A Beautiful Reservoir Created by the Shima River Dam
Lake Okushimako was formed by Shima River Dam, a multipurpose dam built on the upper reaches of the Shima River, which belongs to the Tone River water system.
The dam is a gravity-type concrete dam managed by Gunma Prefecture. With a height of 89.5 meters, it is known as the tallest dam in Gunma Prefecture. Construction began in 1980, and the project was completed in 1999 after approximately 19 years of work.
The dam serves several important purposes, including flood control, water resource management, municipal water supply, and hydroelectric power generation. It therefore plays an important role in supporting both the environment and daily life in the Shima River basin.
The area around the dam has also been developed as a place where visitors can experience nature and learn about the role of the reservoir. The dam is designated by Japan’s Ministry of Land, Infrastructure, Transport and Tourism as a “dam open to the local community”, reflecting its recreational and educational role as well as its infrastructure function.
The Mysterious Beauty of “Shima Blue”
The most striking feature of Lake Okushimako is undoubtedly its extraordinary blue color.
The appearance of the water changes according to sunlight, weather conditions, and the season. On different days, visitors may see shades ranging from brilliant cobalt blue and turquoise to deep indigo.
This phenomenon is commonly known as Shima Blue and has become one of the signature sights of the Shima Onsen area.
Several factors are thought to contribute to the lake’s unusual color. The relatively clear water allows sunlight to penetrate the surface, while tiny mineral particles in the water, including allophane, an aluminum silicate mineral, are thought to scatter light and enhance the blue tones.
Similar natural phenomena can be observed at places such as the Goshiki-numa Ponds in the Bandai area of Fukushima Prefecture. The exact appearance of the water can vary considerably depending on natural conditions, which is part of what makes Shima Blue so fascinating.
When Is Shima Blue at Its Most Beautiful?
The lake is often particularly vivid from around April to May, when snowmelt flows into the reservoir and the water can appear exceptionally clear.
On sunny days, especially in the morning when the angle of the sunlight is favorable, the lake can display an especially striking blue color. However, the precise appearance changes with weather and light, so every visit offers a slightly different view.
Exploring the Lakeshore by Car or on Foot
A scenic road approximately 4 kilometers long circles Lake Okushimako, making it possible to enjoy the landscape by car.
The route is designed as a one-way circuit, with viewpoints and small parks along the way where visitors can stop and take in the scenery.
For those who prefer to explore on foot, walking around the lake takes approximately one hour at a leisurely pace. The quiet forest, fresh mountain air, and sounds of birds create a peaceful atmosphere far removed from the bustle of urban life.
Exploring the area by rental bicycle is another enjoyable option when conditions and local services permit.
Four Seasons at Lake Okushimako
Spring — Fresh Greenery and Flowers
Spring brings cherry blossoms, azaleas, and other seasonal flowers to the area surrounding Lake Okushimako.
The vivid blue of the lake contrasts beautifully with the fresh green leaves covering the mountains. Birds can often be heard in the surrounding forest, adding to the refreshing atmosphere of the season.
Summer — Deep Green Forests and Clear Blue Water
In summer, sunlight illuminates the clear lake water, often making the blue appear especially intense.
The surrounding forests become a deep green, and on calm days the mountains are reflected on the surface of the lake like a mirror. The combination of blue water and lush greenery makes the area particularly attractive for photography.
Autumn — Brilliant Foliage Reflected in the Blue
From approximately mid-October to early November, the mountains surrounding Lake Okushimako are transformed by autumn foliage.
Red, orange, and yellow leaves contrast dramatically with the blue water, creating a landscape that resembles a painting. The lake is one of the notable places around Shima Onsen for enjoying the autumn colors.
Winter — A Quiet World of Snow
In winter, snow covers the surrounding mountains and the landscape takes on a serene, almost dreamlike character.
During particularly cold periods, parts of the lake surface may freeze, creating a striking contrast between the blue water and the surrounding white landscape.
The lakeside road may be closed during the winter depending on weather and road conditions. Even when access around the lake is restricted, visitors may still be able to enjoy views of the winter landscape from areas around the dam, subject to local conditions.
Outdoor Activities on the Lake
Lake Okushimako also offers opportunities to enjoy the natural environment from the water.
Canoeing and stand-up paddleboarding (SUP) are among the activities associated with the lake. Experiencing Shima Blue from the water provides a completely different perspective from viewing it from the shore.
Floating close to the surface allows visitors to appreciate the clarity and color of the water while looking back toward the surrounding mountains. Weather and operating conditions can affect whether water activities are available, so visitors should check local information in advance.
Discovering the “Shima Blue” Lakes Together
Another scenic lake, Lake Shima, can be found closer to the entrance of the Shima Onsen area.
Lake Shima was created by Nakanojo Dam and is also known for its beautiful blue-green water. Although Lake Shima and Lake Okushimako are separate reservoirs, both contribute to the distinctive scenery known as Shima Blue.
Visiting both lakes is a wonderful way to appreciate the changing colors and landscapes of the Shima River valley.
A Scenic Destination to Combine with Shima Onsen
Lake Okushimako is located immediately upstream from the Shima Onsen resort area, making it an easy addition to a hot spring trip.
After enjoying a relaxing soak in the traditional hot springs of Shima, visitors can head farther into the valley to explore the lake, walk through the forest, and admire the vivid blue water.
The contrast between historic hot spring culture, quiet mountain forests, and the striking color of Shima Blue gives the area a distinctive character.
A Mountain Lake Where Nature Creates Its Own Colors
Lake Okushimako is more than simply a reservoir. It is a place where a major piece of modern infrastructure has become part of a spectacular mountain landscape.
Its mysterious blue water, seasonal forests, surrounding mountains, and peaceful lakeside roads offer something different in every season. The lake can be enjoyed as part of a scenic drive, a leisurely walk, an outdoor adventure, or a relaxing excursion from Shima Onsen.
For travelers exploring Gunma’s hot spring country, Lake Okushimako is an unforgettable destination where the beauty of water, mountains, and changing seasons comes together in the remarkable landscape of Shima Blue.