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Nikko NP, Tochigi
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Local legend gives Senjogahara its “battlefield” name: Mt. Nantai’s god became a snake, Mt. Akagi’s a centipede, and Nantai’s descendant Sarumaro won their fight over Lake Chuzenji by shooting the centipede’s eye.
The Yugawa winds through Senjogahara and helped launch Japanese fly-fishing: Thomas Glover’s 1902 brook-trout release was wiped out by a storm, but a 1904 retry succeeded. Locals still call the fish “Parlett trout.”
GPS tracking of deer tagged in neighboring Oze National Park revealed they also use Senjogahara. To keep them out, conservationists built barriers across rivers as well as ordinary fences.
Senjogahara’s pink meadowsweet (hozaki-shimotsuke) looks abundant, but its wild colonies grow only in Oku-Nikko and Kirigamine on Honshu. Here, it brings color to the marsh in summer.
In 1946, postwar food shortages brought settlers to farm part of Senjogahara; marshland was temporarily converted to cropland. By the early 1970s, it had returned to its natural state.
Some drainage channels at Senjogahara are human-made: they were dug in the 1930s to dry marshland for a larch-planting plan. Later conservation teams dammed sections to keep water from draining away.
North American giant coneflower once formed dense colonies at Senjogahara, threatening native willowherb and meadowsweet. Decades of removal drives have sharply reduced it, though it has not been eradicated.
The Ramsar-listed “Oku-Nikko Wetlands” is a four-part designation, not just Senjogahara: its 260.41 hectares include Lake Yunoko, the Yugawa River and Odashirogahara. It was registered in 2005.
Latham’s snipe and stonechats breed in Senjogahara. On one April 2024 survey, Nikko Nature Museum staff recorded 35 bird species around the marsh, including a flock of nearly 20 waxwings.
A lake more than 10 meters deep is thought to have once filled Senjogahara’s basin. Mt. Nantai’s eruptions dammed the river; volcanic debris, sediment and dead water plants gradually built up the marsh.
