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Joshin’etsukogen NP, Nagano
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A field study found 2 pale ash layers around Mount Shiga’s summit craters and dated soil beneath one to within the past 500 years. Researchers infer that steam-driven eruptions likely occurred twice in that period.
Mount Shiga’s lava left an unusual swirl pattern in the landscape. One flow turned northeast, struck a neighboring slope and dammed the Yokoyu River, creating nearby Onuma Pond.
Local legend makes Onuma Pond below Mount Shiga the home of a guardian serpent. After a broken promise and devastating floods, Princess Kurohime plunged into the pond to stop them; each August, the Daija Festival sends a dragon effigy across the lake.
Onuma Pond’s water changed dramatically after a landslide exposed iron-sulfide-rich rock in the Akaishi stream catchment. Historical measurements describe weakly alkaline water in 1910, but a strongly acidic pH of 2.8 in 1926.
Onuma Pond’s outflow travels down the Yokoyu River, joins the Kakuma and Yomase rivers, and then feeds Nakano’s Yatsugo irrigation network—watering rice as well as apple, peach and grape orchards.
Despite its name, Shijuhachi-ike (“48 Ponds”) Wetland contains more than 60 small pools. Nestled between Mount Shiga and Mount Hachiyama, this high-altitude wetland shelters aquatic alpine plants, salamanders and tree frogs.
Between Mount Shiga and Ura-Shiga, a small pond is called Oni no Sumo-ba. Its name comes from its outline, which resembles the fan held by a sumo referee.
Along the Mount Shiga approach, nearby Shibike Pond has little floating islands made from accumulated moss and other plant remains. Sundews—tiny carnivorous plants—and sedges grow on these drifting mats.
At Mount Shiga’s foot, the Otanomousudaira forest includes subalpine conifers more than 200 years old. Researchers have monitored trees, leaf litter and beetles there since 2006; the site is part of Japan’s long-term Monitoring 1000 program.
