Lake Kasumigaura is the second-largest lake in Japan, located 60 km to the north-east of Tōkyō. Lake Kasumigaura is actually the name given to a group of contiguous lakes, which includes the main lake, Nishiura, and two smaller lakes, Kitaura and Sotonasakaura, and also encompasses the rivers connecting them. About 45% of the land surrounding the lake is natural landscape and 43.5% is agricultural land.
The lakes of Kasumigaura once formed a deep marine bay, but have since been cut off from the Pacific Ocean by the sedimentation of adjacent rivers and the progress of tectonic subsidence since the late tertiary period. This lake was famous for its traditional fishery during the Edo period. But today, fishing production has drastically decreased due to water quality deterioration that was partially caused by the closure of the tide gate in 1963 for purposes of desalination.