Shimokubo Dam is a multipurpose dam built on the middle reaches of the Kanna River, a major tributary of the Tone River system. The dam straddles Fujioka City in Gunma Prefecture and Kamikawa Town in Saitama Prefecture.
As an important water resource facility supporting daily life in the Tokyo metropolitan area, Shimokubo Dam plays a major role in flood control and water supply. At the same time, it is a popular sightseeing destination surrounded by beautiful mountain scenery.
The reservoir created by the dam, Lake Kanna, has been selected as one of Japan’s 100 Best Dam Lakes. Its changing scenery throughout the seasons and opportunities for outdoor recreation make it a pleasant place to enjoy nature.
Managed by the Japan Water Agency, Shimokubo Dam is a 129-meter-high concrete gravity dam. It has an effective storage capacity of approximately 120 million cubic meters, with a summer restricted capacity of 85 million cubic meters.
It is the largest of the nine dams in the Tone River system and serves as an important water reservoir for the Tokyo metropolitan area. Its functions include flood control, municipal water supply, industrial water supply, agricultural water supply, and hydroelectric power generation.
One of its most distinctive features is the dam body, which bends toward the upstream side in an L-shaped configuration. With a crest length of 605 meters, it is one of Japan’s longest concrete dams.
This unusual design was created in response to the area’s topographical and geological conditions and represents an example of the engineering solutions used in constructing the dam.
The Kanna River rises at Mount Mikuni, near the borders of Gunma, Saitama, and Nagano prefectures. It flows for approximately 87.4 kilometers through Gunma and eventually joins the Karasu River before flowing into the Tone River.
The river has a drainage basin of approximately 407 square kilometers, and flood control has long been an important issue in the surrounding region.
Lake Kanna is one of the largest artificial lakes in southern Gunma and is surrounded by the beautiful mountains of the Chichibu-Tama-Kai National Park.
In spring, Somei Yoshino cherry trees bloom along the lakeshore, while autumn brings colorful foliage reflected on the surface of the water. Anglers and boating enthusiasts can also be seen on the lake, creating a peaceful atmosphere for enjoying the surrounding nature.
In 1947, the Tone River basin suffered devastating damage from Typhoon Kathleen. In response, the Japanese government began developing comprehensive flood-control measures and formulated plans to construct multiple multipurpose dams throughout the Tone River system.
The Shimokubo Dam project was developed as part of this broader plan. Construction of the main dam began in 1965, and the project was completed in 1968.
Construction required the relocation of 321 households, comprising 364 families, and lengthy negotiations were conducted regarding compensation and relocation.
On the shore of Lake Kanna stands the Bōkyō no Hi monument, erected to express gratitude and remembrance for the residents who were forced to leave their homes. The names of those who relocated are engraved on the monument, quietly preserving an important chapter in the history of the local community.
Shimokubo Dam works together with other dams in the upper Tone River basin to substantially reduce downstream flood flows.
At the dam site, it has the capacity to reduce a flow of approximately 2,000 cubic meters per second to 500 cubic meters per second, contributing significantly to flood safety downstream.
The dam supplies large quantities of water to Tokyo and Saitama Prefecture, supporting both urban life and industrial activities.
Water transported through the Tone-Ozeki Weir is distributed through the Musashi and Asaka water channels as part of the extensive water resource network serving the greater metropolitan area.
Shimokubo also contributes to renewable energy production. Facilities include the Shimokubo Power Station, operated by the Gunma Prefectural Government’s enterprise bureau, with an output of 15,000 kilowatts.
A second facility, the Shimokubo No. 2 Power Station, also uses water released to maintain downstream river flow to generate electricity.
Immediately downstream of the dam lies Sanbaseki Gorge, a designated National Place of Scenic Beauty and Natural Monument.
The gorge is known for its impressive formations of giant and unusual rocks made of crystalline schist. Among its highlights is a group of distinctive stones traditionally known as the “48 Rocks of Sanbaseki Gorge.”
For a period after the dam was constructed, the section of the river directly below the dam became dry because of the reduced natural flow. Since 2001, however, water has been released to maintain the downstream river environment, helping restore the clear stream.
A riverside park has also been developed, allowing visitors once again to enjoy the beauty of the gorge and its flowing water.
Sanbaseki stone is a designated Natural Monument, and unauthorized removal or collection of the stones is prohibited.
Located on a hill overlooking Lake Kanna, Jōmine Park is filled with flowers from spring through autumn.
It is particularly famous for its winter cherry blossoms, which bloom from October through December. The unusual combination of autumn scenery and cherry blossoms makes the park a popular seasonal attraction.
Ocho-ga-Ana is a legendary rock formation with a cave-like opening rising from Lake Kanna. A tragic love story has been passed down in connection with the site.
The quiet rock formation standing above the lake creates a mysterious and atmospheric scene.
In spring, approximately 800 carp streamers, or koinobori, are suspended above the Kanna River, creating a spectacular seasonal sight.
The festival also features sales of local specialties and community events and attracts many visitors.
Held at Jōmine Park during the autumn season, this festival features stalls selling local dishes and regional specialties.
The event is also a popular stop on sightseeing routes from the Tokyo area.
By car, Shimokubo Dam can be reached from the Fujioka Interchange on the Joshinetsu Expressway via Prefectural Route 13 and National Route 462. The area can also be accessed from the Honjō-Kodama Interchange.
By public transportation, take a local bus from Shinmachi Station on the JR Takasaki Line or Gunma-Fujioka Station on the JR Hachikō Line.
Shimokubo Dam and Lake Kanna combine the functions of major infrastructure with the beauty of a mountain landscape shaped by water and nature.
The unusual L-shaped dam structure, peaceful lakeshore scenery, and clear waters of Sanbaseki Gorge offer visitors a fascinating combination of engineering, nature, history, and local culture.
Learning about the dam’s roles in flood control and water supply, as well as the restoration of the downstream environment, provides a deeper understanding of how this major infrastructure project has developed alongside the surrounding communities.
With beautiful seasonal scenery and a rich history behind its construction, Shimokubo Dam and Lake Kanna offer an opportunity to experience both the grandeur of modern engineering and the natural landscapes of southern Gunma.