Lake Inawashiro was formed some 30,000 years ago in a tectonic depression due to the eruption of Mt. Bandai and other volcanoes which dammed rivers by mud flows and topographic changes. It is considered that the original water level of the lake has since been lowered considerably owing to the erosion by the outflowing river, R. Nippashi.

The lake water has been used from ancient times for irrigating rice paddies in the Aizu Basin. An irrigation channel was completed in the 17th century during the Edo period. In 1882, another channel from the lake to the Koriyama Basin was completed to give rise to about 300 km² of newly reclaimed rice fields. One additional channel was constructed in 1915, parallel with the old, to supply the city of Koriyama with water for drinking and industrial use. Since the lake surface is higher than the land surface of the two basins by about 300 m, many hydroelectric power plants have been made along the outflowing river and channels, the electricity being supplied to the Tokyo area.

The lake water is slightly acidic, with a pH value of approximately 5.0, owing to the inflow of acidic water containing sulfuric acid, derived from hot springs and sulfur mines in the drainage basin. Transparency was recorded to be 20 m or more in the early 1930's, but recent measurements revealed its diminishing trend. However, the decrease of transparency is not likely to be caused by the increase in photosynthetic production, since the concentration of chlorophyll α has maintained a low level around 1 micro l^-1 (1).