The Shinano River , known as the Chikuma River in its upper reaches, is the longest and widest river in Japan and the third largest by basin area. 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.

Nagaoka is a city located in Niigata Prefecture, Japan. It is the second largest city in the prefecture, after the capital city of Niigata. As of 4 August 2021, the city had an estimated population of 264,611 in 109,283 households and a population density of 300 inhabitants per square kilometre (780/sq mi). The total area of the city was 891.06 square kilometres (344.04 sq mi).