Shonai Plain - Shonai Heiya

An alluvial plain in northwest Yamagata Prefecture, centered on Sakata City and Tsuruoka City, and extending to Akumi District and Higashitagawa District. The eastern part of the plain borders the Dewa Mountains, covered with volcanic ejecta from Mt. Chokai and Mt. Gassan, while the western part faces the Sea of ​​Japan, separated by the Shonai Sand Dunes. It is about 50 kilometers north to south and 6 to 16 kilometers east to west. The Mogami River runs east to west through the center, emptying into the Sea of ​​Japan, and dividing the plain into Kahoku (formerly Akumi District) and Kanan (formerly Tagawa District). In the past, the plain was an inland lagoon surrounded by the Fukura Hills in the northwest and the protruding Kamo Plateau in the southwest. It was filled in by alluvial fans and deltas formed by the Mogami River and its tributaries, the Aka River, and later became land, forming a low, flat plain. The opening of the Inaba Weir and Kitadate Ozeki Weir in the early modern period, and the success of the sand dune afforestation project in the mid-modern period led to the development of rice fields. Furthermore, after the spread of dry-field horse-drawn plowing in the Meiji period, Sakata became one of Japan's leading areas for monoculture of rice fields. Sakata is also known for the establishment of large landowners such as the Honma family of Sakata, known as the "Sencho Jinushi." Sakata City, located at the mouth of the Mogami River, was a base port for shipping on the Mogami River and the Nishimawari shipping route in the early modern period, and flourished as a distribution center for safflower and Dewa rice, and is now also developed as a coastal industrial city. Tsuruoka City in the south was the castle town of the Sakai clan of the Shonai Domain. It is a specialty of the Shonai oyster, known as Hiratanenashi. There is an oil field (which also produces natural gas) near the northern part of Shonai Town (formerly Amarume Town) in the center of the plain.

[Shige Nakagawa]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

山形県北西部、酒田市、鶴岡(つるおか)市を中心に飽海(あくみ)郡、東田川郡にわたる沖積平野。東部を鳥海山(ちょうかいさん)、月山(がっさん)の両火山噴出物に覆われた出羽山地(でわさんち)に接し、西部は庄内砂丘を隔てて日本海に臨む。南北約50キロメートル、東西6～16キロメートル。中央部を最上川(もがみがわ)が東西に貫流して日本海に注ぎ、河北（旧、飽海郡）と河南（旧、田川郡）とに分ける。古くは北西端の吹浦(ふくら)丘陵と南西端の加茂台地の突出によって囲まれた内湾状の潟湖(せきこ)だったものが、最上川や支流の赤川などが形成した扇状地、三角州によって埋積され、のちに陸化した低平な平野である。近世初期の因幡堰(いなばぜき)、北楯大堰(きただておおぜき)などの開削、近世中期の砂丘植林事業の成功により開田化が進んだ。さらに明治に入って乾田馬耕が普及してからは、日本有数の水田単作地帯となった。千町地主とよばれた酒田の本間家(ほんまけ)など大地主の成立でも知られる。最上川河口に位置する酒田市は、近世には最上川舟運と西廻(にしまわり)航路の拠点港で、紅花(べにばな)や出羽米の集散地として栄え、現在は臨海工業都市としても発展。南部の鶴岡市は庄内藩酒井氏の城下町。平核無(ひらたねなし)として知られる庄内ガキを特産する。平野中央部の庄内町北部（旧、余目(あまるめ)町）付近には油田（天然ガスも産出）がある。

［中川　重］

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