Tenryu River

It originates from Lake Suwa in Nagano Prefecture and flows into the Enshu Nada Sea in the eastern part of Hamamatsu City, Shizuoka Prefecture. Its drainage basin covers three prefectures: Nagano, Aichi, and Shizuoka. It is 213 km long and has a drainage area of ​​5,090 square kilometers. It is a first-class river. Its main tributaries are the Mibu River, Koshibu River, Toyama River, Misakubo River, and Keta River. The river flows south through the Ina Valley basin from Kamaguchi Suimon, west of Lake Suwa, between Tatsuno and Iida. Alluvial fans, river terraces, and tagiri topography are formed on both banks, and there are many orchards and farmland, and irrigation channels have been opened to convert it into rice paddies. After passing the Ina Valley, it enters the Tenryu Gorge, which has the characteristics of a leading river, and is characterized by its meandering river and valleys. During the Keicho era (1596-1615), Suminokura Ryoi opened a waterway on the Tenryu River, promoting shipping by boat and the sending of timber on rafts. Tokimata, Nishido, Kajima and other places flourished as river banks and distribution centers. In the Meiji era, the transportation of ore from the Kune and Minenosawa mines and the development of forest resources led to the development of power sources in the river basin, and in the 1930s and 40s, the Hiraoka and Yasuoka dams were completed, and after World War II, the Sakuma and Akiba dams and Funagira dams were completed in line with the Tenryu East Mikawa Specific Area Comprehensive Development Project. The river supplies water resources to the Toyogawa, Iwata, and Mikatahara irrigation canals as a source of water for power generation, industry, and daily life, but dam disasters such as riverbed shifts caused by sedimentation have also occurred. The downstream plain south of Futamata in Tenryu Ward, Hamamatsu City has repeatedly flooded and been turbulent, forming an alluvial plain between Mikatahara and the Iwatahara Plateau, where rice, pears, persimmons, flowering trees, tea, and other crops are cultivated. Kaketsuka in Iwata City, at the river's mouth, flourished as Kaketsuka Port until the Tokaido Main Line was opened. Enshu Beach and Nakatajima Sand Dunes were formed to the west of the river's mouth, but coastal erosion can also be seen.

[Kitagawa Mitsuo]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

長野県諏訪湖(すわこ)に源をもち、静岡県浜松市東部で遠州灘(えんしゅうなだ)に注ぐ、長野・愛知・静岡の3県に流域をもつ河川。延長213キロメートル、流域面積5090平方キロメートル。一級河川。おもな支流に三峰(みぶ)川、小渋川、遠山川、水窪(みさくぼ)川、気田(けた)川などがある。諏訪湖の西、釜口水門(かまぐちすいもん)からの流れは、辰野(たつの)から飯田(いいだ)までの間は伊那谷(いなだに)の盆地を南流。両岸には扇状地、河岸段丘、田切(たぎり)地形が形成され、果樹園や畑地が多く農業用水路も開かれ水田化が進んでいる。伊那谷を過ぎると先行性河川の性質をもつ天竜峡に入り、曲流と渓谷に特色をもつ。慶長(けいちょう)年間（1596～1615）角倉了以(すみのくらりょうい)が天竜川の水路を開き、舟運を進め、木材の筏流し(いかだながし)も行われた。時又(ときまた)、西渡(にしど)、鹿島(かじま)などは河岸(かし)、集散地として栄えた。明治期には久根(くね)、峰の沢鉱山の鉱石運搬や、森林資源の開発とともに流域の電源開発も進み、昭和10年代に平岡、泰阜(やすおか)ダム、第二次世界大戦後は天竜東三河特定地域総合開発に添い、佐久間、秋葉ダム、さらに船明(ふなぎら)ダムも完成。発電、工業、生活用水源として豊川(とよがわ)用水、磐田(いわた)用水、三方原(みかたはら)用水に水資源を供給するが、堆砂(たいさ)による河床変動などダム災害も発生した。浜松市天竜区二俣(ふたまた)以南の下流平野は氾濫(はんらん)や乱流を繰り返し三方原と磐田原台地との間に沖積地が形成され、米作やナシ、カキ、花木、茶などが栽培される。河口の磐田市掛塚(かけつか)は東海道本線が開通するまで掛塚湊(みなと)として栄えた。河口西方に遠州浜や中田島(なかたじま)砂丘が形成されたが海岸侵食もみられる。

［北川光雄］

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