The Geography and Natural Features of Lake Onuma
A Volcanic Lake Formed Inside Mount Akagi’s Caldera
Lake Onuma was formed within the summit caldera of Mount Akagi. The lake is surrounded by peaks including Mount Jizodake, one of the central volcanic peaks, and Mount Kurohiyama, part of the outer mountain range.
Mount Akagi is a complex volcanic mountain formed through repeated volcanic activity. A major eruption and subsequent collapse approximately 45,000 years ago created a large caldera in the upper part of the mountain.
It is believed that a much larger lake once occupied this caldera. Later volcanic activity formed new volcanic peaks within the caldera and divided the original lake. This geological process eventually produced the present-day Lake Onuma, Kakumanbuchi wetland, and other distinctive features.
The landscape around Lake Onuma therefore provides an opportunity to see the results of the long volcanic history of Mount Akagi.
Size and Shape of the Lake
Lake Onuma lies at an elevation of roughly 1,310 to 1,345 meters, making it one of the higher-elevation lakes in the Kanto region.
The lake is sometimes described as having a long, curved shape resembling a traditional Japanese magatama jewel, while other descriptions compare it to an oval. Its shoreline is approximately 4 kilometers long, and walking around the lake takes about one and a half hours at a leisurely pace.
The lake covers approximately 0.8 to 0.9 square kilometers and reaches a maximum depth of around 16.5 meters. Water visibility is approximately 4 meters under typical conditions.
On the eastern side of the lake is Kotori-ga-shima, a small peninsula-like island where Akagi Shrine stands. The combination of the shrine’s traditional red structures, the surrounding mountains, and the blue waters of Lake Onuma creates one of the most recognizable landscapes on Mount Akagi.
Water Sources and Drainage
The water of Lake Onuma is supplied mainly by rainfall and groundwater emerging from the lake bed. Because of these sources, the lake’s water level is relatively stable throughout the year.
At the northwestern end of the lake, water flows out through the Lake Shiri area and becomes the Numao River. The river descends the western slopes of Mount Akagi before eventually joining the Tone River system, which flows across the Kanto Plain.
The Climate and Seasonal Changes of Lake Onuma
A Cool Highland Climate
Because of its high elevation, Lake Onuma has a much cooler climate than the lowlands around Maebashi. Even during summer, temperatures are generally comfortable, making the area a popular summer retreat.
During summer, the surface water can reach approximately 22–25°C, while the deeper water remains much colder, at around 5°C. This difference in water temperature contributes to the lake’s distinctive aquatic environment.
Winter Ice and the Omiwatari Phenomenon
Winter at Lake Onuma is cold enough for the entire lake to freeze. In a typical year, the lake surface can remain frozen from approximately late December through early April. Ice thickness can exceed 40 centimeters and may reach more than 50 centimeters during particularly cold winters.
During severe winter weather, repeated expansion and contraction of the ice caused by changes in temperature can create large cracks and raised ridges across the frozen surface. This phenomenon is known as Omiwatari, meaning “the crossing of the deity.”
The raised ice formations can sometimes reach around 20 centimeters high and extend across the lake. Their appearance has traditionally been compared to the path left behind when a deity crosses the frozen lake, creating one of the most mysterious sights of winter on Mount Akagi.
The History and Legends of Lake Onuma
The Traditional Name “Ono”
The traditional pronunciation of the name Onuma is said to be “Ono”, while Lake Konuma was traditionally pronounced “Kono.” In modern Japanese, however, the names are increasingly read as “Onuma” and “Konuma.”
Historical records also contain other names for Lake Onuma, including Ishigaki-numa and Kuzuhako-no-ko. These names changed over time according to local usage and historical circumstances before the present name became established.
Myths of the Gods of Akagi and Nikko
Mount Akagi is associated with numerous legends. One of the best known tells of a conflict between the deity of Mount Akagi and the deity of Mount Nikko, also known as the deity of Futara-san.
According to the legend, the two deities battled over territory and water, taking the forms of enormous serpents. Lake Onuma and Lake Chuzenji in Nikko are both associated with this story.
Beyond its mythological meaning, the story has also been interpreted as reflecting ancient conflicts between communities and regional powers over land and water resources. Related place names and legends remain part of the cultural heritage of northern Kanto.
Akagi Shrine and Kotori-ga-shima
A Sacred Center of Mount Akagi Worship
Akagi Shrine stands on Kotori-ga-shima along the eastern side of Lake Onuma. The shrine is closely associated with the ancient religious tradition of worshipping Mount Akagi as a sacred mountain.
Akagi Shrine has a long history and is listed in the Engishiki Jinmyocho, an early 10th-century register of important shrines in Japan. The shrine’s location surrounded by water gives it a particularly distinctive atmosphere.
The vermilion bridge, torii gate, and shrine buildings reflected in the lake are among the most picturesque sights in the Mount Akagi area. The grounds are considered sacred, and visitors can experience a quiet atmosphere quite different from the busier tourist destinations of the lowlands.
A Place Connected with Japanese Literature
Kotori-ga-shima is also associated with Japanese literature. A monument commemorating the novelist Naoya Shiga stands on the island.
Mount Akagi has long attracted writers, poets, and artists, who were inspired by its mountains, lakes, forests, and changing seasonal landscapes.
