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Kushiro Shitsugen NP, Hokkaido
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About 1 km of the walking loop follows the former Tsurui light railway alignment. Trains ran here from 1929 to 1968; now the straight old corridor takes walkers toward the wetland.
Even below −20°C, some Onnenai springs stay unfrozen. These small open-water patches have let birds usually classed as Hokkaido summer visitors, including Japanese Wagtail and Scaly Thrush, be recorded here in winter.
From 2013–2021, staff compiled a 120-species bird checklist for Onnenai. Later, bird-banding confirmed 2 migrants missed by ordinary watching: Eyebrowed Thrush and Japanese Robin, first records for the area.
The shaggy mounds called yachi-bōzu (“marsh monks”) are sculpted by winter frost and spring meltwater: freezing lifts sedge-root clumps, then melting water erodes their sides into hummocks.
Onnenai’s walk crosses reed-and-sedge fen, alder swamp and sphagnum bog. Unlike the river-fed fen, this raised bog is watered mainly by rain and mist—an unusual wetland habitat to reach by boardwalk.
Bog peat shifts, water levels rise and fall, and winter freezes warp the walkway. When sections were rebuilt, rangers chose Hokkaido larch without paint or preservatives, so chemicals would not leach into the wetland.
Threeleaf waterlily (Mitsugashiwa), seen along the Onnenai walkway in June–July, is considered a glacial relict—a survivor from the Ice Age. Its three-part leaves are a handy clue for spotting it.
In July and August, tiny Heike fireflies may flicker beside the Onnenai walkway. Their lights add a nighttime chapter to a landscape visitors often experience by day among reeds, sedges and sphagnum.
Onnenai is an Ainu place-name. A local visitor-center study records several interpretations—“large river,” “old river,” or “river with many tributaries”—so its meaning carries more than one reading.
