Okutone Lake is a man-made lake located in the uppermost reaches of the Tone River in Minakami Town, Tone District, Gunma Prefecture. Surrounded by primeval forests of beech and oak, it is the largest lake in the Okutone area.
Nestled among the deep mountains along the Gunma-Niigata border, the tranquil surface of the lake offers visitors a chance to experience the grandeur of the natural environment. The scenery changes beautifully throughout the seasons, from fresh green foliage in spring to deep summer greenery and vivid autumn colors.
On calm, windless days, the surface of the lake becomes like a mirror, reflecting the surrounding mountains and creating an almost dreamlike landscape.
Okutone Lake was created by the construction of Yagisawa Dam, which was completed in 1967. Together with Lake Dogen, formed by Sudagai Dam, and Lake Fujiwara, formed by Fujiwara Dam, it is known as one of the Three Lakes of Okutone and is the largest of the three.
The reservoir has a total capacity of approximately 240.3 million cubic meters, making it one of the largest dam reservoirs in the Tone River system. It plays an important role in supplying water to the Tokyo metropolitan area and other parts of the Kanto region.
The Tone River originates at Mount Ominakami, at an elevation of 1,831 meters, and has the largest drainage basin of any Class A river in Japan. Located at its uppermost reaches, Okutone Lake serves as an important water resource supporting everyday life and industry across the Kanto Plain.
The reservoir contributes to flood control, agricultural water supply, municipal water supply, and hydroelectric power generation, making it an important part of the region’s water infrastructure.
Yagisawa Dam is an arch-type concrete dam measuring 131 meters in height and 352 meters along the crest. It is a distinctive composite structure combining an arch-type main dam, a gravity-type concrete spillway section, and a rockfill section designed for water sealing.
Viewed from below, the gently curved dam wall rises toward the sky, creating an impressive combination of engineering strength and architectural beauty.
One of the most notable sights is the annual inspection discharge, generally conducted between May and July. During this operation, water is released through the emergency spillway gates to check their performance.
Large volumes of water surge dramatically from the ski-jump-style spillway, sending clouds of spray into the air. The spectacular scene attracts many visitors who come to see the powerful display of water and the massive dam structure.
The area around Okutone Lake is designated as the Tone River Source Area Natural Environment Conservation Region. It is home to a wide variety of valuable plants and animals and is considered an important natural area.
There are no public roads farther upstream from the lake, leaving extensive areas of relatively untouched wilderness. In spring, visitors can see the remaining snow on the Mikuni Mountains in the distance, while autumn transforms the surrounding mountains into a vivid landscape of red and yellow foliage.
The area is also one of Japan’s snowiest regions. In some years, cumulative snowfall can exceed 10 meters. The management road is generally closed during winter, approximately from late November through late April, so visitors should check access conditions before traveling.
Visitors can enjoy outdoor activities such as fishing, canoeing, and boating around Okutone Lake. Seeing a canoe floating against the backdrop of primeval forest creates a peaceful experience that is difficult to find in urban areas.
The area also remains relatively cool during summer, making it an attractive destination for those seeking relief from the heat.
Several renowned hot springs can also be found nearby, including Yunokoya Onsen and Takaragawa Onsen. Together with the Minakami hot spring area, they form one of Gunma Prefecture’s major destinations for hot spring tourism.
Combining outdoor activities in the mountains with relaxation in a traditional hot spring is one of the distinctive ways to experience the Okutone area.
Adjacent to the dam management office, Nature View Yagisawa is an educational facility focusing on disaster prevention, the role and structure of the dam, and the natural environment of the Okutone area.
Through exhibits and video presentations, visitors can learn about the importance of flood control and water resources. The facility can be visited freely and is also a popular educational stop for families.
Yagisawa Power Station is a pumped-storage hydroelectric power station operated by TEPCO Renewable Power. Okutone Lake, formed by Yagisawa Dam, serves as the upper reservoir, while the lake formed by Sudagai Dam downstream serves as the lower reservoir.
The facility has an authorized output of 240,000 kilowatts and contributes to the electricity supply of the Tokyo metropolitan area.
It also has a history of pioneering the introduction of variable-speed pumped-storage generation technology, contributing to the development of hydroelectric power technology in Japan.
In 2005, Okutone Lake was selected as one of Japan’s 100 Dam Lakes following a recommendation by Minakami Town. Its beautiful scenery and importance as a water source were among the qualities recognized in the selection.
Plans for the development of Yagisawa Dam began before World War II, and the project was eventually completed as demand for flood control and water resources increased during Japan’s postwar development.
The dam therefore represents not only important modern infrastructure but also a part of Japan’s history of rapid economic growth and large-scale water resource development.
Okutone Lake is much more than a reservoir. It represents a remarkable balance between a rich natural environment and the water resources that support people’s lives and industries.
The tranquil lake surrounded by primeval forests, spectacular dam discharges, seasonal mountain scenery, nearby hot springs, and outdoor activities come together to create a destination with a distinctive character.
When visiting Minakami Town, consider traveling deeper into the Okutone area to experience the immense scale of one of the Kanto region’s important water sources and the remarkable beauty of its relatively untouched mountain landscape.