The salinity distribution in the southern portion of Gamo Lagoon, Miyagi Pref. is shown to be related to the upstream advance of the surface front of the salt-wedge interface in the adjoining Nanakita River estuary. The salinity level in the lagoon can range from that of ocean water to river water, depending on the position of this front at high tide. Residual waters in the lagoon are forced to concentrate within an inner region with the advance of tide leading to the formation of fronts. Exchange rates for the residual water are nearly 0.25 from the data collected for the spring tide.