In many countries such as Iran, social and industrial developments changed the qualitative characteristics of the river's water quality and leads to excessive pollution. The first step for river water quality management is obtaining information on changes of river water quality in dimensions of time and place and also, determination of major sources of pollutants. WQI is a mathematical and statistical tool for conversion of quantitative values of large quantity of water quality data into single

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... number which presents a simple and understandable tool for qualitative assessment. In current study, samples were collected from stations at up, middle and downstream of three rivers in Mazandaran province (Siahrod River, Haraz River and Babolrod River) in a 2 years interval of 2012-2013 years. The values of NSFWQI (water quality index of America's national health organization) were calculated for all stations and all of the stations were located on the level of unsuitable conditions. According to NSFWQI, the best condition was related to the upstream of Haraz River and the worst condition was related to the downstream of Siahrood River.