Kojima Bay was once an inlet of the Inland Sea, south of Okayama City, Okayama Prefecture, in western Honshu. Due to sedimentation of the Takahashi River, Sasagase River and Asahi River, the inlet has turned into a tidal flat, where extensive reclamation began in the late 16th century. The size of the Bay, which was around 7,000 ha in early Meiji Era (1868-1911), has been reduced to around 2,500 ha.

As the reclaimed land extended toward the lower reaches of irrigation canals, the shortage of water had serious impacts on paddy cultivation. To secure irrigation water and protect reclamation levels from direct exposure to saltwater, the Ministry of Agriculture, Forestry and Fisheries initiated construction of a saltwater barrier in 1956. The reservoir was completed in 1959 and named Kojima-ko. It was the first freshwater lake in Japan constructed at a river mouth, and now it supplies irrigation water for 5,140 ha of reclaimed paddy fields.

Major inflowing rivers are the Sasagase and the Takahashi River. The former has a catchment area of about 300 km² and the latter that of 150 km². These two catchments cover approximately 90% of the total drainage area of the lake. In addition to water supplied by these inflowing rivers, Lake Kojima is fed by irrigation water drawn from the Asahi and the Takahashi River. On an average, the combined volume of water drawn off these two rivers accounts for 20 - 30% of the total inflow. The ratio becomes higher during irrigation periods. Due to opening and closing of the lock and leakage from the levee, saltwater flows into the lake and fills its bottom layer.

One-half of Okayama City and 40% of Kurashiki City are located within the Lake Kojima basin. A rapid growth of population in the basin began after the construction of the weir. The increase of pollutant load caused deterioration of water quality, by wastewater discharged from ordinary households, by industrial facilities and by drainage from agricultural and livestock operations. These pollution sources constitute, respectively, 65-80%, 6-10% and 4-7% of the total waste load discharged into the lake. Two public sewage systems are at work in Okayama City, and one in Kurashiki City. The population served accounts for around 240 thousand persons and its diffusion rate is 28%. At present septic tanks are used by 130,000 persons in the basin (Q).

In December 1985 Lake Kojima was appointed as one of the first five "designated lake" based on the Law Concerning Special Measures for Preserving Lake Water Quality, along with Biwako, Kasumigaura, Inba-numa, and Tega-numa.The current Phase 7 Water Quality Conservation Plan covers from FY2016 to FY2020.