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Reservoirs are key components of water infrastructure that serve many functions (water supply, hydropower generation, flood control, recreation, ecosystem services, etc.). Climate change affects the hydrology of the tributary areas to reservoirs, which may profoundly impact their operation and possibly the reservoirs' water quality, among which the temperature gradient and the total dissolved solids (TDS) are key qualitative characteristics of reservoirs, especially those with irrigation function. This study examines water-quality changes in the Aidoghmoush reservoir (East Azerbaijan-Iran) under climate-change conditions in the period 2026-2039. The temperature and precipitation climate variables are calculated by the HadCM3 model driven by emission scenario A2 in the baseline period (1987-2000), and these variables are then projected over the future period (2026-2039). The average annual runoff under climate-change conditions is simulated by the IHACRES model. The results show the future average annual runoff would decrease by about 1% compared to the baseline conditions. The CE-QUAL-W2 model is employed to simulate the reservoir water quality. It is predicted the surface air temperature would increase by 1.3 degrees C under the climate-change scenario compared to the baseline condition, and the temperatures of the reservoir's surface- and bottom-waters would increase by 1.19 and 1.24 degrees C, respectively. The average TDS near the reservoir surface would increase by 44.5g/m(3) (4.3%) relative to baseline TDS. The TDS near the reservoir surface are projected to be highest in autumn and winter for baseline and future conditions. This research shows that changes due to climate change are potentially severe, and presents a methodology that could assist managers and planners to find optimal strategies of reservoir operation to cope with changes in thermal stratification and TDS. This paper results identify the reservoir levels from which to withdraw water with the best water-quality characteristics.