Lake Tazawa

Lake Tazawa is a nearly round lake some six kilometres across in the city of Semboku, eastern Akita Prefecture. With a maximum depth of about 423 metres it is the deepest lake in Japan; its bed lies well below sea level.

Its clear, cobalt-blue water was once famous for its transparency. In 1940 acidic water from the Tamagawa river, fed by the strongly acidic springs of Tamagawa Onsen, was diverted into the lake for hydropower and irrigation. The lake's pH dropped sharply and most native fish died out, including the endemic kunimasu. Neutralisation works upstream have since improved the water, but the lake has not regained its former fauna.

The lake is the centre of a sightseeing area that also includes the samurai town of Kakunodate and the Nyūtō Onsen hot springs on the slopes of Komagatake.

The lake's round outline and great depth point to a caldera formed around 1.4 to 1.8 million years ago, though its origin is still discussed. Akita-Komagatake, an active stratovolcano, rises to the east. The shore is mostly steep, with few beaches, and only small streams flow in naturally.

Acidification after 1940 wiped out most native fish, including the endemic kunimasu. In 2010 kunimasu were found alive in Lake Sai (one of the Fuji Five Lakes), where eggs had been sent before the loss. The lake's shore forests are mixed broadleaf and cedar.

Legend tells of Tatsuko, a young woman who prayed for eternal beauty, drank from a spring and was transformed into a dragon who became the lake's guardian. In a related tale she married the dragon Hachirō, who visits each winter – which is said to be why Lake Tazawa never freezes.