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Kushiro Shitsugen NP, Hokkaido
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When the Kushiro River rises after rain or spring snowmelt, water can reverse up Lake Takkobu’s outlet and flow back into the lake. Researchers recorded this surprising backflow even during winter thaw.
Lake Takkobu’s south bank preserves a Jōmon shell midden made of Pacific oyster shells; pottery found with it dates the site to about 3,000 years ago. Ancient food scraps remain beside a freshwater lake.
On Lake Takkobu’s south shore, Tapukopu Chashi is one of 11 chashi sites in the nationally designated Kushiro River Basin group. Its Ainu name means “a round hill standing alone”; chashi roles could include ritual or resource-watching, not just defense.
In 1992, native water chestnut covered about 10% of Lake Takkobu; by 2008 it blanketed nearly the whole lake. Recorded aquatic-plant species fell from 24 in 1991 to 9 in 2012.
Rather than clear the lake, restoration crews cut water chestnut in selected plots. After 3 consecutive years of underwater wire-cutting, its cover stayed below 50% in trial areas, where other submerged plants were growing too.
About 40 fish species have been recorded across Lake Takkobu and its connected streams—not just in the lake itself—including rare Ezo hotoke loach and Sakhalin taimen.
Soil in the southern wetland held high phosphorus levels, believed to be linked to past livestock-manure disposal. Crews removed and contained it in winter 2014–15; follow-up monitoring found the treatment reduced nutrient concentrations.
North American signal crayfish have become established in Lake Takkobu and nearby waters. They feed on aquatic plants and bottom-dwelling animals, adding another pressure that conservation monitoring keeps an eye on.
Lake Takkobu is one of 3 coastal-relict lakes on the east side of Kushiro Wetlands. As the sea retreated, the eastern ground sank while the west rose, helping leave these lakes in the low-lying basin.
