Lake Inawashiro was formed some 30-40,000 years ago when a tectonic depression was dammed by a major eruption and pyroclastic flow from Mount Bandai. The water is acidic with a pH value of approximately 5.0, and has a high degree of transparency. The water level was considerably less during the Jomon period as numerous artifacts and ceramic fragments have been found offshore. The lake water is an important source for irrigation in the Aizu region of western Fukushima Prefecture. An irrigation canal was completed during the Edo period and another in 1882. A third canal completed in 1915 supplies the city of Kōriyama with drinking and industrial water. The water also supplies a number of hydroelectric power plants. The lake is also an important tourist and leisure attraction in Fukushima Prefecture.