Hatanagi-I Dam, Gravity dam and hydroelectric station in Shizuoka Prefecture, Japan

Hatanagi-I Dam is a 125-metre hollow-core concrete gravity dam on the Ōi River in Aoi-ku, Shizuoka. Its crest walkway is broken by four gate openings, and vertical contraction joints divide the downstream face. The power station is built inside the dam body, with three turbine-generators set in the gently sloped lower section; switchgear and transmission equipment stand on the right bank just below the dam.

Construction started in the 1950s as part of Japan's post-war infrastructure development, supported by international funding. The project reflected growing demand for reliable energy sources during the nation's rapid economic expansion.

The reservoir behind the dam is named Hatanagi-ko, and its water is used for drinking water, industry and irrigation in Shizuoka Prefecture. The dam is also the trailhead for climbs to Arakawa-dake, Akaishi-dake, Hijiri-dake and Chausu-dake in the Minami Alps, and autumn foliage draws visitors from early to mid-November.

Access is via Shizuoka Prefectural Road 60 from Ikawa Dam, about 27 km, or by Shizuetsu Just Line bus from Shizuoka and Shin-Shizuoka stations (about 3.5 hours, no intermediate stops). The narrow road is prone to rockfalls and closures. Beyond Numazawa Gate, private cars are banned; a shuttle to the mountain lodges runs for guests with a booking.

Water travels through a 5-kilometer tunnel to reach a second dam downstream, creating an interconnected system. This setup allows the same water to be used multiple times for power generation as it circulates between the reservoirs.