Lake Inawashiro

Lake Inawashiro lies at the heart of Fukushima Prefecture, between the Aizu basin to the west and the Nakadōri corridor around Koriyama to the east. With an area of about 103 km² it is Japan's fourth-largest lake, and Mount Bandai rises steeply from its northern shore.

The lake's water is slightly acidic, fed by the Nagase River, which carries acidic water from volcanic springs around Bandai. The acidity limits plankton and keeps the lake unusually clear, giving rise to the poetic name Tenkyō-ko, 'lake of heaven's mirror'.

The lake has long been a water source for the region. The Asaka Canal, completed in 1882 as one of the Meiji government's first large public works, carries its water through the ridge to irrigate the Asaka plain and supply Koriyama. The lake drains west through the Nippashi River into the Agano River and the Sea of Japan.

The lake occupies a basin formed by faulting and subsidence, whose outlet was blocked by volcanic deposits from Bandai and neighbouring volcanoes. The shores are mostly gently sloping sandy beaches, especially on the north and east sides, while the west shore is rockier.

The acidic water supports few fish and little plankton. In winter the lake is an important stopover and wintering site for whooper swans and ducks from Siberia. Improving water quality in recent decades – as acidity has gradually decreased – has brought worries about eutrophication.

The lake is closely associated with Noguchi Hideyo, born on its shore in 1876, whose portrait appeared on Japanese banknotes. As a summer retreat for the imperial family and Aizu nobility in the Meiji era, the north shore gained villas such as Tenkyōkaku.