The Shinano River in Niigata City. The Shinano River (信濃川, Shinano-gawa), known as the Chikuma River (千曲川, Chikuma-gawa) in its upper reaches, is the longest and widest river in Japan and the third largest by basin area (behind the Tone River and Ishikari River). It is located in northeastern Honshu, rising in the Japanese Alps and flowing generally northeast through Nagano and Niigata Prefectures before emptying into the Sea of Japan. It is designated as a Class A river.