Shinano River. The Shinano River (信濃川, Shinano-gawa) is the longest and largest river in Japan. It flows from Nagano Prefecture to Niigata Prefecture. It is called the Chikuma River (千曲川, Chikuma-gawa) in Nagano Prefecture. The river rises from Mount Kobushi on the border of Saitama, Yamanashi and Nagano, and flows northwest to meet the Sai River from Matsumoto.