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Akan-Mashū NP, Hokkaido
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Kussharo’s water was acidic (pH 4) in 1960, but by 1998 had naturally climbed above 7, despite acidic Yukawa flowing in. As water quality improved and fish were released, trout returned—some growing past 50 cm.
In an Ainu legend, hero Otashitonkuru harpoons a giant amemasu that capsizes boats. The fish thrashes so hard it drags a mountain into the lake: that mountain becomes Nakajima, and its wriggles are said to cause local quakes.
Kussharo’s winter omiwatari form in a two-step squeeze: night cooling cracks the ice and water refreezes; daytime warming expands the sheet, raising ridges. Direct observations overturned a century-old explanation.
At Sunayu, a little digging in the lakeside sand releases warm spring water. In winter, geothermal heat keeps pockets of shore from freezing, giving migratory whooper swans rare open water beside the frozen lake.
Kussharo comes from Ainu kutchar, meaning “lake outlet.” At that literal outlet, the Kushiro River begins a 154-km journey through Kushiro Wetland to the Pacific, with no dams or weirs on its main stem.
Nakajima, Japan’s largest island within a lake, covers 5.7 km² and has a 12-km shoreline. It is itself a double volcano, with a central lava dome reaching 355 m.
Wakoto Peninsula was once a volcanic island in Kussharo. Sediment washed from the Osappu River gradually built a sandbar that joined it to the southern shore—so today’s peninsula began as a separate island.
Pottery found near Kussharo is nearly 7,000 years old, while the first written mention of Kussharo Kotan dates to 1858. Explorer Matsuura Takeshiro’s map even preserved Ainu names for village districts.
Two chashi near Kussharo’s river outlet are among 4 Teshikaga sites in the nationally designated Kushiro River Basin group. These Ainu enclosures may have served as defenses, gathering places, or ceremonial sites.
Kussharo’s “Kussie” mystery began with a 1973 report from workers who saw 2 black shapes, each 4–5 m long, swimming about 300 m offshore. TV crews and underwater searches followed; no lake monster has been confirmed.
Black-and-white rounded pebbles on Kussharo’s northeastern shore look like Go-game stones, inspiring the name Goishi-hama, or “Go-stone beach.” Local guides say collecting has made the best examples scarcer.
