Lake Inawashiro

This lake is located almost in the center of Fukushima Prefecture. The lake shore is included in Inawashiro Town to the north and east, Koriyama City to the south, and Aizuwakamatsu City to the west. It has an egg-shaped outline stretching 14 km north to south and 10 km east to west, with an area of ​​103.32 square kilometers (fourth largest in Japan), a lake surface elevation of 514 meters, a maximum depth of 93.5 meters, and a shoreline length of 49 kilometers. The formation history of the lake basin is not simple, beginning with crustal movement from the end of the Tertiary Period to the beginning of the Quaternary Period (activity of the Kawageta Fault, which bounds the eastern edge), followed by the formation of a fault angle basin. In parallel with the deposition of the welded tuff layer of Mt. Aizu Nunobiki on the southwestern edge, and the formation of Mt. Nekomagatake and Mt. Bandai on the northern edge, a lake that could be called the original Lake Inawashiro was born in this area at a level about 60 meters lower than the current lake level. After that, the Okinajima mudflow was pushed out and dammed in the northwestern part of the lake basin, causing the lake level to rise rapidly to about 530 meters, and at the same time, the area of ​​the lake became larger than the current lake level (Old Lake Inawashiro, about 20,000 years ago). During this period, the current Akai Yachi, the Hara area, most of the Kohoku Plain, and the lowlands to the east and south of the lake were below the lake level, and the lake shoreline had the characteristic of a ria type with many bends. Subsequently, due to renewed erosion by the draining river, the Nippashi River, the lake level gradually dropped to its current height.

Looking at the conditions around the lake shore from a geographical perspective, first on the north shore, the Nagase River, which carries gravel from upstream, forms a delta. Part of this is the Tenjinhama swimming beach. The rest of the lake is a shallow, muddy shore where swans visit in winter. On the west shore, there are tourist spots such as Nagahama, Okinajima, Jurokukyo, Gin-no-hama and Sakkahama, as well as a power generation intake that uses water from Lake Inawashiro and the historic Tonoguchihara battlefield from the Boshin War, and National Route 49 runs along the lake shore for four kilometers. On the south bank, small plains with sandy beaches stretch out near the mouths of the Tokonatsu, Suga, and Funazu rivers, while natural landscapes include the sand spit of Oni Numa, the lake cave at Byobu Rock, and the columnar joints at Tateishi. At the northern end of the east bank, in Jōko, there is the intake of the Asaka Canal, the old canal channel, and the Shidahama Lake swimming beach. Because the lake is slightly acidic, there are not many fish species.

[Yoshio Nakamura]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

福島県のほぼ中央にある湖。湖岸の北と東は猪苗代町、南は郡山市(こおりやまし)、西は会津若松市に含まれる。南北14キロメートル、東西10キロメートルほどの卵形の輪郭をもち、面積103.32平方キロメートル（全国第4位）、湖面高度514メートル、最大深度93.5メートル、湖岸線の長さは49キロメートルである。湖盆の形成史は単純ではなく、第三紀末から第四紀初めころの地殻運動（東縁を限る川桁(かわげた)断層の活動）に始まり、断層角盆地の生成を端緒とする。南西縁での会津布引山(ぬのびきやま)溶結凝灰岩層の堆積(たいせき)、北縁での猫魔ヶ岳(ねこまがだけ)、磐梯山(ばんだいさん)の形成などと並行して、この付近に現湖水面より60メートルほど低い位置に原猪苗代湖ともいうべき湖水が誕生した。その後、湖盆北西部に翁島泥流(おきなじまでいりゅう)の押し出しと堰(せ)き止めが行われ、湖面は急速に上昇してほぼ530メートルに達し、同時に面積も現湖水面よりも広いものとなった（古猪苗代湖。約2万年前）。この時期、現在の赤井谷地(あかいやち)、原(はら)付近、湖北平野の大部分、湖東、湖南の低地などが湖面下にあって、湖岸線は屈曲の多いリアス式の特色を示していた。その後、排水河川の日橋川(にっぱしがわ)の侵食復活によって湖面が徐々に低下して現在の高度になった。

　湖岸付近の状況を地誌的にみると、まず北岸には、上流から砂礫(されき)を運び出す長瀬川(ながせがわ)がデルタを形成する。その一部が天神浜(てんじんはま)湖水浴場である。その他の部分は浅い泥質の湖岸で、冬季ハクチョウが飛来する。西岸には、長浜、翁島、十六橋(じゅうろっきょう)、銀の浜、崎川浜(さっかはま)などの観光スポットや、猪苗代湖の水を利用する発電用取水口、戊辰(ぼしん)戦争の戸ノ口原古戦場などがあり、国道49号が湖岸を4キロメートルにわたって走っている。南岸には常夏(とこなつ)川、菅(すが)川、舟津(ふなづ)川の河口付近に砂浜を伴う小平野が広がる一方、鬼沼の砂嘴(さし)、屏風(びょうぶ)岩の湖食洞、立石(たていし)の柱状節理などの自然景観がある。東岸北端の上戸(じょうこ)には安積疏水(あさかそすい)の取入口、旧疏水路、志田浜湖水浴場などがある。弱酸性湖なので魚種はあまり多くない。

［中村嘉男］

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