used for waterborne trade based on archaeological finds, including coin hoards and imported pottery, found in the headwaters of the Shinano. One of the main inland roads in Honshu followed the Shinano valley inland to the Japanese Alps. The location of the road indicates that the river played a vital role in local trade. The Chikuma valley also provided a link across the island of Honshu. The confluence of the Chikuma and Sai rivers, called the Kawanakajima Plain, was the location a series of battles in the sixteenth century as part of the Sengoku period. The most significant of the battles was fought on 18 October 1561 between the Takeda clan and Uesugi clan, and resulted in severe casualties to both sides. The battles have become well known in Japanese military history, mentioned in literature, woodblock printing and movies. The Shinano River system has a long history of hydroelectric power generation, with the first hydroelectric power plant in Japan being located in the system. During the construction of a plant on the Shinano River in the Niigata Prefecture in July 1922, it was reported by Tokyo's Yomiuri Shimbunon 29 July that Korean workers were being mistreated and murdered. The Shinano River incident involved 1,200 workers, approximately 600 of whom were Korean, who were subjected to regular mistreatment by their Japanese foremen. The mistreatment included malnutrition, forced labour up to sixteen hours a day and severe beatings for those who tried to escape. News articles at the time alleged that up to 100 Koreans were killed during construction and that local farmers had reported seeing bodies floating down river. This incident occurred following the annexation of Korea by Japan and was partially responsible for the development of trade unions for Korean workers in Japan. Two Japanese naval cruisers, one launched in 1911 and the second in 1938, were named after the Chikuma River. A 1940s aircraft carrier was named after the old Shinano Province. Course The Chikuma River rises in the northeastern foothills of Mount Kobushi in the Japanese Alps on the border of Saitama, Yamanashi and Nagano Prefectures, partially in the Chichibu Tama Kai National Park It joins with the Yochi River and flows roughly north to join the Yu River () then turns northwest into an intermontane basin at Nagano City where it is joined by the Sai River from Matsumoto and the Hoshina River (). The Chikuma then