Kasumi-ga-ura (lake of Kasumi) is the second largest lake of Japan located about 60 km northeast of Tokyo. The general name "Kasumigaura" is used to include altogether the main lake (Nishiura), two attached lakes (Kitaura and Sotonasakaura) and the rivers connecting them. These lakes once formed a deep marine bay, but have since been cut off from the sea by the sedimentation of adjacent rivers and the progress of tectonic subsidence since the late tertiary period. The present maximum depth is only 7 m in both Nishiura and Kitaura.

The lake water has long been used for irrigation of paddy fields in the surrounding alluvial plain and upland fields on diluvial terraces covered by thick deposits of volcanic ash. With the increase in population and industrialization around the lake's drainage basin in recent years, however, the need for rational and effective use of its water resource of some 850 million tons became pressing. The Kasumigaura Development Project thus started in 1971 for water use and flood control, and was followed by another program for improving river head areas of the lake. These enabled supplies of the lake water to the Kashima Coastal Industrial Zone, the Tokyo metropolitan area, and other new needs.

Since shallow lakes are liable to rapid eutrophication, Kasumigaura is also suffering from severe blooms of Microcystis every summer which have seriously damaged carp culture in the lake and caused foul smelling of tap water annually. Ibaraki Prefectural Government legislated the Eutrophication Prevention Act in 1981, and is making efforts to check further progress of eutrophication by reducing nitrogen and phosphorus loadings of the lake.(Q)

In December 1985 Lake Kasumigaura was appointed as one of the first five designated lakes based on the Law Concerning Special Measures for Preserving Lake Water Quality, along with Biwako, Inba-numa, Tega-numa, and Kojimako. In 1996, it was designated as GEMS/Water Trend Station and the long term monitoring has been implemented by the Center for Environmental Biology and Ecosystem Studies in collaboration with the Center for Regional Environmental Research. Please refer to "Links" in the footer to find Lake Kasumigaura Database. (2017)