Onga River

A first-class river that flows north through eastern Fukuoka Prefecture. It originates from Mt. Umami (978 meters) in the southern Sangun Mountains and Mt. Hiko (1,199 meters) on the border with Oita Prefecture, and merges with the Honami River and the Kamagawa River in Iizuka City to form the main Onga River, which flows into the Hibikinada Sea in Ashiya Town. It is 61 kilometers long and has a drainage area of ​​1,026 square kilometers. It has a triangular drainage basin that is wide in the upper reaches and narrow in the lower reaches, which is due to the complex arborescent river system formed by numerous tributaries such as the Hikosan River, Chuganji River, and Inunaki River, and the numerous basins that develop in the upper reaches. The river basin is widely distributed with small hills of the Paleogene layer containing coal beds, and from the Meiji period to recent years, more than 300 coal mines were in operation, making it the largest coalfield in Japan. Together with the Horikawa Canal, which was completed in 1763 (13th year of the Horeki era) from Katsuki in the middle reaches to Dokai Bay, it played a major role in transporting Chikuho coal, rice, and other goods, but after the railway was opened in 1891 (24th year of the Meiji era), it declined, and water transportation by Kawahirata (Gohei-fusen) disappeared. Currently, it is an important water source for the water supply and industrial water of Kitakyushu City and other areas, and the Rikimaru Dam upstream of the Yagiyama River, Jin'ya (Nakamotoji River), Hata (Kurokawa River), and Kuboshiro Dams have been constructed, and the river mouth weir was completed in 1980 (55th year of the Showa era). During the coalfield's heyday, water pollution caused by coal washing was severe, but progress is currently being made in purifying the water.

[Masaaki Ishiguro]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

福岡県東部を北流する一級河川。三郡(さんぐん)山地南部の馬見山(うまみやま)（978メートル）や大分県境の英彦(ひこ)山（1199メートル）などに源を発し、飯塚(いいづか)市で穂波川(ほなみがわ)と嘉麻川(かまがわ)とが合流して遠賀川本流となり、芦屋(あしや)町で響灘(ひびきなだ)に注ぐ。延長61キロメートル、流域面積1026平方キロメートル。上流部で広く、下流部に狭い三角形状の流域をもつが、それは彦山(ひこさん)川、中元寺(ちゅうがんじ)川、犬鳴(いんなき)川など多数の支流が複雑な樹枝状河系を形成し、上流部に多くの盆地が展開することによる。流域には炭層を含む古第三紀層の小丘陵が広く分布し、明治時代から近年まで300余りの炭鉱が操業、日本最大の炭田地域であった。1763年（宝暦13）に完成した、中流の香月(かつき)から洞海(どうかい)湾までの堀川運河とともに筑豊炭(ちくほうたん)や、米などの物資輸送に大きな役割を果たしたが、1891年（明治24）の鉄道開通後は衰退し、川艜(かわひらた)（五平太船）による水運は消滅した。現在では北九州市などの上水道、工業用水の重要水源となっており、八木山(やきやま)川上流の力丸(りきまる)ダムをはじめ、陣屋(じんや)（中元寺川）、畑(はた)（黒川）、久保白ダムなどが建設され、河口堰(ぜき)も1980年（昭和55）完成した。炭田最盛期には洗炭による水質汚濁が著しかったが、現在では浄化が進んでいる。

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