The Ōi River flows from the Akaishi Mountains, the branch of the Japanese Southern Alps which form the border between Shizuoka, Nagano and Yamanashi prefectures.

[1] These mountains, with peaks ranging from 2,000 to 3,000 metres (6,600 to 9,800 ft), are characterized by heavy rainfall (up to 3,000 millimetres (120 in) per year) and deep V-shaped valleys.

In cases of bad weather or high waters, they were forced to stay several days (or even several weeks) beside the river at post stations such as Shimada-juku or Kanaya-juku.

During the early 1930s, Ōigawa Electric had completed a number of dams on tributary branches of the Ōi River, but work was stalled by the Great Depression, and came to a halt during World War II.

After the end of World War II, the American occupation authorities ordered the dissolution of Nippon Hassoden, which was divided into regional power companies.