The Watarase River is a major waterway located in the northern Kanto region of Honshu, Japan. It is a principal tributary of the Tone River and spans approximately 106.7 kilometers (66.3 miles) in length, draining a basin area of about 2,150 square kilometers.
Geography The river originates from Mount Sukai, situated on the border of Gunma and Tochigi Prefectures within Nikko National Park. From its source, it flows through the mountainous terrain of the Ashio area before entering the Kanto Plain. It passes through several major cities, including Kiryu and Ashikaga. The river eventually reaches its confluence with the Tone River at the boundary between Koga in Ibaraki Prefecture and Kazo in Saitama Prefecture.
Environmental History The Watarase River is historically significant due to the Ashio Copper Mine incident, which is recognized as Japan's first major case of industrial pollution. During the late 19th and early 20th centuries, toxic runoff—primarily copper and sulfuric acid—from the Ashio Copper Mine contaminated the river, causing extensive damage to downstream agriculture, fisheries, and public health. This environmental disaster led to a major social and political movement spearheaded by Tanaka Shozo, a member of the Japanese Diet who advocated for the rights of the affected residents.
Infrastructure and Management To mitigate flooding and manage the remnants of the mining pollution, the Japanese government constructed the Watarase Retarding Basin (Watarase Yusuichi) in the early 20th century. Located at the lower reaches of the river, it is one of the largest flood-control basins in Japan, covering approximately 3,300 hectares.
In 2012, the Watarase Retarding Basin was designated as a Ramsar Site, a wetland of international importance. The area serves as a critical habitat for a diverse range of migratory birds, plants, and insects, balancing its industrial-historical legacy with modern ecological conservation efforts.
