Estuary water quality is greatly influenced by the ocean, particulaly the influences from ocean currents. Ocean currents have a very large thermal capacity, especially the Kuroshio and Tsushima warm currents. The Kuroshio, Oyashio and Tsugaru warm currents meet offshore from the Sanriku. Therefore, the water quality in bays along Sanriku coast can change with these three ocean current conditions.In this investigation, it was found that (1) for an average year, the coast southern limit of Tsugaru warm current (summer) and northern limit of Kuroshio current (winter) are about 40° N (Miyako), (2) Kuroshio waters intruded into Onahama are affected by the Kuroshio route at Hachijyojima (33.1° N, 139.78° E), (3) from the middle of September to January, yearly water temperature deviation is small.