To know the annual variations of water quality in the Shihwa Lake, we investigated water qualities from October 1996 to November 2002. The salinity of surface water in the Shihwa Lake before water gate operation ranged below 14psu. After 1998, salinity rose 29psu at surface water due to continuous water gate operation and the stratification was weakened except summer. However, despite large amounts of surface water discharge and input of outer seawater, the water quality of the Shihwa Lake did not improve. We suggest that the effects resulted from the load of a great amount of pollutants into inner reservoir after the event of rainfall in summer and a dense phytoplankton bloom throughout the year.