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Shiretoko NP, Hokkaido
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Lava did the dam-building: a flow from Mt. Tencho blocked a basin to create Lake Rausu. At 0.43 km², it is Shiretoko National Park’s largest lake—yet its maximum depth is just 2.1 m.
In 1991, botanist Teruo Katsuyama found Carex stylosa at Lake Rausu—the sedge’s first recorded occurrence in Japan. Later named Rausu-suge, it was a new national record, not a species new to science.
A 2009 survey found Carex livida at Ayamegahara, then only its 3rd known Japanese location. Researchers counted 60–70 plants there and about 10 at Pond Five; Japan’s 2025 Red Data Book lists it as Vulnerable.
On the northeast shore, a peat layer 1.34 m thick contains 3 volcanic-ash layers. Dating and peat-growth rates suggest the wetland began forming 2,500–3,000 years ago—a bog that keeps a long record.
The lake sits at 731 m, yet Shiretoko’s nature guides describe high moor and alpine plants around it. The contrast is striking: alpine flora grows far below the peninsula’s highest ridges.
At Lake Rausu, visitors are asked to walk straight through the muddy trail, not step around it. Detours can crush fragile wetland plants, so rubber boots help keep footsteps on the path.
The trail passes 4 smaller ponds before reaching Lake Rausu. Mountain avens and cotton grass bloom in the wetlands in early summer; in autumn, the marsh grasses turn red and gold.
