Lake Suwa

Lake Suwa lies in the Suwa basin in central Nagano Prefecture, shared by the cities of Suwa and Okaya and the town of Shimosuwa. It is small by area but important: the lake is the source of the Tenryū River, which flows some 200 kilometres south to the Pacific, and it has been the focus of the Suwa region's religion, fishing and industry for centuries.

The lake is very shallow – nowhere deeper than about seven metres – and freezes in cold winters. When the ice cracks and heaves into a ridge running across the lake, it is called omiwatari, 'the god's crossing', and is interpreted by the priests of Yatsurugi Shrine; records of the phenomenon go back to the 15th century and are now valued as one of the world's longest climate series.

Around the lake stand the four shrines of Suwa Taisha, one of Japan's oldest shrine complexes, and the hot-spring town of Kamisuwa. The lake's fireworks festival on 15 August is among the largest in Japan.

The Suwa basin lies where two of Japan's great fault systems meet: the Itoigawa-Shizuoka Tectonic Line, which marks the western edge of the Fossa Magna, and the Median Tectonic Line. Movement along the faults opened a pull-apart basin, which filled with water to form the lake. Sediment from inflowing rivers has made it shallow, and land reclamation has reduced its area over the past century.

Shallow and fed by a densely populated basin, Lake Suwa suffered severe algal blooms from the 1960s to the 1980s. Sewerage and conservation measures have improved the water considerably. The lake is known for wakasagi (pond smelt), once exported as eggs to lakes across Japan, and in winter it attracts ducks and swans.

Suwa Taisha is dedicated to Takeminakata, the deity who in legend crosses the frozen lake from the upper shrine to visit his consort Yasakatome at the lower shrine – the omiwatari. The Onbashira festival, held every six years in the years of the tiger and the monkey, sees giant fir trunks dragged down the mountains and raised at the corners of each shrine.