parameters(P<0.05). Nutrient budget determination indicates that the concentration of phosphate, nitrite, nitrate and ammonia at inlet are higher than outlet of the dam reservoir.
Conclusion: The calculated (WQI) showed good water quality. Based on the results of NSFQWI calculations, the dam reservoir water quality is suitable for various purposes.