Salt water from the sea flows up-and down-ward in tidal rivers due to tides. When mixing in a tidal river is relatively weak, a saltwater wedge is formed in it. The Kita river, the objective field in this study, located in eastern part of Kyushu island in Japan is in this state. When external forces are applied in an ecosystem, then changes might occur in response to any impact. The ecosystem in such a river is to be with more impact, because its aquatic zone in the tidal range is fragile and high in productivity. This study attempts to investigate the behavior of Deiratonotus japonicus, known as a scarce kind in the world, in association with salt wedge movement, to show relationships between water quality (ex. salinity, pH, turbidity, temperature, and dissolved oxygen) and hydraulic factors (ex. velocity and salt water intrusion) and to study the environmental condition on the habitat of Deiratonotus japonicus. Based on field observations, the habitat of Deiratonotus japonicus has a wide distribution correlated to salinity in the longitudinal direction in the tidal range.
