A retarding basin holds the water temporary during heavy rain. It then slowly releases the water through natural or man-made drainage channels. The Watarase retarding basin (hereafter WRB) is known as Japan's largest retarding basin, which is located in the central part of Kanto Plain, some 70 km north of Tokyo.
WRB was constructed from 1910 to 1927 to decrease floods of the Watarase, Tomoe and Omoi rivers, but there is another reason to make it. WRB was the control basin for settling mineral poison from Ashio Copper Mine. As I mentioned before, the incident of Ashio mining pollution occurred around Watarase River in Tochigi and Gumma prefectures at the end of the 19th and the beginning of 20th century. The mine was closed in 1973 after declining copper production. Currently, the Watarase River has become clean and WRB was registered as a wetland under the Ramsar Convention in 2012.
To visit this large artificial basin, please take the Tobu-Nikko Line and get off at Yagyu Station. Yagyu is a small station, where only local trains stop. The operation interval is about every half an hour. Most of the southbound trains are bound for Minami-Kurihashi. My vehicle was a 4-car train, the EMU 10030 series. It is Tobu's standard commuter train launched in 1988. The electric control system is an old chopper type, but the 10030 series has brand new bead-press stainless-steel bodies, bolster-less bogies and auxiliary power units. The air compressors are also new ones and silent.
