Ibigawa River

Along with the Kiso River and Nagara River in the Chubu region, the Kiso River is one of the so-called Kiso Three Rivers, which form the Nobi Plain downstream. It is 121 km long and has a drainage area of ​​1,840 square kilometers. It originates in Tokuyama Valley in Ibigawa Town, Ibi District, Gifu Prefecture, and flows through a deep gorge in the Etsumi Mountains, which have an annual precipitation of over 3,000 mm, forming the Ibi Gorge and emerging into the Nobi Plain. It then flows south through the lowest part of Nishino, combining with its tributaries the Kasu River, Neo River (Yabu River), and Makita River, and merges with the Nagara River at its mouth, passing east of Kuwana City, and pouring into Ise Bay. In the northwestern part of the Nobi Plain, the Ibi River forms an alluvial fan together with the Kasu River and Neo River, and rice farming has developed there since ancient times. Downstream, natural levees and deltas have developed, but for a long time the area was a humid, ring-shaped area with a constant risk of flooding. Since 1755 (5th year of the Horeki era), and particularly from the mid-Meiji period, efforts have been made to overcome disasters through projects such as diversion of the Kiso and Nagara rivers, river improvement work, and land improvement, and the ring-shaped area has undergone a major transformation. Meanwhile, upstream, Yokoyama Dam (Ibigawa Town) was completed in 1964 (Showa 39), which is intended for multi-purpose use such as flood control as well as power generation, and further upstream, the large-scale Tokuyama Dam was completed in 2008 (Heisei 20). The area around Ibi Gorge is part of the Ibi-Sekigahara-Yoro Quasi-National Park.

[Masanobu Ueshima]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

中部地方、木曽(きそ)川、長良(ながら)川とともに、下流に濃尾平野(のうびへいや)をつくる、いわゆる木曽三川の一つ。延長121キロメートル、流域面積1840平方キロメートル。岐阜県揖斐郡揖斐川町の徳山谷に源を発し、年降水量3000ミリメートル以上の越美(えつみ)山地に深い峡谷をうがって流れ、揖斐峡をなし、濃尾平野に出る。さらに、支流の粕(かす)川、根尾(ねお)川（藪(やぶ)川）、牧田川などをあわせながら、西濃の最低部を南流し、河口で長良川と合流、桑名市の東を過ぎて、伊勢湾(いせわん)に注ぐ。揖斐川は、濃尾平野の北西部では、粕川、根尾川などと扇状地をつくり、古くから稲作農業の発達をみた。その下流では自然堤防、三角州を発達させているが、長い間低湿かつ水害の危険性の絶えない輪中(わじゅう)地域であった。1755年（宝暦5）以後とくに明治中期から木曽、長良との三川分流、河川改修、土地改良などの事業により、災害の克服が図られ、輪中は大きく変貌(へんぼう)している。一方、上流では発電利用だけでなく、洪水調節などの多目的利用を目的とした横山ダム（揖斐川町）が1964年（昭和39）に、さらに上流でも2008年（平成20）大規模な徳山ダムが完成した。なお、揖斐峡付近は揖斐関ヶ原養老国定公園の一部。

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