The Shimanto-gawa is a river in western Kochi Prefecture, Japan. The headwaters of the Shimanto River start in Miyama and it winds 196 kilometers down into Tosa Bay. The river is considered one of the cleanest and most beautiful rivers in Japan, along with the Nagara River in Gifu Prefecture and the Kakita River in Shizuoka Prefecture.. The river is certified as one of Japan's 100 remarkable waters by Ministry of Environment.It has no big dams and is the last free flowing river without banks in Japan. The river also has 47 chinkabashi that are. bridges without parapets in order not to be washed away by floods. The bridges sink with the water level when the river floods.