With a total area of nearly 3,300ha, the Watarase Retarding Basin is Japan’s largest retarding basin, spanning the four prefectures of Tochigi, Ibaraki, Saitama, and Gunma. The huge heart-shaped Lake Yanaka lies gently gathering water at the southern edge. In addition to water sports such as wind surfing, canoeing, yachting, and other water sports, as well as cycling and fishing, the area is also known for its almost 260 recognized species of birds, as well as the falcons that live there all year, even in the winter.
Watarasegawa downstream portion once Akama Swamp Ishikawa Swamp, Akashibu Swamp Maehara swamp in the whole area, further Itakura swamp include, more than peripheral to the topographical large wetlands lower flood is retarding the natural, the embankment has become surrounded by the flood control basin.
The second mineral poison Investigation Committee of the Imperial Japanese government, Ashio copper mine of Watarase, was established in 1903 to investigate the occurrence of the Ashio mineral poison incident. Mineral poison, rather than measures, were provided for the purpose, flood, but prevention is the purpose, of the 1903 second mineral poison Investigation Committee of the Imperial Japanese government, Ashio copper mine of Watarase. It was constructed in response to a proposal to build a retarding basin in the Segawa River’s lower reaches, and it was evident that the goal was to prevent mine pollution.
It is located within a river and is governed by the Ministry of Land, Infrastructure, Transport, and Tourism. The mine poisoning has diminished more than 100 years after the Ashio copper mine catastrophe, and the area is now mostly used for hydraulic control and water use. However, the amount of mineral poison that flows upstream has diminished, and the soil of the retarding basin still contains a significant amount of heavy metals like copper as of 2020.
Location: Fujioka, Fujioka Town, Tochigi City, Tochigi Prefecture