What Is Okunikko? Senjogahara, Lake Yunoko and the Ramsar Marsh
This guide is for visitors who want to know what Okunikko is before they go. It covers the marshes and lake that make up the Ramsar-listed Oku-Nikko wetlands, the volcano behind them, Lake Chuzenji and its waterfalls, the wildlife you may meet, and where to ask for help in an emergency.
We used the Ministry of the Environment's Ramsar leaflet for the wetlands, its Nikko National Park leaflet, the Nikko City Tourism Association's travel pages and the Japan National Tourism Organization's safety pages. All pages were read on 2026-10-04. We name no hotels or shops.
Where Okunikko is
The Ramsar leaflet places the wetlands at an altitude of about 1,400 meters, 120 km north of Tokyo. It says they lie in a valley surrounded by mountains above 2,000 meters, including Mount Nantai at 2,484 meters and Mount Shirane at 2,577 meters. The Nikko National Park leaflet says the part of the park above the Iroha-zaka slope is called Okunikko.
The Ramsar wetlands
The Oku-Nikko-shitsugen wetlands were designated under the Ramsar Convention in November 2005. The leaflet gives a total area of 260.41 hectares. That is made of Lake Yunoko at 35.71 hectares, Senjogahara at 174.68 hectares, the Yukawa river at 5.3 hectares and Odashirogahara at 44.72 hectares. All lie in Nikko City. The leaflet describes the site as one of the most prominent high moors in Japan and notes it is next to the World Heritage site of Nikko.
The tourism association gives a larger figure for Senjogahara: about 400 hectares. The difference probably comes from what is counted inside the formal designation, but neither source explains it, so we give both figures.
- Senjogahara was once a lake dammed by an eruption of Mount Nantai. Layers of dead water plants built up over it to form a high moor. The leaflet says more than 100 species of wetland plants have been identified, among them hare's tail cotton grass and Japanese azalea. Electric fences protect the plants because too many deer damage the vegetation.
- Odashirogahara lies next to it and is in transition toward grassland. The leaflet says temporary lakes appear after rain, and that wetland and grassland plants grow together.
- Lake Yunoko is a small volcanic lake of 35 hectares. The leaflet says its condition is relatively good because there are no man-made buildings in its catchment. Hot spring water from its north shore supplies the waterfront hotels. The lake drains through Yutaki Falls, which the leaflet notes for its scenery and for its Japanese rhododendron.
The tourism association adds a story about the name Senjogahara, which means field of battle. It says the myth tells of the god of Mount Nantai and the god of Mount Akagi fighting over Lake Chuzenji. The association describes this as the reason given for the name.
Lake Chuzenji and the waterfalls
The Nikko National Park leaflet names Lake Chuzenji, Kegon Falls and Nikko Toshogu Shrine as famous sights of the park. The tourism association says Kegon Falls is counted as one of the three famous waterfalls of Japan. It also lists the five famous Nikko falls as Kegon, Kirifuri, Urami, Ryuzu and Yudaki. These are the association's own groupings, and the leaflet says nothing on the subject.
The association's traffic article describes Lake Chuzenji's west shore at Senjugahama and the early morning light there in late September and early October. Private cars cannot reach Senjugahama, so people walk or ride the bus. Our guide to getting to Okunikko explains the buses.
Trees and walks
The national park leaflet describes a short tour of giant trees. It lists a Japanese oak by Lake Sainoko that resembles the Thousand-Armed Kannon, and a Japanese elm at Senjugahama. It says a paved walking path of about 90 minutes leads from the oak toward Senjugahama Beach. A katsura tree at Yumoto Onsen is also listed, and the leaflet says its fallen leaves give out a sweet caramel-like scent.
Wildlife
The leaflet says that on the hybrid bus ride you may spot deer, monkeys and Japanese black bears. It says wild animals usually keep their distance from hikers but are not afraid of the quiet buses. Do not feed or approach wild animals. We did not read the bear safety rules for hikers, so check with the national park office or a visitor center before you walk in the marsh.
If you need help
The Japan National Tourism Organization's safety pages list 110 for police and 119 for fire and ambulance. Its Japan Visitor Hotline runs 24 hours a day, every day. The Ministry of the Environment's Nikko National Park page gives the park office telephone number for the Nikko City area as 0288-54-1076, a public office line.
What we could not verify, and therefore left out
- Current bear activity and trail closures. The park office is the place to ask.
- Fees and hours for boat cruises on Lake Chuzenji.
- Why the two sources give different areas for Senjogahara.
Related guides
- How do you get to Okunikko and avoid the Iroha-zaka jam?
- When and where can you see the autumn leaves in Okunikko?
Sources
All URLs retrieved 2026-10-04.
- Ramsar Sites in Japan 29: Oku-Nikko-shitsugen (Ministry of the Environment, English PDF) — (area, plants, formation)
- Nikko National Park leaflet (Ministry of the Environment, English PDF) — (Okunikko, trees, hybrid bus)
- Nikko National Park (Ministry of the Environment) — (park office contact)
- Iroha-zaka and Lake Chuzenji traffic, 2026 edition (Nikko City Tourism Association, in Japanese) — (Senjogahara size and name, falls)
- Autumn leaf status (Nikko City Tourism Association, in Japanese) — (spot list)
- Safety tips for travelers (Japan National Tourism Organization) — (110, 119 and hotline)