Ashio was the location of a copper mine that caused a major pollution problem in Japan at the beginning of the 20th century. Subsequent environmental problems related to the mine are still evident along the river, in Tochigi, Gunma and Ibaraki Prefectures. In 1907 the miners here rioted. During World War II a POW camp was based here to supply slave labour to the copper mines. 
The following communities agreed to seek the permission of the governor of the prefecture to merge on March 1, 2006:
- Nimura Kazuo. (1997) "The Ashio Riot of 1907: A Social History of Mining in Japan" Archived September 7, 2012, at Archive.is, American Historical Review, Vol. 104, No. 3, June 1999.
- "We Were Determined To Get Home".
- Nimura Kazuo. (1997) The Ashio Riot of 1907: A Social History of Mining in Japan(Ashio bōdō no shiteki bunseki translated by Terry Boardman and Andrew Gordon). Durham, N.C.: Duke University Press. ISBN 978-0-8223-2018-0 (paper)
- Nikkō official website (in Japanese)
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