The purpose of this study is to clarify the characteristics of chemical compositions of river water and its spatial distribution in the Oga Peninsula, Akita Prefecture, Northeast Japan. Water samples were collected at 42 sites during 2011-2012. Samples for major ions were collected two times during summerautumn (June-November) and winter (December-January). For comparison, spring water was collected from 5 sites. pH, temperature, electric conductivity (EC) and oxidation reduction potential (ORP), were simultaneously measured at each station during water sampling. The analytical results of water show that river water is characterized by water of Na-Cl type. The Na-Cl type river water in the Oga Peninsula contains high sodium (20-30 mg/L) and chloride (30-40 mg/L) ion concentrations, respectively. The source of Na and Cl in river water could be sea-salt aerosols, though some amount of Na is derived from bed rock. The Oga Peninsula are derived into five area according to characteristics of river water chemistry.