Rainbow trout (Oncorhynchus mykiss) is one of the most-widely cultivated cold fresh water fish in the world. Gamasyab River, with a length of about 200 Km, is one of the longest rivers of Iran. The aim of this research is to determine the effects of trout farm effluents on this river water. From a total of 24 trout farms in the area, four farms were randomly selected and three sampling stations were chosen at each selected farm. Measured water quality parameters were temperature (T), total suspended solids (TSS), total dissolved solid (TDS) dissolved oxygen (DO), pH, five-day biochemical oxygen demand (BOD5), chemical oxygen demand (COD), ammonia-nitrogen (NH4-N), nitrate-nitrogen (NO3-N) and phosphates (PO4-P). Based on the laboratory analysis, DO in the river water was more than 6 mg/l in all cases. There were signiﬁcant differences in six variables of T, BOD, COD, NO3-N, NH4-N, and TSS of all water samples taken from the second sampling stations between dry and wet season. The DO concentration in Gamasiab River decreased between the water inlet (first stations) and outlet (second stations), but increased in third station. Trout farm effluents had significant impact on the TSS content of the river. This significant increase in TSS concentrations is expected to have occurred due to cleaning or harvesting activities on any of the farms. The results of the study showed that, in all of season, the amount of TSS in the second stations was not equal to the standards of the Department of Environment Protection (40 mg/L) for discharging into river systems.