Katanuma is a remarkable volcanic lake located in Naruko Onsen, Osaki City, Miyagi Prefecture. Known as one of Japan’s notable highly acidic volcanic lakes, it offers a fascinating landscape shaped by volcanic activity.
The lake is surrounded by mountains and forests, and its surface can appear in different shades depending on the weather and sunlight. On a clear day, the water may appear sky blue, emerald green, or deep cobalt blue, creating an almost otherworldly scene.
Katanuma is located within Kurihara Quasi-National Park and features a walking trail along the lakeshore. Depending on the season and operating conditions, visitors can also enjoy activities such as boating and stand-up paddleboarding (SUP).
Katanuma is traditionally believed to have been formed by a major eruption of the Naruko volcanic group in 837. It is surrounded by an approximately 7-kilometer-wide outer volcanic rim.
The lake lies at an elevation of approximately 306 meters and has an oval shape with a circumference of about 1,380 meters.
Volcanic activity can still be detected around the lake. Fumaroles—openings in the ground that release volcanic gases—can be found along the lake bottom and shore. Some of these gases contain hydrogen sulfide, creating a characteristic sulfur-like smell around the area.
One of Katanuma’s most fascinating features is the way its water appears to change color depending on weather conditions and the angle of sunlight.
Under different conditions, the lake can appear sky blue, emerald green, or a rich cobalt blue. The surrounding mountains and seasonal vegetation add even more color to the landscape.
During the seasons of fresh green foliage in spring and colorful leaves in autumn, the contrast between the lake and surrounding forest becomes especially beautiful. The unusual colors and volcanic landscape also make Katanuma a popular destination for photographers.
The water of Katanuma is extremely acidic, with a pH of approximately 2.4. In the past, the lake was recorded at a pH as low as 1.4 and was once described as having the world’s highest level of acidity.
Because of these unusually acidic conditions, fish do not live in the lake. Nevertheless, the surrounding environment supports specialized organisms that have adapted to the harsh conditions.
For visitors, this provides an unusual opportunity to see how volcanic activity can influence an entire ecosystem. Katanuma is not simply a beautiful blue lake; its unusual water chemistry is an important part of what makes the landscape distinctive.
Despite the highly acidic environment, organisms such as the Sanyusurika, certain types of phytoplankton, and benthic diatoms can be found in and around the lake. These organisms have adapted to the unusual conditions and contribute to a distinctive local ecosystem.
The vegetation around the lake includes forests of Japanese red pine and konara oak, along with areas of dwarf bamboo and Japanese pampas grass.
One particularly notable plant is Yamatanuikane, a species considered native to the Tohoku region. Large communities of this unusual plant grow around Katanuma, particularly in the volcanic environment where sulfurous gases are present.
The plant communities add another layer of interest to the landscape and demonstrate how plants can adapt to distinctive volcanic environments.
The origins of Katanuma are mentioned in one of Japan’s ancient historical records, the Shoku Nihon Koki. An account from 837 describes major volcanic activity in the Naruko area, including mountains burning, valleys becoming blocked, rocks collapsing, and trees being broken.
The record also describes the formation of a new lake, and this event is traditionally associated with the birth of Katanuma.
Katanuma is also associated with a fascinating dragon deity legend. According to local tradition, a female dragon deity lived in the lake and appeared in spring to weave cloth.
A small shrine known as the Ryujin Hokora, or Dragon Deity Shrine, remains beside the lake. The site reflects the traditional connection between local communities, natural features, and spiritual beliefs.
The sulfur-rich environment around Katanuma attracted attention from the Edo period onward. Sulfur from the area was later mined from the Meiji period through the Showa period.
During World War II, a sulfur refinery was built in the area, giving the lake and its surroundings an industrial role in addition to their natural significance.
Later in the Showa period, Katanuma was developed as a tourist destination. Walking paths and a rest house were established, and the lake gradually became one of the notable attractions of the Naruko Onsen area.
A walking trail of approximately 1.3 kilometers runs around the lake, allowing visitors to enjoy a leisurely circuit of Katanuma.
The trail provides changing views of the colorful lake surface, surrounding volcanic mountains, and seasonal vegetation. In spring, fresh green leaves create a refreshing atmosphere, while autumn brings vivid foliage to the surrounding forest.
The relatively short distance makes the lakeshore walk an enjoyable addition to a sightseeing trip around Naruko Onsen.
Depending on the season and operating conditions, visitors can enjoy activities such as boating and SUP (stand-up paddleboarding) on the lake.
SUP allows participants to glide across the calm water while looking closely at the surrounding mountains and sky reflected on the lake surface. It can be an especially memorable way to experience Katanuma from the water.
Visitors should follow local safety instructions and check current operating conditions before planning water activities, as conditions and availability can vary.
Katanuma is generally closed during the winter from late November through late April. During the snowy season, however, special snowshoe tours may be offered using the area’s abundant snowfall.
These tours provide a completely different experience from visiting during the warmer months, allowing participants to explore the quiet winter landscape surrounded by snow.
Location: Naruko Onsen, Osaki City, Miyagi Prefecture
Nearest station: Approximately 4 kilometers from Naruko-Onsen Station on the JR Rikuuto Line.
By car: Approximately 10 minutes from Naruko-Onsen Station.
On foot: The route includes steep sections, so walking from the station requires caution and appropriate footwear.
Parking: Free parking is available near the lakeshore.
Naruko Onsen is one of Japan’s well-known hot spring destinations, offering abundant hot spring water and a variety of mineral compositions. A visit to Katanuma pairs naturally with a relaxing stay in the onsen town.
Naruko Gorge is a spectacular scenic area particularly famous for its autumn foliage. The steep valley walls and colorful forest create an impressive seasonal landscape.
Naruko Dam is another nearby destination where visitors can enjoy views of the dam reservoir and surrounding mountains.
Katanuma is a unique destination where volcanic forces, unusual water chemistry, nature, history, and local traditions come together. Its strikingly colored water is the first thing many visitors notice, but the lake has much more to discover.
The highly acidic water, specialized ecosystem, unusual plant communities, ancient volcanic history, and dragon deity legend all contribute to the lake’s distinctive character.
Visitors can walk around the lakeshore, admire the changing colors of the water, enjoy seasonal scenery, or experience the lake through activities such as boating and SUP when available.
When visiting Naruko Onsen, be sure to include Katanuma in your itinerary. Its mysterious volcanic landscape offers a memorable glimpse into the powerful natural forces that have shaped the mountains of Miyagi.