The aim of this study was to assess the potential impacts of climate change on hydrology and stream water quality for a 6642 km2 mountainous dam watershed in South Korea using the Soil and Water Assessment Tool (SWAT). The model was calibrated for three years (1998-2000) and validated for another three years (2001-2003) using daily streamflow data at three locations and monthly stream water quality data at two locations. For future evaluation, the MIROC3.2 HiRes and ECHAM5-OM climate data by Special Report on Emissions Scenarios (SRES) A2, A1B, and B1 of the Intergovernmental Panel on Climate Change (IPCC) were adopted. The future biased data (2007-2099) were corrected using 30 years (1977-2006, baseline period) of weather data, and downscaled by the change factor (CF) statistical method. The future (2020s, 2050s, and 2080s) watershed hydrology and stream water quality were evaluated based on the 2000 data. The MIROC3.2 HiRes A1B 2080s temperature and precipitation showed an increase of +4.8¡C and 34.4%, respectively, based on the 2000 data. The impacts of projected future climate change scenarios on the evapotranspiration, groundwater recharge, and streamflow were increases of +23.1%, +28.1%, and +39.8%, respectively. The future sediment load showed a general tendency to decrease in the A2, A1B, and B1 emission scenarios of the two GCM models. The increase of the future T-N load may come from the increase of the subsurface lateral flows from January to June and the groundwater recharges from January to July. The future T-P load showed an increase of +19.6% in the 2080s under the MIROC3.2 HiRes A1B scenario and a decrease of -48.4% in the 2050s under the ECHAM5-OM B1 scenario. © 2011 American Society of Agricultural and Biological Engineers ISSN 2151-0032.