Hasamagawa

It is one of the tributaries of the Kitakami River that flows through northern Miyagi Prefecture. It originates at the southeastern foot of Mount Kurikoma, but in the upper reaches it splits into the Ichihasama River, Ninohasama River, and Sansasa River from the south and flows eastward, merging in the eastern part of Kurihara City to form the Hasama River. In Tome City it meanders through low-lying alluvial land and merges with the old Kitakami River east of the Nono Dake Hills. It is 87 kilometers long and has a drainage area of ​​855 square kilometers. The riverbed gradient is extremely small between Sanuma and Oyachi in Sako Town, Tome City, so when the flow rate of the Kitakami River increases due to heavy rain, the river cannot flow into the Kitakami River, making it a flood-prone area, and the flood control ponds of Kabukuri Numa, Naga Numa, Izu Numa, Uchinuma, and Oyachi were created. After World War II, Hanayama Dam was constructed upstream of the Hitasako River and Kurikoma (Gozan) Dam was constructed upstream of the Misako River as part of the Kitakami River Comprehensive Development Plan, and a new course for the Shinasako River was opened directly south from Sanuma, preventing flooding and reclaiming many of the valleys and marshes. The seeps of the Misako River are a national natural monument as a breeding ground for Genji fireflies. Izu-numa and Uchinuma were designated as Ramsar Convention wetlands in 1985 (Showa 60), and Kabukuri-numa and the surrounding rice fields in 2005 (Heisei 17).

[Kiyotaka Sakaida]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

宮城県北部を流れる北上川(きたかみがわ)支流の一つ。栗駒(くりこま)山南東麓(ろく)に発するが、上流部は南から一迫川(いちはさまがわ)、二迫川(にのはさまがわ)、三迫川(さんはさまがわ)に分かれて東流し、栗原市東部で合流して迫川となる。登米(とめ)市では低平な沖積地を蛇行し、篦岳(ののだけ)丘陵東方で旧北上川に合流する。延長87キロメートル。流域面積855平方キロメートル。登米市迫町佐沼から大谷地(おおやち)の間は河床勾配(こうばい)が著しく小さく、大雨などで北上川の流量が増えると北上川に流入できず洪水常襲地となり、蕪栗(かぶくり)沼、長沼、伊豆(いず)沼、内沼や大谷地の遊水池が生じた。第二次世界大戦後、北上川総合開発計画により一迫川上流に花山(はなやま)ダム、三迫川上流に栗駒（五山）ダムが建設され、佐沼から真南に新流路の新迫川も開かれたので、洪水は防止され、谷地や沼の多くは干拓された。三迫川の沢辺(さわべ)はゲンジボタルの発生地として国の天然記念物となっている。なお伊豆沼と内沼は1985年（昭和60）に、蕪栗沼と周辺水田は2005年（平成17）に、それぞれラムサール条約登録湿地となった。

［境田清隆］

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