A 3-D coupled hydrodynamic and ecological model of ELCOM-CAEDYM was applied to Urayama Reservoir to study the effects of climate change on the thermal structure and water quality in the reservoir as well as those downstream. We evaluated the differences between present period (2002-2010) and future period (2062-2070). Scenarios were estimated using JRA25 and GFDL-CM3 with CO₂ emission scenario of RCP4.5 by downscaling of WRF model and a hydrological model of YHyM/BTOPMC. Future surface and hypolimnetic water temperatures were predicted to rise by 2.9°C and 1.6°C, respectively. As a result, a stronger thermal stratification was expected. In the reservoir, significant increase of future SS concentrations by more frequent and bigger floods and decrease of phytoplankton biomasses by high water temperature, hydrologic condition and light limitation were also predicted. In the downstream of the reservoir, less frequent cold-water release and more frequent warm-water release were both predicted. Moreover, more frequent and high turbid water releases were predicted due to more frequent and bigger floods in the future.