This is a man-made lake, completed in 1947 by damming up the Sagami River in its middle course. The river comes from Yamanaka-ko, one of the Five Lakes of Fuji, and flows into Sagami Bay on the Pacific side of the island of Honshu. The drainage basin of the lake is therefore large for its size, having more than 300 times the surface area of the lake itself. With an effective storage of some 48 million tons, water for city and industrial use in such big cities of Kanagawa Prefecture as Yokohama and Kawasaki, for irrigating upland fields on the Sagamihara Plateau, and for hydroelectric power generation can all be provided by the lake.

Excess water is discharged through the dam gate in time of flood, but otherwise it passes through a duct 24 m under the water surface, mainly to a hydroelectric power station. During the warm season from late spring to early fall, as is often the case with many other reservoirs used for hydroelectricity, two thermoclines are observed at depths corresponding, respectively, to the bottom of normal metalimnion and the level of subsurface outlet. The inflowing river water forms a continuous density current leading to the subsurface discharge between these two thermoclines. Most of the inflowing nutrients may therefore not be available for biological production during this period.

In spite of such hydrographical constraints, the large amount of nutrient loading from the wide drainage basin has furthered the progress of eutrophication in Sagami-ko since its construction. Spring blooms of diatoms lead to clogging trouble in the filtration bed, while the growth of certain blue-green algae often gives an unpleasant smell to tap water. To suppress the bloom of Microcystis, which has appeared every summer since 1967, a device for vertical water mixing and aeration is being tested, together with other measures (1).