Tenryu River

Tenryū River 天竜川
Upper Tenryu River in Nagano
Basin
Main source	Lake Suwa, Nagano Prefecture, Japan 759 m (2,490 ft)
River mouth	Philippine Sea 0 m (0 ft)
River system	Tenryū River
Basin size	5,050 km2 (1,950 sq mi)
Physical characteristics
Length	132 mi (212 km)
Discharge	Average rate: 135 m3/s (4,800 cu ft/s)

The Tenryū River (天竜川, Tenryū-gawa^?) is a river in central Honshū, Japan. With a length of 213 km (132 mi), it is Japan's ninth longest river.

Its source is Lake Suwa in the Kiso Mountains, near Okaya in Nagano Prefecture. It passes Aichi Prefecture, and flows through western Shizuoka Prefecture

The Tenryū River is the only river exiting Lake Suwa. It follows a generally southern course. The upper reaches of the river in the Ina Basin of Nagano Prefecture is a rich agricultural area. The river exits through a gap between the Kiso Mountains (Central Alps) and the Akaishi Mountains (Southern Alps), which forms the border between Shizuoka and Nagano Prefectures . This area is characterized by heavy rainfall (up to 3000 mm per year) and deep V-shaped valleys. Continuing south through Shizuoka Prefecture, the river drains a wide coastal plain noted for fruit and rice production. The city of Hamamatsu is near the river mouth at the Philippine Sea.

The Tenryū River is mentioned in Nara period records as the Violent Tenryū (暴れ天竜, Abare-Tenryū^?) for its fast, turbulent flow and its propensity to flood. The upper portion of the river (in Shinano Province) was referred to as the Tenryūgawa, whereas the lower portion (in Tōtōmi Province) was often referred to as the Aratama River (麁玉川, Aratama-gawa^?). In various entries in the national historical chronicle Shoku Nihongi, flooding of the river is mentioned in the years 710 CE and 765 CE. Records through the Heian period and Kamakura period are sparse, but from the Muromachi period, increasing efforts at flood control were made by various warlords and landholders along the river by construction of dikes, levees, and channels.