Lake Inawashiroko, located almost in the center of Fukushima Prefecture, is the main entrance to Bandai-Asahi National Park. It is the fourth largest lake in Japan, and is also called Lake Tenkyoko, or heaven's mirror lake, because its surface reflects the shape of Mt. Bandai-san like a heavenly mirror. Around Nagahama on the northern shore, you can enjoy nature's beauty from season to season: spring with its verdure and wild birds; camping, water skiing, boardsailing, and lake bathing in summer; in autumn the scarlet-tinged leaves; and try winter sports and watch migrating swans in the wintertime. It is frequented by visitors all the year round. The world renowned Dr. Noguchi Hideyo (1876-1928), famous for his research into yellow fever and whose portrait is depicted on the 1,000-yen note, was born in Inawashiro, and in his parents' home has been preserved as the Noguchi Hideyo Memorial Hall, where many of his favorite items and letters are displayed.
Lake Inawashiroko was formed some 30,000 years ago in a tectonic depression due to the eruption of Mt. Bandaisan 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. Nippashigawa.
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-era. 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 a has maintained a low level around 1 micro l-1.