Ōi River

Ōi River 大井川
The upper Ōi River meets its tributary Sumata
Basin features
Main source	Mount Aino, Akaishi Mountains, Japan 3,198 m (10,492 ft)
River mouth	Suruga Bay 0 m (0 ft)
Basin size	1,280 km2 (490 sq mi)
Physical characteristics
Length	168 km (104 mi)
Discharge	Average rate: 76.40 m3/s (2,698 cu ft/s)

The Ōi River (大井川, Ōi-gawa^?) is a river in Shizuoka Prefecture, Japan.

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. These mountains, with peaks ranging from 2000–3000 meters, are characterized by heavy rainfall (up to 3000 mm per year) and deep V-shaped valleys. The river follows a generally southern course, with seven wide bends in its central region, before exiting into Suruga Bay in the Philippine Sea.

The Ōi River is mentioned in Nara period records as forming the border between Tōtōmi and Suruga Provinces. However, due to shifting of the course of the river over the centuries, by the late Muromachi period, this was not always the case.

During the Edo period, the Tōkaidō developed as the major highway linking Edo with Kyoto, and daimyōs from the western domains were forced to travel on a regular basis to Edo to attend to the Shogun in a system known as sankin kōtai. However, the Tokugawa shogunate prohibited the building of bridges over major rivers as a security measure, and in the case of the Ōi River, even ferryboats were forbidden. As depicted in contemporary ukiyoe prints by artists such as Hokusai, travelers crossed the river on bearers' shoulders or on horseback. 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.