This lake and Tega-numa are shallow sister lakes which are more or less the same in their geological origin, physiographic conditions and recent environmental changes. Inba-numa had been a W-shaped lake of about 2,900 ha until the 1950's, but was completely reformed by the reclamation project which was started in 1963 and completed in 1969 for the purpose of flood control, rice field improvement and water resource development. The present lake consists of two parts, North and West Lake connected with each other by creeks, measuring 1,155 ha in the summed area.

The lake is extensively utilized as a source of water for irrigation and industrial and urban waterworks. It serves as one of the prefectural nature parks providing for fisheries and recreational activities.

This lake experienced progressive eutrophication since the beginning of the 1970's when the population in its catchment area also began to increase rapidly. This was associated with the blooms of blue-green algae, overgrowth of water chestnuts, unpleasant odor of tap water, and other troubles. Although the domestic wastewater from an increasing number of households is the main cause of eutrophication, some 60% of the total 54,000 inhabitants of the catchment area have not yet been covered by the existing sewerage system. Therefore, together with educational campaign to the public, various measures similar to the case of Tega-numa are being taken to reduce nutrient loads carried into the lake.