Changes of seasonal and vertical water quality was analyzed with physico-chemical data from Soyang and Paldang river-reservoir system in Korea during the 1996 to 1998. In Soyang river-reservoir system, the water column was well stratified, which narrow epilimnion layer of 5 to 10 m depth in spring to summer enlarged gradually about 40 m depth in fall as going to times. In contrast, metalimnion layer tended to be narrow during the same period. Water temperature of hypolimnion was maintained about $5^{circ}C$ continuously throughout the year. DO of the epilimnion layer was supersaturated from spring to summer, however, it was decreased to 75% at the epilimnion layer and $45{sim}50%$ at the hypolimnion layer at the late fall. The lowest conductivity of below $50;{mu}S/cm$ was observed at the metalimnion layer during thesummer to fall. In Paldang river-reservoir system, the water column wag well mixed layer throughout the year, although water temperature was changed seasonally from $5^{circ}C$ in February to $28^{circ}C$ in July. Water temperature between upper and lower layer was different about $5^{circ}C$ from late spring (May) to early fall (September). DO was over and less saturated in upper and lower layer during the early summer to early fall, respectively. Conductivity was decreased to $90;{mu}S/cm$ in lower layer of below $4{sim}5;m$ depth during the late spring to early fall and that of upper layer of above 10 m depth decreased to about $100;{mu}S/cm$ during the late fall (November) and early spring (March). Retention time of Soyang river-reservoir system was much longer than that of Paldang river-reservoir system. Chlorophyll a, T-N and T-P concentration in Paldang river-reservoir system were higher than that of Soyang river-reservoir system by a factor of 2.7, 1.2 and 2.6, respectively. Algal blooming was deeply affected by the nutrients than the retention time.