 With the completion of the Sakuma Dam, one of the tallest dams in Japan, on the main stream of the Tenryū River, developers turned their eyes to the confluence of the Ōnyugawa (大入川), a major tributary of the Tenryū River, and the Ōchisegawa (大千瀬川) in Aichi Prefecture.The Ōnyugawa was also of interest in that it was prone to flooding.Preliminary design work began in 1962, with construction beginning in 1969 after some opposition from the 100 households who needed to be relocated, and over increasing controversy regarding the issues of dams on the Tenryū and Ōi rivers rapidly filling with sand and silt from the mountains upstream, with the resultant reduction of the amount of sand and silt reaching the river mouth creating problems with coastal erosion.