Matsumoto River which runs through Hagi City in Yamaguchi Prefecture is a river being subject to the influence of the tidal motion of the Japan Sea. This paper is to report on the investigations of the characteristics about the water quality of the tidal river. The conclusions of the investigations (1) Matsumoto River has an estuary of highly stratified type and has a so called "SALINITY WEDGE"with well-defined interface. (2) The salinity of the river surface decreases towards the upstream with exponential