Ōi River
The potential of the Ōi River valley for hydroelectric power development was realized by the Meiji government at the start of the 20th century. The Ōi River was characterized by a high volume of flow and a fast current. Its mountainous upper reaches and tributaries were areas of steep valleys and abundant rainfall, and were sparsely populated. From the 1930s through the 1960s, numerous concrete gravity dams had been constructed on the main flow of the Ōi River, and to its various tributary streams.