Japanese Lakes
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JAPANESE LAKES Takashi Asaeda1, Ryuichiro Shinohara2 1 Institute for Environmental Science and Technology, Saitama University, Saitama, Japan 2 National Institute for Environmental Studies, Tsukuba-City, Ibaraki, Japan
Synonyms Inland water bodies of Japan; Reservoirs of Japan Definition Lake. A lake is an inland and permanent body of water containing a considerable amount of fresh or saline water. Reservoir. A reservoir is an artificial lake. Eutrophication. A heavy load of nutrients in a body of water. Algal bloom. A great increase of phytoplankton in a body of water as a result of eutrophication. Natural lakes and their formation Many Japanese lakes are geologically isolated natural waters, not connected to an ocean or river. Some of them were formed by intrusion of seawater into the offshore land after the last glacial age. Shallow inner bays were formed; sediments transported by river inflows were then accumulated in the bays and isolated the water from the sea. Therefore, many lagoons, such as Lake Kasumigaura, Lake Saroma, Lake Shinji, and Lake Ogawara, are located along the coastal line. Among the five lakes around Mt. Fuji (Lake Motosu, Lake Shojin, Lake Sai, Lake Kawaguchi, and Lake Yamanaka), Lakes Onuma, Chuzenji, and Inbanuma are, in fact, rivers that were dammed with accumulated debris from the surrounding volcanoes. The accumulation of decaying plant materials also interrupted river flows and created inundated areas.
Lakes created by this mechanism - Lake Ozenuma, for instance - are relatively small. Since Japan is situated in a volcanic zone, many lakes have been created due to its geological features. Diastrophism is a mechanism by which many lakes, such as Lakes Biwa, Suwa, and Kizaki, were formed. The development of these lakes is associated with seismic faults. Lakes formed in volcanic craters or calderas are called crater lakes. Lakes Tazawa, Ashino, Towada, Inawashio, Akan, and Kuttara belong to this group and are generally deep with relatively small catchment areas. Lakes Mikazukinuma and Nougaike in Kiso and Lake Barato on Hokkaido were created due to severe river erosion. Figure 1 shows the major Japanese lakes and their origins.
Reservoirs Since 8 CE, more than 200,000 irrigation ponds have been constructed in Japan. Seventy percent of these ponds were constructed in western Japan, as in the Kinki district (near Osaka) and around the Seto Inland Sea, due to the scant annual precipitation of this area. The largest pond, Mannoike, with an area of 1.4 km2, was constructed in 9 CE. These irrigation ponds are of two major types: dammed rivers and swamps surrounded by embankments. The latter group is usually flat and shallow and has a special ecosystem. From ancient times, various types of management practices, such as drying up (Maezono and Miyashita, 2004) or harvesting of overgrown plants, have been conducted by local people to maintain these bodies of water. Therefore, the ponds provide suitable habitats to various kinds of aquatic flora and fauna. They have high biodiversity and can harb
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