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Rishiri Rebun Sarobetsu NP, Hokkaido
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Two magma–seawater blast craters joined to form a gourd-shaped basin about 1.5 km across. Today, it holds Rishiri Island’s largest marsh and 2 ponds: Otatomari and crescent-shaped Mikazuki.
A core beneath the marsh held a fossil oyster reef dated 7,380 ± 70 radiocarbon years BP—evidence seawater once reached this volcanic basin during Jōmon-era sea rise. The peatland formed later, about 4,000–4,500 years ago.
Pollen preserved in peat records a makeover: between 4,500 and 3,500 years ago, reed-and-sedge ponds gave way to a Sphagnum- and heather-rich bog. Researchers suggest rain and sea fog help nourish it.
Rishiri thistle grows nowhere beyond the island, and its large flower heads point upward—unlike the drooping Chishima thistle. Numaura’s marsh and pond margins are among its local habitats.
Otatomari Pond, one of the marsh’s 2 lakes, doubles as a migration stopover: an Environment Ministry ranger reports ducks resting here during spring and fall passage, with whooper swans appearing occasionally.
Nearby Numaura Observation Deck is nicknamed Shiroi Koibito Hill: it faces Mt. Rishiri, whose snowy profile appears on the wrapper of Hokkaido’s famous Shiroi Koibito cookies.
The town designated Numaura a natural monument on October 26, 1979, recognizing its wetland plants. Among them is sundew, a tiny carnivore that catches insects in sticky droplets on its leaves.
A 2015 Environment Ministry ranger report describes local residents, the island nature-information center and park volunteers joining annual pulls of invasive Rudbeckia around Otatomari Pond, one of Numaura’s 2 waters.
