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Much domestic sewage, industrial wastewater and many agricultural drainages will enter the Qarahsoo River in itsflow path. Given that each river has a capacity to accept pollutants and self-purification to a certain extent. nowadays, knowing the process of changing the quality and self- purification ability of rivers is very important. In this study, the Qual2kw model's performance was investigated in the assimilative capacity in 2015 to the Qarahsoo River in Ardebil province between Aladisge to Arbabkandi stations with a length of 90.6 km.for this purpose, first, the calibrated model of Qual2kw simulated parameters change of NO₃, BOD₅, DO, pH and temperature for two months of January and July, and then it was compared with observational data in Aladizgeh, Samian, Anzob and ArbabKandi stations. The best model simulation for pH parameter was obtained and in the next ranks, the parameters were, respectively NO₃, BOD₅, DO, and Temperature. In addition, the quality of the water of the Qarahsoo River with the NSFWQI water quality index was monitored during the months of April and January 2015 in the range of stations from Neiragh to Kangerlu according to the samples taken by the Environmental Protection Agency. The results showed that the water quality of the Qarahsoo River in the month of January is in the middle class and in good quality in April. The Kangerlu station with the index of 56 in January had the lowest water quality along the river.

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Introduction: Nowadays Water pollution is one of the most important problems in the world and environmental concerns are the major sources of surface water. Shahrood River is one of the most important rivers in Qazvin province. Materials and Methods: Sampling was done in two seasons of November 96 (winter) and July 97 (summer) during two times and three replications from 7 stations including Taleghan, Alamoot, RajaeeDasht, Razmian, Loshan, Loshan 1 and 2. Qualitative parameters included: DO, pH, Total Solids, BOD, Turbidity, Temperature, Phosphate, Nitrate and Fecal Coliform. The data were determined using NSFWQI and water quality index of each river section measured. Results: The results showed that the mean TDS was 180.66 to 329.33 mg/l, pH 8.01 to 8.55, BOD 0 to 2.8 mg/l, DO 5 to 7.5, Coliform 1618.33 to 27300, and Turbidity of 0.73 to 114.9 NTU in summer the TDS 184.66 to 451mg/l, pH 7.73 to 8.55, BOD 0 to 1 mg/l, DO 9 to 13.7, Coliform 162/33 to 1653/333, And the Turbidity is 0.72 to 262.66 in winter. Conclusion: The water quality status of summer in station 1 was good in other stations moderate and in winter season station 1 and 4 were in good range and in other stations was moderate. As a result, Shahrood River water is good and medium quality and due to lower water quality of homes, agricultural lands near the river are also causes of increased Nitrate and Phosphate in winter due to agricultural and urban effluents around the river. As a result, river water is somewhat suitable for drinking and needs purification and the Industrial agents around the river have no effect on the water quality.