Tega-numa derived from a valley eroded into the deluvial plateau during the last glacial period about 20,000 years ago, which became invaded by the sea in the hypsithermal period (5,000 years B. P.) and further left as an isolated freshwater body owing to the recession of the sea and the blocking by river washouts.

The lake is regarded as a great asset for the residents of Chiba Prefecture, since it provides an annual amount of 1.7 million cubic meters for irrigation, industrial and city water, besides serving as the fields for fisheries and recreation. The influx of immigrating population from the Tokyo metropolitan area, however, resulted in the rapid eutrophication of the lake since the early 1970's, though the water quality tended to remain more or less the same during the last several years. The eutrophication caused frequent blooms of blue-green algae (Microcystis) and reduced fish catch. Rice plants irrigated with the nutrient-rich lake water became liable to fall on the ground at harvest time due to excessive vegetative growth.

Only 34% of the total population of 400,000 in the catchment area (150 km²) are provided with sewerage facilities, and unregulated discharge of household gray-water became a major source of pollutant loads carried into the lake. The quality of effluents from industrial plants is regulated by setting limitation standards. Sewerage systems are being extended, while other measures for improving inflowing water quality are promoted, e.g. the purification of stream water by gravel filters, removal of nutrients by water hyacinth cultivation and contact filters, introduction of kitchen drain filters for reducing organic waste load discharged from individual households, etc.