Lake Suwa

A lake in the Suwa Basin in central Nagano Prefecture. It is a tectonic lake formed in the depression of the Fossa Magna, which stretches from Itoigawa City to Suruga Bay. It has an area of ​​12.9 square kilometers, a circumference of 17 kilometers, an elevation of 759 meters above sea level, and a maximum depth of 7.6 meters. The Miyagawa River and Kamigawa River, which flow down from the Yatsugatake Volcano, have accumulated sediment intensively, and the lake has shrunk considerably since geological times. It is the largest lake in the prefecture, but is an old lake with a small depth compared to other inland lakes in Japan. The lake water has become polluted and its transparency has been extremely low, but Nagano Prefecture has been working to protect the water quality and has achieved results in recent years. Many rivers flow into the lake, and the lake level and outflow are regulated by the Kamaguchi sluice gate of the Tenryu River, which flows out from the southwestern part of the lake shore. During the Meiji and Taisho periods, the lake was used as a water source for silk-reeling industries in Okaya and Suwa, but today, the lake shore is one of Japan's leading precision manufacturing areas, with the camera and watch industries thriving after being evacuated during World War II. Fishing is carried out by carp and crucian carp, and smelt is farmed. In winter, smelt fishing, where fishermen drill holes in the ice to catch them, is a seasonal sight on the lake. Hot springs bubble up in the lake, forming the Kami-Suwa Onsen hot springs town on the north shore. When the lake freezes over in the bitter cold of late January, a phenomenon called "Omiwatari" occurs. A crack appears in the south-southwest direction from Shimo-Suwa, and an ice bank forms along the crack. This occurs due to the large temperature difference between day and night, and local legend has it that it is the path of love that the male god of the upper Suwa Taisha shrine travels to the female god of the lower Suwa Taisha shrine.

Furthermore, the sea of ​​Suwa is also a utamakura (poem pillow) that is often mentioned in ancient poetry.

[Hiroyoshi Kobayashi]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

長野県中央部、諏訪盆地にある湖。糸魚川(いといがわ)市から駿河(するが)湾に至るフォッサマグナの陥没地にできた構造湖で、面積12.9平方キロメートル、周囲17キロメートル、湖面標高759メートル、最深部7.6メートル。八ヶ岳(やつがたけ)火山から流下する宮川や上川(かみがわ)の土砂の堆積(たいせき)が激しく、地質時代からみるとかなり縮小している。県ではいちばん大きいが、日本の内陸湖としては水深の小さい老齢湖である。湖水の汚染が進み、透明度はきわめて低かったが、長野県は水質保全に取り組み、近年は成果を得ている。湖に流入する河川は多く、湖岸南西部から流出する天竜川の釜口水門(かまぐちすいもん)によって湖面の高さや流出量を調節している。明治、大正期には湖を用水源として岡谷(おかや)市、諏訪市などに製糸工業が発達したが、現在は第二次世界大戦中に疎開したカメラ、時計などの工業が発展し、湖岸は日本有数の精密工業地をなしている。漁業はコイ、フナなどの漁獲やワカサギの養殖が行われ、冬期、氷に穴をあけて釣るワカサギ釣りは湖の風物詩である。湖中には温泉がわき、北岸に上諏訪温泉(かみすわおんせん)街を形成する。1月下旬の厳寒期に湖が全面結氷すると、「御神渡り(おみわたり)」とよばれる現象が生じる。下(しも)諏訪から南南西方向に割れ目ができ、割れ目に沿って氷堤ができる。昼夜の温度差が大きいために生じるものであるが、地元では、諏訪大社の上社の男神が下社の女神のもとに通う恋路であると言い伝えている。

　なお、「諏訪の海」として古歌に多く詠まれる歌枕(うたまくら)でもある。

［小林寛義］

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