A new monitoring system for the observations of spatial and temporal movement of halocline occurring at boundary between fresh water (the low-salinity water) and salt water (the high-salinity water) has been developed and applied since 1992. The saline wedge (salt-water intrusion along the bottom of river) has been observed at the lower stream of River Gonokawa of Shimane Prefecture since 1997, and when discharge rate decreased below 50 m^3/s in 1999, we observed for two months that the head of saline wedge invaded up to 7.2 km. A groundwater well was installed at the riverbank for the observation of groundwater system. The well is 50 m in length and penetrates the alluvial sand and gravels. The preliminary results from this observation well indicate that the boundary between saline and fresh water in adjacent subsurface aquifer system moves downward when the discharge rate at the river increases. Consequently the movement of saline wedge is positively related with the boundary between saline and fresh water in subsurface aquifer system.