Saline water intrusion into the Teshio and Sarobetsu Rivers(Japan)was measured on July 6 and 7, 2004, using a SC-3 acoustic profiling system. This system is an improved 200-kHz precision echosounder, that records the distribution of the halocline and the thickness of the salt-water layer as profile records. At the Teshio River, saline intrusion was observed over an area up to 20 km from the rivermouth, while the halocline boundary, located about 4 m below the water surface, was clear. Beyond the salt-invaded area, saline water was found trapped in man-made deep areas to max. 22.5 km upstream of the river mouth. Measured salinities were 33 psu at the head of the wedge, and more than 31 psu in the deep areas. The data suggest that in the Teshio River mixing of saline and fresh water is very weak. In the Sarobetsu River, three watermasses different in salinity(more than 30, 10～16 and 7～0 psu) were demonstrated acoustically. High-salinity water can easily invade the Sarobetsu River from the Teshio River. On the contrary, running freshwater from the Sarobetsu River is interrupted by that of the Teshio River, yielding a watermass of intermediate salinity in the lower Sarobetsu River. At Lake Panke, the intermediate.salinity water invades periodically beyond the barrier at the lake mouth(less than 1 m in depth)at high flow tide.