Nobi Plain - Nobi Heiya

The Kiso River is a large plain that includes Owari Province (Aichi Prefecture) and Mino Province (Gifu Prefecture). It is a typical alluvial plain with an area of ​​about 1,800 square kilometers, 40 kilometers east to west and 45 kilometers north to south. The eastern edge is the Owari Hills to the Yagoto Hills in Nagoya City, the western edge is the Yoro Mountains, the northern edge is the southern end of the Hida Mountains of the Paleozoic strata, and the southern edge is the Ise Sea. The origin of the plain is the Yoro Fault (late Tertiary period) that formed on the eastern edge of the Yoro Mountains, that is, a fault angle basin, into which the Ise Sea intruded, and furthermore, the sediment discharged by the so-called Kiso Three Rivers, Kiso, Nagara, and Ibi, and the Shonai River, accumulated, forming a plain that became land, and can be said to be a typical structural basin. The Yoro Fault is part of the Tsuruga-Isewan Tectonic Line, intersects the Median Tectonic Line at right angles, and forms the narrowest part of the island of Honshu. The thickness of the sedimentary layers below the Yoro Fault is a whopping 1,800 meters, including the Tertiary layers, and the alluvial and diluvial layers alone are about 400 meters thick. The Yoro Fault Escarpment is the lowest point, and Nagoya City to the east is the highest point, making it a topography that is high in the east and low in the west. It is only natural that the Kiso River system approaches each other nearly to join in the lower reaches.

The plain on the left bank of the Kiso River is the Owari Plain, and compared to the Mino Plain on the right bank, which is a plain of waju (circular rings), the Owari Plain is unique in that it has almost no waju (circular rings). In particular, since the construction of the sturdy levee "Okakoizutsumi" (built in 1601) on the left bank of the Kiso River, the damage caused by floods has been avoided.

The topography of the Owari Plain is divided into three areas: the Inuyama alluvial fan, the natural levees, and the delta/reclaimed land.

Inuyama alluvial fan is one of the largest in Japan, with a radius of about 12 km from Inuyama city at its apex. It is an impoverished alluvial fan with a layer of coarse gravel at the bottom and a thin layer of sandy loam on the surface. After the Meiji period, it was reclaimed as a mulberry field and silkworm farming became popular. Currently, the cities of Inuyama, Konan, Komaki, Fuso Town, and Oguchi Town are developed.

The natural levee area spreads in front of the Inuyama alluvial fan, with its southern limit extending as far as the delta and reclaimed land, and currently includes the cities of Ichinomiya, Iwakura, Inazawa, Kitanagoya, and Kiyosu, as well as the town of Toyoyama in Nishikasugai County. It has long been the center of Owari Province, and is now a modern industrial urban area through which the Tokaido Main Line and Nagoya Railroad Main Line pass. The natural levee is a slight elevation of sandy soil created on the old riverbed during the turbulent times of the Kiso River, and is accompanied by a low, damp depression known as a back marsh. The soil is fine sandy loam, suitable for agriculture, and the area above the natural levee is used for settlements and fields, while the back marsh is used for rice paddies.

The delta and reclaimed land is a muddy alluvial lowland south of the line connecting Biwajima and Tsushima. It is a rice paddy area including the delta at the mouth of the old rivers of the Kiso River from the turbulent era and the Shonai River that flow into Ise Bay, and new rice fields reclaimed from the Edo period. Furthermore, reclaimed land is being developed on the bay coast in front of it to expand the Nagoya Port waterfront industrial zone as part of the Ise Bay Typhoon Reconstruction Project.

The area is subject to severe land subsidence, with annual subsidence rates reaching as much as 12 centimeters in the 1970s. However, restrictions on groundwater pumping have meant that this trend is now easing. However, as the area is a water town and sits at zero meters above sea level, it faces unique drainage problems.

[Gohei Ito]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

木曽川(きそがわ)を挟んで尾張(おわり)国（愛知県）、美濃(みの)国（岐阜県）を含む大平野をいう。面積約1800平方キロメートルの典型的な沖積平野で、東西40キロメートル、南北45キロメートル。東縁は尾張丘陵から名古屋市の八事(やごと)丘陵、西縁は養老山脈、北縁は古生層の飛騨(ひだ)山地南端、南縁は伊勢海(いせのうみ)である。成因は、養老山脈の東縁に生じた養老断層（第3紀末）すなわち断層角盆地に、伊勢海が湾入、さらに木曽、長良(ながら)、揖斐(いび)のいわゆる木曽三川と庄内(しょうない)川の流出する土砂が堆積(たいせき)し陸化した平野で、代表的構造盆地といえる。養老断層は敦賀(つるが)・伊勢湾構造線の一環をなし、中央構造線とは直交し、本州島の最狭部を形成。養老断層下の堆積層の厚さは第三紀層を含めると実に1800メートル、沖積層と洪積層だけで約400メートル。この養老断層崖(がい)を最低地とし、東方名古屋市を最高地とする東高西低型の地形である。木曽三川が下流部で合流に近い接近をするのも自然の理といえる。

　木曽川左岸の平野は尾張平野で、右岸の美濃平野が輪中(わじゅう)平野であるのに比べると、ほとんど輪中がみられないのが一大特色である。とくに木曽川左岸に堅固な堤防「御囲堤(おかこいづつみ)」（1601築堤）ができてからは洪水の害を免れるようになった。

　尾張平野の地形は、犬山扇状地、自然堤防地帯、デルタ・干拓地帯に三分される。

　犬山扇状地は犬山市を頂点に半径約12キロメートル、日本最大級の扇状地で、下部は粗大な砂礫(されき)層、表層に薄く砂壌土が覆っている乏水性扇状地である。明治以降、桑畑に開墾され養蚕が盛んであった。現在、犬山市、江南市、小牧市、扶桑(ふそう)町、大口町などを発達させている。

　自然堤防地帯は、犬山扇状地の前面に広がり、南限はデルタ・干拓地帯までで、現在、一宮(いちのみや)市、岩倉市、稲沢市、北名古屋市、清須市と西春日井(にしかすがい)郡豊山(とよやま)町などを発達させ、古くから尾張国の中心、現在は東海道本線、名古屋鉄道本線の貫通する近代工業都市地帯である。自然堤防は木曽川の乱流時代の旧河道につくられた砂質土壌の微高地で、後背湿地とよばれる低湿な凹地を伴っている。土壌は細かい砂壌土で農業に適し、自然堤防上は集落と畑、後背湿地は水田に利用されている。

　デルタ・干拓地帯は、枇杷島(びわじま)・津島を結ぶ線以南の泥層の沖積低地である。伊勢湾に注ぐ木曽川乱流時代の旧河川と庄内川の河口のデルタと、おもに江戸時代の干拓新田を含む水田地帯である。さらに前面の湾岸には、伊勢湾台風復興事業の一環として、名古屋港臨海工業地帯の拡張のための埋立地が造成されている。

　地盤沈下の激しい地区で、1970年代には年間12センチメートルもの沈下量を示したが、地下水揚水の規制によって現在では緩和傾向にあるが、水郷、ゼロメートル地帯のため特有の排水問題を抱えている。

［伊藤郷平］

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