: Increasing greenhouse gases in recent decades, has changed the climate balance on Earth. This phenomenon affects atmospheric and hydrological parameters such as temperature, precipitation and as a result water quality in water resources. Considering that the water quality of a lake is affected by the characteristics of its basin, so climate change can alter the water quality status in water bodies by changing the amount of nutrient load entering the lake from the basin. In this study, the climate model HadCM3 under A2, B2 scenarios and climate model CanESM2 under rcp2.6, rcp4.5, rcp8.5 scenarios were used to investigate the effects of climate change on nutrient load of Mahabad dam watershed and Mahabad dam eutrophication. Considering the period covered by large-scale climatic parameters (NCEP), calibration and validation period for HadCM3 model was 1988-2003 and 2004-2012, and for CanESM2 model was 1988-1996 and 1997-2001, respectively. After calibrating the model, temperature and precipitation during the two periods of 2020-2040 (P1) and 2040-2060 (P2), using statistical downscaling model (SDSM), were predicted for the under study area. A2, B2, rcp2.6, rcp4.5 and rcp8.5 scenarios showed +1.62%, +1.00%, +0.29%, +1.99% and 3.33% temperature change and -7.51%, -4.52%, +10.78%, 0.00% and -2.50% precipitation change in P2 compared with P1. After predicting the temperature and precipitation, the impact of changes in these parameters on runoff, sediment load and nutrient load of the watershed was investigated using the hydrological watershed model (SWAT). In order to calibrate the SWAT model, SUFI2 module of SWAT-CUP software was used. According to the available observational data, calibration and validation period for runoff was 2004-2012 and 1988-2003, calibration and validation period for sediment load was 2003-2009 and 2010-2012, and calibration and validation period for nutrient load was the beginning 8 months and last 4 months of 2011, respectively. According to the results, the prevailing trend of runoff and nutrient load entering the reservoir would be decreasing, and only rcp2.6 scenario in P2 compared with P1 shows a rising trend. After forecasting the climatic parameters and nutrient load from the watershed, the status of water quality and thermal stratification in Mahabad dam reservoir was studied. For this purpose, the reservoir model (CE-QUAL-W2) was used. Calibration and validation period for this model was the beginning 8 months and last 4 months of 2011, respectively. According to the results, the trend in changes of temperature and the concentration of phosphorus in the reservoir, in P2 compared with P1 is increasing and the most significant variations in reservoir water temperature was observed during summer, that the increase in thermal stratification intensity and depth of the thermocline layer, causes phosphorus release in anaerobic conditions, thereby reducing the quality of water in the reservoir. As a conclusion, according to the results of this study, climate change will worsen the eutrophic state of the Mahabad dam. Keywords: Climate Change, Mahabad Dam, SDSM, SWAT, CE-QUAL-W2