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Nikko NP, Tochigi
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Senjogahara’s “Thunder God” is a bird: Latham’s snipe migrates from Australia to breed here, then dives with its tail spread, making a rattling, thunder-like sound that earned it the Japanese nickname Kaminari-shigi.
A legend, not a recorded human battle, explains Senjogahara’s name. Tochigi’s version has Mt. Nantai’s deity become a giant snake and Mt. Akagi’s a centipede, defeated when the archer Sarumaro shoots its eye.
A crowd of futasuji-chō butterflies on the boardwalk may be seeking salts, not nectar: males gather on animal droppings for sodium and ammonia. In Tochigi Prefecture, this butterfly lives only in Oku-Nikko.
These planks are conservation equipment, not just a convenient route. By the 1960s, heavy visitor traffic had left bare, trampled patches; boardwalks and barriers helped keep feet off the marsh and vegetation recovering.
Before it was a marsh, Senjogahara was a volcanic dammed lake over 10 meters deep. Volcanic deposits, sediment from surrounding mountains and layers of dead aquatic plants gradually filled it in.
A 2022 government contract to rebuild part of the boardwalk specified two ways to move materials: by helicopter and on workers’ shoulders. Even maintaining this visitor path takes some serious legwork.
Senjogahara’s deer barriers account for long-distance visitors: GPS tracking in neighboring Oze showed that deer also use this marsh. The protection measures include barriers across rivers and gates where paths cross the fence.
Senjogahara contains low, intermediate and high moors—wetland types shaped by different water and nutrient conditions. That means the plant communities can change across a single marsh, not just from season to season.
The pink blooms of Hozaki-shimotsuke cluster in places where deeper soil has built up from deposited sediment. Along the boardwalk, their patches can mark subtle changes in the wetland’s ground and vegetation.
Senjogahara’s international wetland status is part of a four-part site, not a designation for this marsh alone: the 260.41-hectare “Oku-Nikko Wetlands” also includes Yunoko, Yukawa and Odashirogahara.
