The Seki River (関川) is a river in

Niigata Prefecture is a Prefectures of Japan, prefecture in the Chūbu region of Honshu of Japan. Niigata Prefecture has a population of 2,227,496 (1 July 2019) and is the List of Japanese prefectures by area, fifth-largest prefecture of Japan by geographic area ...

and

Nagano Prefecture is a landlocked prefecture of Japan located in the Chūbu region of Honshū. Nagano Prefecture has a population of 2,052,493 () and has a geographic area of . Nagano Prefecture borders Niigata Prefecture to the north, Gunma Prefecture to the ...

Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...

References

Rivers of Niigata Prefecture Rivers of Nagano Prefecture Rivers of Japan {{Japan-river-stub