Things to Do at Lake Onuma
Lakeside Walks and Nature Observation
A walking trail follows the shoreline of Lake Onuma, offering an easy way to experience the scenery. A complete circuit is approximately 4 kilometers and takes around one and a half hours at a relaxed pace.
The trail passes through forests and along the water, allowing visitors to enjoy changing views of the lake and the sounds of birds. The highland air is particularly refreshing, offering a welcome contrast to the urban environment of the Kanto Plain.
Boating and Fishing
During the warmer months, boating and fishing are popular activities on Lake Onuma.
The lake is home to fish including smelt, crucian carp, carp, and Japanese dace. Fishing is enjoyed by both local anglers and visitors looking for a peaceful outdoor experience.
Ice Fishing for Smelt
One of the most famous winter activities at Lake Onuma is ice fishing for smelt.
When the lake has frozen sufficiently, anglers set up small tents on the ice, drill holes through the frozen surface, and fish through the openings. The colorful tents scattered across the white lake have become a characteristic winter scene on Mount Akagi.
Winter Recreation
The slopes around Mount Jizodake have also been used for winter recreation. Skiing became particularly popular in the area during the 1970s, when Mount Akagi was one of the better-known winter destinations accessible from the Kanto region.
Today, visitors can enjoy various forms of seasonal winter recreation depending on weather and operating conditions, including snow activities and skating-related experiences.
A Gateway to Hiking on Mount Akagi
Exploring the Mountains Around the Lake
Lake Onuma is an important base for exploring the mountains of Akagi. The mountain range includes Mount Kurohiyama, Mount Komagatake, Mount Jizodake, Mount Arayama, Mount Nabewari, Mount Suzutake, and Mount Choshichiroyama.
Mount Kurohiyama reaches 1,828 meters and is recognized as one of Japan’s 100 Famous Mountains. Hiking routes around Mount Akagi range from relatively accessible trails to more demanding mountain walks.
Lake Onuma serves as a convenient starting point for many of these routes. Hikers can enjoy the mountain scenery before returning to the lakeside to rest or explore the surrounding attractions.
Seasonal Scenery and Events
Fresh Greenery in Spring and Early Summer
From spring into early summer, the forests around Mount Akagi are covered in fresh green leaves. In June, the area around the Akagi White Birch Ranch becomes especially colorful when Rhododendron japonicum, commonly known as azalea, comes into bloom.
The vivid orange-red flowers cover the highland landscape, and seasonal azalea festivals are held in the area.
Autumn Foliage
Autumn brings brilliant colors to the forests surrounding Lake Onuma. Trees such as Japanese oak, beech, and white birch turn shades of red, orange, and yellow.
The reflections of the colorful trees on the lake create especially beautiful views, making autumn one of the most popular seasons for visiting Mount Akagi.
Summer Festival and Fireworks
In early August, the Akagiyama Summer Festival is held around the time of the annual festival of Akagi Shrine.
Traditional events and a fireworks display take place around the lake. Fireworks reflected on the dark surface of Lake Onuma create a memorable summer evening in the mountains.
Kakumanbuchi Wetland
A Highland Wetland Known as “Little Oze”
Kakumanbuchi is a small wetland located southeast of Lake Onuma. With a circumference of approximately 500 meters, it is sometimes called “Little Oze” because of its beautiful highland wetland environment.
A wooden boardwalk allows visitors to walk through the wetland while observing plants and wildlife at close range.
Depending on the season, visitors may see plants such as sphagnum moss, sundews, and Nikko kisuge daylilies. The scenery changes throughout the year, offering a peaceful and intimate experience of the highland ecosystem.
The Natural Environment of Lake Onuma
A Rich Lake Ecosystem
Lake Onuma supports a variety of aquatic life, including smelt, carp, crucian carp, and Japanese dace. Amphibians such as Japanese common toads and Montane brown frogs can also be found in the surrounding environment.
Waterbirds, including spot-billed ducks, visit the lake and surrounding wetlands. Together, the lake, forests, wetlands, and mountain slopes form an important highland ecosystem.
Water Quality and Environmental Conservation
As tourism increased, changes in the water quality of Lake Onuma became an environmental concern at certain times. The local authorities have therefore established environmental water-quality standards and implemented measures to help protect the lake.
Following the 2011 Fukushima nuclear accident, restrictions were temporarily placed on taking caught smelt home because of concerns about radioactive contamination. Subsequent testing confirmed that the fish met safety requirements, and restrictions on taking the fish home were lifted in 2015.
A Highland Lake Where Nature, History, and Culture Meet
Lake Onuma on Mount Akagi is much more than a scenic mountain lake. Its volcanic landscape, sacred traditions, seasonal scenery, and outdoor activities come together to create one of Gunma’s distinctive highland destinations.
Visitors can walk around the lake, visit Akagi Shrine, explore Kakumanbuchi Wetland, hike the surrounding mountains, go boating or fishing in summer, and experience the frozen lake and smelt fishing in winter.
The lake also offers an opportunity to discover the cultural history of Mount Akagi, including ancient mountain worship and the legends associated with the deities of Akagi and Nikko.
Whether you visit for the fresh summer air, brilliant autumn foliage, snowy winter scenery, or the quiet atmosphere of the shrine beside the lake, Lake Onuma offers a peaceful escape into the natural landscape of northern Kanto.