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Oze NP, Fukushima / Gunma
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Ozenuma’s natural outflow travels through Ozegahara and the Agano River to the Sea of Japan. Since 1949, some lake water has also been diverted by tunnel into the Katashina–Tone system, which reaches the Pacific.
During the Edo period, traders met on Ozenuma’s shore: Aizu travelers brought rice and sake, while people from Numata carried salt and oil. Coins found near Sanpei-shita are reminders of this mountain trade.
When a power company secured water rights in 1922, lakeside lodgekeeper Chōzō Hirano moved his family to Oze and challenged the plans. The Oze Preservation Alliance, formed in 1949, became the forerunner of the Nature Conservation Society of Japan.
Lake Ozenuma is believed to have formed about 8,000 years ago, when a huge debris avalanche from Mt. Hiuchi’s southern slope blocked the Nushiri River. Older explanations credited lava; newer studies favor the landslide.
The Oze-numa thistle is a Japanese endemic, and Lake Oze is its scientific type locality. Its upright purple flower heads grow in wetlands around Oze and appear in August.
North American waterweed was first recorded in Ozenuma in 1981; by September 1984, it had spread across almost the entire lake. Its underwater beds later shrank and rebounded, prompting decades of monitoring.
In 1914, Oze pioneer Chōzō Hirano acquired fishing rights in the lake and partnered with landowner Chinosuke Yokota in a fish-farming business—a very different chapter in the history of today’s quiet lakeshore.
Boats once crossed Lake Ozenuma, but they were discontinued and fishing was banned in 1967. That same year, officials adopted a formal “Plan to Protect Oze”—so the stillness has a conservation history.
Beside Ozenuma, the landmark Three Larches stand on a small mound called Oze-zuka. Local legend—not established history—says it marks the grave of a retainer who died while fleeing with Prince Mochihito.
