Kitakami River

It originates from Mt. Nanashigure (1,063 meters) in the northern part of Iwate Prefecture, flows south through Iwate Prefecture, and empties into Oppawa Bay in the eastern part of Miyagi Prefecture. It is the largest river in the Tohoku region. It is a first-class river. It has a length of 249 kilometers and a drainage area of ​​10,150 square kilometers. It flows through the low-lying Kitakami lowlands between the Kitakami highlands, which are mainly made up of Paleozoic formations, to the east, and the Ou Mountains, which are mainly made up of Neogene formations, to the west. It is divided into lower and middle reaches by the Kozenji Gorge, which is about 20 kilometers long and cuts through the Iwai Hills on the border between Iwate and Miyagi Prefectures, and into the middle and upper reaches near Morioka. The width of the alluvial lowlands increases below Morioka, forming the main lowland area of ​​Iwate Prefecture, and cities such as Morioka, Hanamaki, Kitakami, Mizusawa, and Ichinoseki are located there. In the lower reaches, the river flows south, forming a gentle gorge along the Kitakami Highlands to the east, and to the west an extremely low-lying alluvial plain spreads out. Rice is the main agricultural product in the middle and lower reaches, but in the upper reaches, the rate of paddy fields is low, and in addition to rice cultivation, dairy cows are raised.

The name Kitakami River is said to come from the name of a river in "Hitakami Province," i.e., the Hitakami River. Since ancient times, when this region was called Hidaka Province, the Kitakami River has been used as the main transportation route. In the early modern period, the northern half of the river basin became the territory of Morioka Domain, and the southern half became the territory of Sendai Domain. At the beginning of the early modern period, the Sendai Domain carried out major renovations of the downstream area, divert one side of the river that flows directly into Oiha Bay from Kanomata in Ishinomaki City into Ishinomaki Bay and the other into Oiha Bay, relocating the confluence of the Hasama River and merging it with the Eai River. As a result, rice from the northern part of the Sendai Domain and Morioka Domain was able to be collected in Ishinomaki via the Kitakami River, and then transported by sea to Edo. Domain storehouses were established along the river basin, and river ports developed, with domain rice storehouses and official residences built in Ishinomaki, where goods from Edo and Kamigata gathered and prospered. The repair work also encouraged the development of new fields in northern Miyagi Prefecture. Boat traffic on the Kitakami River became even more active after the Meiji Restoration, as various restrictions imposed during the old domain era were removed, and the canal served as a major artery for the Tohoku economy. In conjunction with plans to build Nobiru Port at the mouth of the Naruse River in central Miyagi Prefecture, the 12.8-kilometer-long Kitakami Canal was completed in 1882 (Meiji 15), linking Ishinomaki at the mouth of the Kitakami River with Ishii Lock Gate, eight kilometers upstream. This was a booming business for a time, but the objectives could not be achieved due to the failure of the Nobiru Port construction project and other factors. When the Tohoku Line between Ueno and Morioka was completed in 1889, shipping on the Kitakami River declined, and many of the river ports downstream, which were not served by rail, were left behind in prosperity. At the beginning of the Showa era, the Kitakami River ran south from Yanaizu, Tsuyama Town, Tome City, using its old, straight course to connect with the Oiha River at Iinogawa in Ishinomaki City, and this became the main stream, and the course to Ishinomaki came to be called the Old Kitakami River. In 1951 (Showa 26), the National Land Development Act designated the Kitakami River basin as a special region, and from 1953, many multipurpose dams were constructed on the main and tributaries for soil and water conservation, power generation, and irrigation, promoting comprehensive regional development. In 1975, the Kitakami Ozeki Weir was completed downstream in Ishinomaki City, Miyagi Prefecture, for flood control, irrigation, and drinking water. In addition, a flood retarding basin is currently under construction in Ichinoseki City to prevent flooding of the Kitakami River. The tsunami caused by the Tohoku Pacific Ocean Earthquake on March 11, 2011 (Heisei 23) traveled upriver for about 50 kilometers from the river mouth.

[Yuji Goto]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

岩手県北部、七時雨山(ななしぐれやま)（1063メートル）に源を発し、岩手県を南流して宮城県東部の追波湾(おっぱわん)に注ぐ東北一の大河。一級河川。延長249キロメートル、流域面積1万0150平方キロメートル。東に古生界を主とする北上高地、西に新第三系を主とする奥羽山脈があり、その間の低平な北上低地を流れる。岩手・宮城県境の磐井(いわい)丘陵を切る約20キロメートルの狐禅寺(こぜんじ)峡谷により下流部と中流部に、盛岡付近で中流部と上流部に分けられる。盛岡を下ると沖積低地の幅が広がり、岩手県の低地の主要地域を形成し、盛岡、花巻、北上、水沢、一関(いちのせき)などの都市が分布する。下流部では東の北上高地沿いに緩い峡谷をつくって南流し、西にきわめて低平な沖積地が広がる。中・下流部では稲作中心の農業が行われるが、上流部は水田率が低く、稲作に加えて乳用牛の飼育が行われている。

　北上川の名は「日高見国(ひたかみのくに)」の川、すなわちヒタカミ川に基づくといわれている。この地方が日高見国といわれた古代から、北上川は最大の交通路として利用されていた。近世には、流域の北半が盛岡藩領、南半が仙台藩領となったが、仙台藩では近世初めに下流部の大改修を行い、追波湾へ直接注ぐ河道を、石巻(いしのまき)市鹿又(かのまた)から、一方は石巻湾へ、一方は追波湾へ流し、迫(はさま)川の合流点を付け替え、江合(えあい)川を合流するなどの工事を実施した。これにより、仙台藩領北部と盛岡藩の米が北上川を通じて石巻に集められ、江戸へ海上輸送されることになった。流域には藩庫が置かれ、河港が発達し、石巻には藩の米蔵や役人の屋敷が設けられ、江戸、上方(かみがた)からの物資が集まるなど繁栄した。改修工事は宮城県北部の新田開発をも促した。北上川の舟運は、明治維新後は旧藩時代のさまざまな制約が取り除かれてさらに活発化し、東北経済の大動脈の役割を果たした。宮城県中部、鳴瀬(なるせ)川河口の野蒜築港計画(のびるちくこうけいかく)に伴い、北上川河口の石巻から8キロメートル上流の石井閘門(こうもん)とを結ぶ全長12.8キロメートルの北上運河が1882年（明治15）完成した。これは一時活況を呈したが、野蒜築港の失敗などにより目的を達成できなかった。1889年に東北線上野―盛岡間が完成すると、北上川の舟運は衰退し、鉄道が通らない下流部の河港の多くは繁栄から取り残された。昭和の初め、北上川は登米(とめ)市津山(つやま)町柳津(やないづ)から南へ直線的旧流路を利用して石巻市飯野川(いいのがわ)で追波川と結ばれ、以後これが本流となり、石巻への流路は旧北上川とよばれることになった。1951年（昭和26）の「国土開発法」により、北上川流域は特定地域の指定を受け、1953年から本・支流に治山治水、発電、灌漑(かんがい)のための多くの多目的ダムが建設され、総合的な地域開発が進められた。1975年には、下流部の宮城県石巻市に治水、灌漑、上水用として北上大堰(おおぜき)が完成した。また、北上川の洪水を防止する目的で、一関市では遊水地が建設中である。2011年（平成23）3月11日の東北地方太平洋沖地震では津波が河口から約50キロメートル上流まで遡上した。

［後藤雄二］

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