Okazaki Plain - Okazaki Heiya

A plain that spreads across the central part of Aichi Prefecture, in the Yahagi River and Sakai River basins. Commonly known as the Nishimikawa Plain, it is the second largest plain in the prefecture. It stretches 20 kilometers east to west, 36 kilometers north to south, and covers an area of ​​approximately 700 square kilometers. It is said to be a mecca for the automobile industry, and is undergoing rapid urbanization. The north and east are bounded by the Mikawa Mountains, which are made of granite, and the Hazu Mountains, which are made of Ryoke metamorphic rocks. The western border is the Sakai River, and the southern border is Mikawa Bay. The main components of the plain are flat Pleistocene (Pleistocene) terraces and alluvial plains that spread widely along the right bank of the Yahagi River. The terrace surface on the right bank is the hilly Fujioka surface (Tertiary Period) at the top, followed by the Pleistocene Miyoshi, Koromo, and Hekikai terraces, with the Koromo and Hekikai surfaces making up the main part of this plain. The Koromo surface, which is somewhat dissected, is 80 to 20 meters above sea level and covers the automobile city of Toyota, while the flat Hekikai surface is 20 to 5 meters above sea level and covers the Anjo city area and the four Kinuura cities (Kariya, Chiryu, Takahama, and Hekinan). The alluvial plain is the downstream plain of the Yahagi River, which is narrow upstream at the border of Okazaki city, and the downstream Nishio Hazu area is a vast plain with many natural levees created by the turbulent flow of the Yahagi River, an elevated river. Additionally, there are newly reclaimed rice fields in the zero-meter zone along the coast of Mikawa Bay, and in the area of ​​the Hazu Mountains approaching the sea there is a small sandy beach that has become a swimming spot. The former main stream of the Yahagi River was called Yahagi Furukawa, but in 1605 (Keicho 10) the Hekikai Plateau was excavated and the course was changed, and the current Yahagi River became the main stream. The climate is typical of the Tokai climate zone, with an annual precipitation of 1,400 mm, making it the driest area in the prefecture. Formerly a grain-producing region known as the Denmark of Japan, the area has seen rapid urbanization and industrialization in recent years, and it now occupies a major part of the Chukyo Industrial Zone.

[Gohei Ito]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

愛知県中央部、矢作川(やはぎがわ)、境川流域に展開する平野。一般に西三河平野(にしみかわへいや)とよばれ県下第二の広い平野である。東西20キロメートル、南北36キロメートル、面積約700平方キロメートル。自動車工業のメッカといわれ、都市化が激しく進んでいる。北部と東部は花崗(かこう)岩の三河山地と領家(りょうけ)変成岩の幡豆山地(はずさんち)で囲まれ、西境は境川まで、南部は三河湾となる。平野を構成する主体は、矢作川右岸に広く展開する平坦(へいたん)な更新世（洪積世）の段丘面と沖積平野である。右岸の段丘面は、丘陵性の藤岡面（第三紀）を最上位段とし以下更新世の三好(みよし)面、挙母(ころも)面、碧海(へきかい)面の段丘面で、当平野を構成するのは挙母面と碧海面が主体である。やや開析の進んでいる挙母面は標高80～20メートルで、自動車の都市豊田(とよた)市域を、平坦な碧海面は標高20～5メートルで、安城(あんじょう)市域を中心に衣浦(きぬうら)四市（刈谷(かりや)、知立(ちりゅう)、高浜、碧南(へきなん)）を発達させている。沖積平野は矢作川下流平野で岡崎市街部を境に上流は狭く、下流の西尾(にしお)幡豆地区は広大な平野で、天井川の矢作川が乱流時代につくった多くの自然堤防列が分布する。また三河湾岸にはゼロメートル地帯の干拓新田があり、幡豆山地の海に迫る地区には小砂浜があり、海水浴場になっている。矢作川旧本流は矢作古川(ふるかわ)であったが、1605年（慶長10）に碧海台地を開削して流路変更をし、現在の矢作川が本流となった。気候は東海式気候区の典型で、年降水量は1400ミリメートルで県下最寡雨地域である。かつては、日本のデンマークとよばれた穀倉地帯であったが、近年は都市化、工業化が著しく、中京工業地帯の主要部を占めている。

［伊藤郷平］

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