This paper aims to understand the effects of a turbidity flow intrusion on eutrophication in Daecheong Dam Reservoir. CE-QUAL-W2, a two-dimensional hydrodynamic and water quality model, is applied. The elevation of the reservoir water surface is used to validate the hydrodynamic model parameters and maximum fluctuations in the water surface elevations reaches about 1 m in the reservoir. During the heavy storm season, July, the thermocline submerged to less than 30 m below the surface. The thickness of the thermocline also reduced to 10 to 15 m. While the average TSS in June, the beginning of the monsoon was still low but it peaked in July due to heavy rainfall. Vertical profiles of the TSS regime in July indicated higher concentration in upper water layers and then the regime moves gradually downward in accordance with the time lapse. Due to the dam spillway opening, high concentrations of TSS attributed to storm turbidity ascended to the upper water layer by following the upward current movement and then, the regime precipitated to a layer below 30 to 40 m after September.