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Joshin’etsukogen NP, Nagano
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A landslide exposed pyrite-rich rock above the Akaishi stream sometime between 1910 and 1926. Acidic runoff transformed Onumaike; its water measured pH 2.8 in 1926, and the pond remains too acidic for fish.
In local legend, the pond’s guardian serpent must circle a lord’s castle 7 times to win Kurohime’s hand. The lord secretly plants upside-down swords in the path; wounded, the serpent unleashes a flood, and Kurohime leaps into the pond to stop it.
Onumaike’s color can shift from cobalt blue to emerald green as the light changes. The shore may look clear while deeper water appears blue—a beautiful reminder that the view depends on both sunlight and depth.
This mountain pond is also part of Nakano’s farming story: its outflow joins rivers feeding Hakkago irrigation, one of several sources watering 515 hectares of fields and orchards growing rice, apples, peaches and grapes.
Each August, the pond’s serpent legend becomes a lake ceremony: people carry a colorful dragon effigy across the water by boat. The festival’s later parade and fireworks are held away from the pond, at Shiga Kogen’s resort center.
In 2023, Onumaike’s level fell about 6 metres after an outlet gate failed during irrigation-system work. The water returned to full level by August 31—a striking reminder that this celebrated wild landscape is also part of a working water system.
Despite its name, Onumaike is Shiga Kogen’s largest lake: it covers about 0.23 square kilometres and reaches roughly 26 metres deep. Lava from Shiga Mountain blocked a river to form it.
