Abstract
The quality of surface water is one of the most important concerns in many parts of the world. Regular analysis of surface water quality helps accurate management to reach acceptable quality for drinking water; however, its quality may be affected by seasonal variations. The aim of this study was to evaluate the microbiological quality of the water from the Ahar River and Sattārkhān dam (Iran) in different seasons. The physicochemical properties and the count of saprophytic, ammonifying, denitrifying, nitrite-fixing (aerobic and anaerobic), and coliforms were investigated in the samples during different seasons of a year. For this purpose, 4 stations in the river and 9 stations in the dam were selected for sampling. The mean count of saprophytic bacteria in the water of the River varied between 2.76 log CFU/ml (winter) and 4.66 log CFU/ml (fall). Mean total coliforms significantly differed (p≤0.05) among different seasons. At the entrance region of river water to the dam, the mean count of saprophytic bacteria and coliforms generally was more than it was in other stations. The distribution of ammonifying bacteria in the water of the dam was similar to saprophytes. The highest mean count of nitrogen-fixing and denitrifying bacteria was detected in the summer season. The conditions of the Sattārkhān dam were more favorable to Azotobacter sp. than those of Clostridium pasteurianum.The results of the present study indicated that seasonal variations and physicochemical parameters of the water of River and dam are directly or indirectly affect the levels of indicator bacteria. So, regular monitoring of rivers during different seasons of the year can lead to better water quality for human consumption and ultimately improve public health.

Key words: River, Microbiology, Water, Microbial Quality, Clostridium.