To understand the characteristics of the salinity distribution, salinity data were observed by depth at 18 stations in the downstream of the Seomjin river. In order to analyze the characteristics of long-term variations of the salinity, observed data from 2003 to 2017 were collected and compared. According to the results of the analysis, the distribution of the isohaline due to the difference of the tidal height moved to the upstream of the Seomjin river at high tide, and the travel distance depend on the flow rate. In the case of normal flow rate(15-30 cms), the movement of the isohaline due to the difference of the tidal height was about 5 to 7 km, and when the low level flow rate (< 15 cms), the isohaline is located upstream compared to the normal flow rate and the isohaline moves 1 to 2 km by tides. Changes of the salinity distribution caused by tides were affected up to 25 km from the entrance of the Seomjin river. It was estimated that the flow rate of more than 40 cms was required to maintain the salinity of 5-15 psu in the habitat of the Corbicula japonica.