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Hidakasanmyaku Erimo Tokachi NP, Hokkaido
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Erimo’s heritage sources record the Ainu name Kamuy-to, “lake of the gods.” Travelers on the old Saruru Trail were traditionally expected to keep silent while the sacred lake was in view.
Best known as Heart Lake, Toyoni was also called Bateiko, “Horse-hoof Lake.” Erimo’s local legend—not a geological account—says a 1798 gust swept explorer Kondō Shigezō’s retainer and horse over a cliff, after which the lake gradually took on a hoof-like shape.
At about 260 m above sea level, Toyoni is a surprisingly low home for a pika, usually associated with cool, rocky high country. A park ranger once spotted a young one darting back and forth with leaves for its den.
Toyoni’s isolated Japanese crayfish population is genetically valuable. Erimo designated the lake’s crayfish a municipal natural monument in 2020, giving this unusual lake resident special protection.
Toyoni is a natural dammed lake: rock from a mountainside collapse blocked a stream. Though it covers just 0.03 km², it reaches 18.6 m deep—a surprisingly deep little lake.
Toyoni has no surface outflow. Its water seeps underground and reappears farther away as a spring feeding a tributary of the Saruru River—so its water leaves without a visible outlet stream.
Toyoni can be glimpsed from Numa-mi Pass on the Saruru Trail, opened by the shogunate in 1799. Surveyor Ino Tadataka crossed the trail in 1800; later, explorer Matsuura Takeshirō traveled it with Ainu guides.
A 1998 study found that Toyoni’s deepest water lost its oxygen from July to October, while phytoplankton grew mainly below the surface. The lake’s unusual water chemistry also made it useful for studying acid-rain effects.
