Yashima Marsh, also known as Yashimagahara Marsh, is one of Japan’s most remarkable high-elevation wetlands. Located at an altitude of approximately 1,630 meters in the northwestern part of the Kirigamine Highlands in central Nagano Prefecture, it is renowned for its rich natural environment and exceptional scientific value. The marsh was designated a National Natural Monument in 1939 and has been carefully protected ever since.
The origins of Yashima Marsh date back approximately 12,000 years. Peat has accumulated here at a rate of roughly one millimeter per year, creating a peat layer that is now about 8.05 meters thick. Walking across the marsh offers a rare opportunity to experience a landscape that has been gradually shaped by nature over thousands of years—a truly living record of the past.
Kirigamine, with a highest elevation of 1,925 meters, is a vast highland of gentle hills and open grasslands at the heart of the Yatsugatake-Chushin Kogen Quasi-National Park. The area is particularly famous for its expansive scenery and alpine vegetation.
Kirigamine is home to three important wetlands: Yashimagahara Marsh, Kurumayama Marsh, and Odoriba Marsh, also known as Ike-no-Kurumi. Each is protected as an important natural plant community. Among them, Yashimagahara Marsh is the largest, covering approximately 43.2 hectares with a circumference of about 3.7 kilometers.
The marsh extends across Suwa City and Shimosuwa Town. Its location along the Venus Line also makes it relatively easy to visit, attracting hikers, nature lovers, photographers, and travelers seeking the distinctive scenery of Japan’s highlands.
Yashimagahara Marsh is particularly valuable because it represents the southern limit of Japan’s high-elevation raised bogs. A raised bog develops over an extremely long period as a former lake gradually becomes a wetland and peat accumulates until the surface rises above the surrounding land.
The story began with a small lake. Sediment flowed in from the surrounding mountains while aquatic plants flourished in the water. Over time, the lake gradually filled with sediment and plant remains.
Because the highland climate is cool, dead plants decompose slowly. Instead of completely breaking down, they accumulate as peat. The wetland initially developed as a low marsh, where groundwater supplied relatively nutrient-rich conditions suitable for reeds, cattails, sedges, and other plants.
As more peat accumulated, the surface gradually rose above the surrounding area. Groundwater could no longer reach the center as easily, and the wetland became increasingly dependent on rainwater. The environment became acidic and nutrient-poor, creating the conditions characteristic of a raised bog.
Sphagnum moss plays a central role in the formation of Yashima Marsh. As many as 18 species of sphagnum have been identified here, a level of diversity said to rival that of the much larger Kushiro Marsh in Hokkaido.
Sphagnum grows in small mounds and depressions, creating subtle variations in the surface known as bultes and schlenken. As these formations develop repeatedly over time, they produce the gently undulating landscape characteristic of a raised bog.
The peat layer at Yashima Marsh also has a clearly defined structure. The lower layers contain peat formed from reeds and sedges, the middle layers are associated with Molinia-dominated wetland vegetation, and the upper layers consist mainly of sphagnum moss. This clearly preserved cross-section provides important information for scientific research into the formation and development of wetlands.
Approximately 360 species of plants have been identified around Yashima Marsh. Some even carry the name “Kirigamine,” reflecting their close association with the highland. Despite the harsh environment, delicate flowers appear throughout the warmer months, adding vivid color to the expansive landscape.
In June, brilliant orange renge azaleas begin to color the highlands. In July, the iconic Nikko kisuge daylilies bloom across Kirigamine, transforming the landscape into a sea of yellow. This spectacular display attracts photographers and nature enthusiasts from many places.
From spring into summer, delicate plants such as cotton grass, bog cranberry, and small rhododendrons appear across the marsh. As early autumn arrives, the grasses gradually change color, creating a soft, warm landscape known as kusa-momiji, or autumn-colored grassland.
The rich wetland environment also provides a habitat for a wide variety of wildlife. Visitors may encounter amphibians such as the Schlegel’s green tree frog, as well as numerous insects and wild birds.
These species have adapted to the cool, demanding conditions of the highlands, offering visitors an opportunity to observe the remarkable resilience of nature in a mountain wetland environment.
A well-maintained walking trail circles the marsh, making it accessible even to visitors without extensive hiking experience. The complete loop is approximately 3.7 kilometers and takes around 1.5 to 2 hours at a leisurely pace.
The trail is suitable for couples and families as well as individual travelers. Along the way, the scenery constantly changes, offering views of the marsh, surrounding mountains, flowers, and small wetland plants.
On a clear day, the water reflects the sky, clouds, and surrounding mountains, creating a refreshing highland panorama. When mist rolls across the marsh, however, the atmosphere changes completely. The landscape becomes quiet and mysterious, sometimes appearing almost otherworldly.
The fact that the scenery can look dramatically different depending on the weather and time of day is one of Yashima Marsh’s greatest charms.
The distant mountains may naturally attract your attention, but the plants growing beside the trail are equally fascinating. Small wetland species and softly colored thistles can be found close to the path, revealing a delicate beauty that is easy to overlook.
For visitors seeking a longer hike, routes continue from the marsh toward the summit of Mount Kurumayama and other parts of the Kirigamine Highlands.
Before or after walking around the marsh, consider visiting the Shimosuwa Town Yashima Visitor Center Azamikan. The center provides an introduction to the natural environment and formation of Yashima Marsh, including a model showing the cross-section of its peat layers.
Exhibits also introduce the plants, birds, and animals that inhabit the highlands. The research room on the second floor presents seasonal footage of the surrounding natural environment, making it a useful place to learn about current conditions before setting out on the trail.
Taking a little time to understand the landscape here can make the subsequent walk much more meaningful, as visitors can see the wetland not simply as beautiful scenery but as the result of thousands of years of natural processes.
Yashima Marsh is a natural heritage site that has developed over approximately 12,000 years. However, its environment is not immune to change. Increasing forest growth around the marsh and changes in rainfall patterns have been identified as factors that may contribute to drying of the wetland.
For this reason, visitors also play an important role in protecting the marsh. Staying on designated trails, avoiding the collection or damage of plants, and respecting the fragile wetland environment are essential to preserving this remarkable landscape for future generations.
A walk through Yashima Marsh is more than a simple highland stroll. It is an opportunity to experience a landscape that has been quietly developing since prehistoric times. Surrounded by the vast scenery of the Kirigamine Highlands, visitors can sense the extraordinary depth of Japan’s natural environment and the passage of time preserved within this ancient wetland.