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Chubu-Sangaku NP, Nagano
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On June 6, 1915, a steam-driven eruption sent mud and rocks from Yakedake across the Azusa River. Its 4.5m debris barrier was overtopped within hours, but the pond behind it remained.
Taisho Pond’s birth came with a river-wide disaster: red-black floodwater, logs and boulders damaged riverside bridges, and fish downstream died. The muddy surge reached Matsumoto, about 30km away, in just 2 hours.
In 1927, Taisho Pond became the intake for Kasumizawa Power Station, reached by a 7,656m waterway and a 454m drop. The old Kama Tunnel was dug to haul project materials; its road later opened vehicle access to Kamikochi.
Introduced brown trout are common around Taisho and Myojin ponds, while native iwana are now reported mainly upstream of Yokoo. A Kamikochi study found brook trout compete with char and brown trout may prey on them.
The familiar gray snags are drowned trees: the 1915 lake flooded forest along the Azusa’s banks. Early photographs show many more standing trunks than remain today, as the pond and its landscape have gradually changed.
This broad new waterbody was apparently first called Azusa-ko, or “Azusa Lake.” After local forestry officials surveyed the eruption’s aftermath, the name Taisho Pond caught on.
Surveyors measured the newborn pond in July 1915: 1,540m along its longest axis, 0.39km² in area and just 4.5m deep at maximum. Early photographs show many more standing snags than today.
Sydney Powers expected the river to wear down the volcanic barrier, leaving marsh within a few years. The pond is still here; silt had cut its hydro-storage capacity to 1/9 by 1976, prompting dredging in 1977.
The pond’s dredged sand even became a school project: local elementary pupils once gave visitors small bottles of it and asked for ideas about how the sediment could be reused.
