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Towada-Hachimantai NP, Akita
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At Ōbuki, the trail’s main spring, about 9,000 liters of 98°C water surge out every minute—enough to feed a 3-meter-wide hot stream. The path lets you watch a river begin at a single vent.
First collected at Tamagawa in 1898, the radium-bearing mineral hokutolite was named after Taiwan’s Beitou hot springs. Its deposit became a Natural Monument in 1922 and a Special Natural Monument in 1952.
Tamagawa’s extraordinary acidity comes chiefly from hydrochloric acid linked to volcanic gases, rather than sulfuric acid. At the source, the water measures pH 1.05—remarkable chemistry flowing right beside the trail.
Geothermal heat warms the ground beside the path into a natural rock bath: visitors lay down a mat and cover themselves with a blanket. Some spots reach 40–50°C, so the earth—not a spa heater—does the warming.
From 1940, Tamagawa’s acidic water was diverted to Lake Tazawa for hydropower and irrigation; the lake acidified and its native kunimasu vanished. The fish was rediscovered in Lake Saiko in 2010.
Downstream, gravity carries the acidic water through reaction tanks filled with 5–20 mm limestone. As the stone dissolves and neutralizes the water, a conveyor replenishes it—using about 40 tonnes of limestone each day.
As spring water cools downstream, dissolved materials form tiny deposits called yu-no-hana, or “hot-water flowers.” They are harvested and processed as bath additives—a local product made by the spring’s own chemistry.
A 1930 guide lists Shibukuro Onsen, Shikayu (“Deer Bath”) and Sukayu (“Vinegar Bath”), reporting that ordinary bathers avoided the place for fear of poisonous gas. What is now a famous spa was once feared.
