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Shikotsu-Tōya NP., Hokkaido
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Sunlight, weather and viewing angle can turn the lake from emerald to cobalt blue. During spring thaw, floating shards of broken ice have even been likened to tiny sea floes.
Lake Okotanpe’s outflow drops as a waterfall into the Okotanpe River, which descends roughly 360 m before reaching Lake Shikotsu. This little lake sits high above its famous neighbor.
A visit used to be a real trek: in 1955, the Tomakomai Minpo newspaper ran an “Okotanpe Lake exploration” outing, and the old riverbank route took about 1.5 hours to reach the shore.
A 1964 fisheries paper documents a surprising experiment here: researchers transplanted first-generation hybrids of chum salmon and kokanee (himemasu) into Lake Okotanpe.
This is no volcanic crater: eruptions of nearby Mount Eniwa sent rock across the Okotanpe River, damming it to create a lake. Okotanpe is a natural lava-dammed lake.
Okotanpe began as the Ainu name of the river, not the lake. Hokkaido researchers interpret o-kotan-uni-pe as “the river with a dwelling at its mouth”—a meaning carried into the lake’s name.
Okotanpe is grouped with Onneto near Lake Akan and Shinonome by Lake Shikaribetsu as one of Hokkaido’s “Three Hidden Lakes.” The lookout reveals just part of this forest-ringed lake.
The forest above this lake is not merely scenery: Japan’s park plan records Ezo salamanders in the water, a sedge-rich wetland on the western shore, and mixed native forest around it.
Despite its vivid hues, the lake has very little microscopic algae: Hokkaido researchers link chlorophyll-a near 1 μg/L to slightly acidic, sulfate-rich water and low phosphorus.
There are no farms or towns in its upstream basin: Hokkaido researchers classify all 8.62 km² of Lake Okotanpe’s catchment as forest or other natural land.
