Chikugo River

It is the largest river in Kyushu, spanning four prefectures: Fukuoka, Saga, Kumamoto, and Oita, and flowing into the Ariake Sea. It is also known as the "Tsukushi Jiro." It is a first-class river with a length of 143 kilometers and a drainage area of ​​2,860 square kilometers. The Oyama River, which originates in the outer rim of Mt. Aso, and the Kusu River, which originates in Mt. Kuju, join in the Hita Basin to become the Mikuma River, which flows through the narrow Yoake Gorge in the mountainous area on the border between Oita and Fukuoka prefectures, and enters the Chikushi Plain, where it becomes the Chikugo River. The river then meanders, merging with tributaries such as the Sata River, Koishiwara River, Kose River, Takara River, and Homan River, before flowing through the Chikugo and Saga Plains and emptying into the Ariake Sea in Yanagawa City. The upper reaches are covered with volcanic deposits from the Aso and Kuju mountains, creating a complex geology, and numerous hot springs, such as Tsuetate and Amagase, are found here. Reforestation is also popular, and Hita's forestry industry is famous. The middle and lower reaches are made up of Paleozoic Sangun metamorphic rocks, and the plains are covered with sedimentary layers from the late Pleistocene (Pleistocene) to early Holocene (Alluvial Epoch), but there is little development of alluvial fans or natural levees. The delta plain downstream is characterized by a water town landscape with countless creeks developing into a network, and rice paddy agriculture has developed rapidly, making it Kyushu's largest grain-producing region.

As the upstream area is in the Kyushu mountainous region where there is a lot of rain, there is a lot of water, and flooding has occurred frequently since ancient times, so flood control works were started by the Saga Domain in the Edo period, but there was a lack of consistency, and modern repair work only began in the 1880s. Full-scale repair work began after a flood in 1921 (Taisho 10), but after a major flood in 1953 (Showa 28), drastic flood control measures were taken from 1957 onwards, including the construction of Matsubara and Shimouke dams, and the basic construction implementation plan was revised in 1973, and it remains in place to this day.

The water was also used for agricultural purposes early on, and since the Edo period, numerous weirs and irrigation channels have been constructed, such as the Oishi Weir in Ukiha City, Fukuoka Prefecture, the Yamada Weir in Asakura City, and the Tokoshima Weir in Tachiarai Town, bringing the total irrigated area to approximately 55,000 hectares, but the Egawa Dam was built in 1975, and the Chikugo Ozeki in Kurume City in 1983, and its use as water for urban areas, including Fukuoka City, is also progressing. Water transportation has also been thriving since ancient times, with river ports such as Wakatsu (Okawa City) and Hita developing, and numerous ferries at Kanegae, Aokijima, and other locations, but these have now almost disappeared due to the construction of bridges. On the left bank of the river's mouth, Okawa City is home to a thriving furniture industry, while Jojima Town in Kurume City is home to a thriving sake brewing industry. To transport these products, the Japan National Railways Saga Line (abolished in 1987) and the Chikugo River Bridge (completed in 1935) were built as Kyushu's only lift-type movable bridge. Near the river's mouth, reclaimed land has spread, making use of the tidal flats of the Ariake Sea. Nori farming and fishing for fish and shellfish unique to the Ariake Sea were also popular, but in recent years environmental pollution and a decline in living organisms have become serious problems. The area has potential hydroelectric power of approximately 300,000 kilowatts, and there are 23 power plants with a total maximum output of approximately 220,000 kilowatts (2004).

[Masaaki Ishiguro]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

福岡、佐賀、熊本、大分の4県にまたがり、有明海(ありあけかい)に注ぐ九州第一の大河で、「筑紫次郎(つくしじろう)」の異名をもつ。延長143キロメートル、流域面積2860平方キロメートルの一級河川。阿蘇(あそ)外輪山に源を発する大山川(おおやまがわ)と、久住(くじゅう)山に源を発する玖珠川(くすがわ)とが日田(ひた)盆地で合流して三隈川(みくまがわ)となり、大分・福岡県境山地の狭隘(きょうあい)な夜明峡谷(よあけきょうこく)を経て、筑紫平野に出、筑後川となる。その後、佐田(さた)川、小石原(こいしわら)川、巨勢(こせ)川、高良(こうら)川、宝満(ほうまん)川などの支流を合流しながら蛇行を続け、筑後平野、佐賀平野を貫流、柳川(やながわ)市で有明海に注ぐ。上流部は阿蘇、久住などの火山群の噴出物によって覆われ、複雑な地質を構成し、杖立(つえたて)、天ヶ瀬(あまがせ)など多数の温泉が分布、植林も盛んで日田の林業は有名である。中・下流は古生代の三郡(さんぐん)変成岩などからなり、平野部は更新世（洪積世）末期から完新世（沖積世）初期の堆積(たいせき)層がこれを覆っているが、扇状地、自然堤防の発達はあまり著しくない。下流の三角州平野は無数のクリークが網状に発達した水郷(すいごう)景観が広がり、水田農業の発展が著しく、九州一の穀倉地帯である。

　上流が九州山地の多雨地であるために水量が多く、古来水害が多発したため、治水工事は江戸時代に佐賀藩により始められたが、一貫性に乏しく、明治20年代に入って近代的な改修工事が開始された。1921年（大正10）の洪水を契機に本格的な改修工事に入ったが、1953年（昭和28）の大水害により1957年以降松原、下筌(しもうけ)両ダム建設を含む抜本的治水対策がたてられ、1973年には工事実施基本計画を改定、現在に至っている。

　農業用水としての利水も早く、江戸時代より福岡県うきは市の大石堰(おおいしぜき)、朝倉(あさくら)市の山田堰(やまだぜき)、大刀洗(たちあらい)町の床島堰(とこしまぜき)など多数の井堰、用水路などが築かれて総灌漑(かんがい)面積は約5万5000ヘクタールに及ぶが、1975年には江川(えがわ)ダム、1983年には久留米(くるめ)市に筑後大堰が建設され、福岡市をはじめとする都市用水としての利用も進んでいる。水運も古くから盛んで、若津(わかつ)（大川(おおかわ)市）、日田などの河港を発達させる一方、鐘ヶ江(かねがえ)、青木島(あおきじま)など多数の渡しがあったが、現在では架橋が進みほとんど消滅した。河口左岸の大川市は家具工業、久留米市城島(じょうじま)町は酒造業が盛んであり、その運搬のために国鉄佐賀線（1987年廃止）筑後川橋梁(きょうりょう)（1935年完成）は九州唯一の昇開式可動橋であった。河口付近では有明海の干潟を利用して干拓地が広がる。また、ノリ養殖をはじめ、有明海特有の魚貝類の漁も盛んであったが、近年は環境汚染と生物の減少が深刻な問題となっている。包蔵水力は約30万キロワットで、発電所が23か所で総最大出力は約22万キロワット（2004）である。

［石黒正紀］

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