Elegant Dam Architecture Reminiscent of a European Castle
One of Shimagawa Dam’s most distinctive features is its elegant and impressive design. The exterior incorporates decorative elements resembling stone masonry, while structures such as the spillway feature details reminiscent of a European castle.
Although the dam is a massive concrete structure, considerable attention was given to creating a design that harmonizes with the surrounding natural landscape. The result is a dam that can be appreciated not only as an important infrastructure facility but also as a sightseeing attraction.
The view from below the dam is particularly impressive, making it a popular spot for photography. Inside the structure is an inspection passage known as a gallery. Tours of the interior are available by reservation, offering visitors a rare opportunity to see the hidden structure of a large dam while learning about how it operates.
Okushima Lake and the Mysterious “Shima Blue”
The reservoir created by Shimagawa Dam is known as Okushima Lake. Located at the deepest end of the Shima Onsen area, the lake has a circumference of approximately 4 kilometers and is surrounded by beautiful mountain scenery.
A one-way road circles the lake, allowing visitors to enjoy the scenery at a leisurely pace by car. Those exploring on foot can walk around the lakeshore in approximately one hour, making it an enjoyable destination for a nature walk or light hike.
The greatest attraction of Okushima Lake is its mysterious blue water. Depending on the conditions, the lake can appear cobalt blue, turquoise, or deep indigo. This beautiful phenomenon is known as “Shima Blue” and attracts visitors and photographers from near and far.
The most vivid blue colors are often seen around April and May, when snowmelt flows into the lake. On clear mornings, sunlight reaches the surface of the water and can create an especially brilliant blue landscape.
The color and appearance of the lake change depending on the season, weather, and time of day. This ever-changing scenery is one of the reasons visitors can enjoy a different view each time they return.
Why Does the Lake Look Blue?
The water of Okushima Lake is exceptionally clear, and its blue appearance is thought to be related to the way sunlight interacts with the transparent water. The clearer the water, the more strongly certain wavelengths of light can contribute to the blue appearance of the lake.
Fine particles called allophane, a type of aluminum silicate mineral, are also thought to scatter light in the water and contribute to the vivid blue coloration.
A similar phenomenon is often associated with Goshiki-numa in the Urabandai area of Fukushima Prefecture. The striking color of Okushima Lake is therefore an example of a fascinating landscape created through the interaction of water, minerals, sunlight, and the surrounding natural environment.
Beautiful Nature in Every Season
Okushima Lake is a scenic destination that offers a different atmosphere throughout the year. Each season brings its own colors and landscapes, making the lake rewarding to visit more than once.
Spring – Fresh Greenery and Azaleas
In spring, fresh green leaves begin to emerge across the surrounding mountains, while azaleas and other flowers bloom around the lakeshore.
The vivid greenery and colorful flowers create a beautiful contrast with the blue surface of the lake. The period from April to May, when snowmelt flows into the reservoir, is often considered one of the best times to see the vivid blue of the lake.
Summer – Clear Water and Deep Green Mountains
During summer, sunlight shines across the lake, making the exceptionally clear water glow with an almost dreamlike blue color.
The deep green mountains surrounding the lake are reflected on the surface, creating scenery that resembles a giant natural mirror. On calm, windless days, the water can reflect the sky so clearly that the boundary between the lake and the surrounding landscape seems to disappear.
Autumn – Brilliant Foliage and Shima Blue
From mid-October to early November, the surrounding mountains turn red, orange, and yellow as autumn foliage reaches its peak.
The vivid autumn colors combined with the blue lake create a landscape reminiscent of a painting. Lookout areas and parks have been developed along the lakeshore, allowing visitors to sit back and enjoy the scenery at their own pace.
Winter – A Quiet World of Snow and Blue
In winter, snow covers the surrounding mountains and Okushima Lake takes on a serene, almost dreamlike atmosphere.
When temperatures become particularly low, parts of the lake may freeze. The contrast between the white snow-covered landscape and the blue water creates a striking winter scene.
The lakeside loop road may be closed during winter, depending on conditions. Even then, visitors can enjoy views of the snow-covered lake from the dam itself.
Outdoor Activities on the Lake
Okushima Lake also offers opportunities to enjoy outdoor activities in harmony with nature. Among the most popular are canoeing and stand-up paddleboarding (SUP).
Seeing the famous Shima Blue from the surface of the lake offers a completely different perspective. Being close to the water makes it easier to appreciate its clarity and striking color.
Walking paths and rest areas are also available around the lakeshore, making the area suitable for nature observation and hiking.
The surrounding forests are home to wildlife such as Japanese macaques and Japanese serows. Exploring the area provides an opportunity to appreciate the richness of the local natural environment.
The History of Shimagawa Dam
Shimagawa Dam was constructed as part of Gunma Prefecture’s Shima River Comprehensive Development Project.
A smaller dam, Nakanojo Dam, had already been built along the Shima River for purposes including erosion control and power generation. However, an additional large-scale dam farther upstream was considered necessary to improve flood control and secure water resources throughout the river basin.
Construction began in 1980 and took approximately 19 years to complete. Shimagawa Dam was finally completed in 1999.
The dam is a multipurpose facility serving several roles, including flood control, water supply, water utilization, and hydroelectric power generation.
Immediately downstream of the dam, the Hinata-mikami Power Station generates up to approximately 1,000 kilowatts of hydroelectric power, contributing to the local energy supply.
A Scenic Destination to Visit Alongside Shima Onsen
Shimagawa Dam and Okushima Lake are ideal destinations to combine with a visit to Shima Onsen.
After leaving the hot spring town and driving farther into the mountains, the mysterious blue waters of Okushima Lake suddenly come into view. The dramatic change from the traditional hot spring town to the expansive mountain landscape is an experience that leaves a strong impression on many visitors.
After relaxing in the hot springs, a walk along the lakeshore provides another opportunity to unwind while surrounded by nature.
A Scenic Landmark in Harmony with Nature
Shimagawa Dam plays an important role in flood control and water resource management, but it has also become a fascinating sightseeing destination.
The mysterious blue waters of Okushima Lake, the mountains changing color with the seasons, and the distinctive architecture of the dam come together to create a unique landscape.
Shimagawa Dam and Okushima Lake are among Gunma Prefecture’s most memorable scenic destinations, offering visitors the chance to experience the beauty of nature alongside an impressive example of modern infrastructure.