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Lake Tazawa
is a caldera lake in the city of Semboku, Akita Prefecture, northern Japan. It is the deepest lake in Japan at . The area is a popular vacation area and several hot spring resorts can be found in the hills above the lake. Akita Prefecture's largest ski area, Tazawa Ski Area, overlooks the lake.
==Hydrology==
Lake Tazawa has a surface elevation of 249 meters, and its deepest point is 174.4 meters below sea level. Due to this depth, there is no possibility that the lake is frozen in the dead of winter. At , it is slightly deeper than Lake Shikotsu in Hokkaidō (423.4 meters), and is the 17th deepest lake in the world. Lake Tazawa has no natural inflow or outflow, and in 1931, had a measured transparency of 31 meters, comparable with Lake Mashu, but with abundant aquatic organisms. However, due to the construction of hydroelectric power plant facilities and agricultural runoff, coupled with an influx of highly acid water from Tamagawa Hot Spring, transparency has be reduced to less than 4 meters, and by the late 1940s the lake had become so acidic (pH 4.3) that it could no longer support irrigated agriculture. Starting in 1972, the Japanese government has been attempting to rectify the acidity problem through introduction of lime, with a new facility completed in 1991. However, in the year 2000, the lake still had an acidity of 5.14 at the 200 meter depth, and 4.91 at the 400 meter depth, indicating that full recovery has not yet been achieved.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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