Imaichi Dam, Gravity dam in Tochigi Prefecture, Japan

Imaichi Dam is a concrete gravity structure on the Togawa River in Tochigi Prefecture, built to serve as the lower basin of a pumped storage power facility. It measures 177 meters across and rises 75.5 meters above the riverbed.

Tokyo Electric Power Company started construction in 1979 and finished the dam in 1986 as part of its hydroelectric infrastructure expansion. The project responded to growing energy demands in the region during that period.

The name Imaichi comes from the former city of Imaichi, now part of Nikkō. The dam's reservoir is the lower basin of the Imaichi Pumped Storage Power Station, whose three Francis pump turbines each produce up to 350 MW. The power station sits in an artificial cavern 100 m below ground between the two dams, and the rock excavated there was used as aggregate for the dam's concrete.

From the Imaichi Interchange on the Nikkō-Utsunomiya Road, follow National Route 121 toward Kinugawa Onsen, then turn up the Togawa valley; the dam comes into view along the river. The former TEPCO Kinugawa Land visitor centre by the route, which ran tours of the dam and power station by free advance booking, closed in spring 2011.

The power station is not inside a mountain but in an artificial cavern 100 metres below ground between the two dams, reached by an egg-shaped excavation rather than the usual mushroom profile. Rock removed during digging was crushed and used as aggregate in the dam's concrete. It was Japan's first TEPCO hydro plant to send power at 500,000 volts.