History of Aimata Dam
The construction of Aimata Dam was prompted by the devastating damage caused by Typhoon Kathleen in 1947. Following the disaster, comprehensive flood-control measures were considered necessary throughout the Tone River system.
The Flood Control Investigation Committee of the Economic Stabilization Board recommended the construction of multipurpose reservoirs. As a result, a plan was developed to construct nine multipurpose dams throughout the Tone River system, with Aimata Dam being one of them.
Preliminary surveys along the Akaya River began in 1948. In 1952, the project was transferred to Gunma Prefecture as part of the Akaya River Comprehensive Development Project, and full-scale construction of the dam began.
The main structure was completed in 1956. However, during the initial reservoir filling test, water leakage was discovered through bedrock on the left bank. Concrete lining and other measures were carried out to address the problem.
Construction was finally completed in 1959, and Aimata Dam began its role as an important multipurpose facility in the Tone River basin.
Functions and Purposes of Aimata Dam
Aimata Dam serves several important purposes, including flood control along the Akaya River and the Tone River downstream of their confluence, maintaining water supplies at the Kurihashi area of Saitama Prefecture, and hydroelectric power generation.
Flood Control
The dam was designed to reduce the planned flood discharge based on the flooding experienced during Typhoon Kathleen. A planned flow of 650 cubic meters per second is regulated to approximately 330 cubic meters per second.
Through this flood-control function, the dam contributes to reducing the risk of flooding along the downstream river system.
Hydroelectric Power Generation
Aimata Dam also supports hydroelectric power generation. The main facilities include Aimata Power Station, with an authorized output of 7,300 kW, and Momono Power Station, with an authorized output of 6,200 kW.
An additional facility, Aimata No. 2 Power Station, with an output of 120 kW, was later added. These hydroelectric facilities are managed by the Gunma Prefectural Bureau of Public Enterprises.
Lake Akaya and the Surrounding Area
Lake Akaya
The reservoir created by Aimata Dam is known as Lake Akaya. Today, the lake is one of the best-known scenic destinations in the Minakami area and is closely associated with Sarugakyo Onsen.
The original Sarugakyo hot spring area was located along the Akaya River. When the dam was constructed, the former hot spring district was submerged by the reservoir and relocated to the lakeshore.
A new hot spring town was developed around Lake Akaya, and numerous hotels and traditional Japanese inns were established. Today, Sarugakyo Onsen is one of Gunma Prefecture’s well-known mountain hot spring resorts.
Tourism and Access Around Lake Akaya
National Route 17, historically known as the Mikuni Kaido, runs near Lake Akaya. Continuing toward Niigata leads across Mikuni Pass.
The route has been an important connection between the former provinces of Echigo and Kozuke since ancient times. During Japan’s Warring States period, Uesugi Kenshin is said to have traveled this route repeatedly during his campaigns into the Kanto region.
The surrounding area is also close to other popular destinations, including Minakami Onsen, Mount Tanigawa, and the ski resorts of Yuzawa Town in neighboring Niigata Prefecture. This makes Lake Akaya and Sarugakyo a convenient base for exploring the wider mountain region.
Lake Akaya as One of Japan’s 100 Dam Lakes
In 2005, Lake Akaya was selected as one of the “100 Dam Lakes of Japan” by the Dam Reservoir Environmental Development Center, following a recommendation from Minakami Town.
The designation recognizes the scenic and recreational value of dam reservoirs throughout Japan. Lake Akaya is particularly attractive because its calm waters are surrounded by the forests and mountains of northern Gunma.
The Planned Kawako Dam
After Aimata Dam was completed, water-resource development continued throughout the Tone River system in response to the growing population of the Tokyo metropolitan area.
However, water shortages became increasingly frequent during the 1980s, while a trend toward lower rainfall also became more apparent. In response, the former Ministry of Construction promoted an Emergency Water Demand Countermeasure Dam Project designed to include additional capacity for periods of drought.
As part of this initiative, plans were developed for the construction of Kawako Dam upstream on the Akaya River.
Plans for Kawako Dam
Kawako Dam was planned near Kawako Onsen, upstream from Aimata Dam. The proposed reservoir would have had a total storage capacity of approximately 76 million tons.
However, water demand in the Tokyo metropolitan area did not increase as expected, while calls for the review and restructuring of public works projects became stronger. As a result, the Kawako Dam construction plan was ultimately cancelled.
Aimata Dam and the Changing Landscape of Minakami
The history of Aimata Dam is closely connected with the development of Minakami and the wider Tone River basin. The dam transformed the landscape of the Akaya River valley while creating Lake Akaya, which has since become an important scenic and tourism resource.
The relocation of Sarugakyo Onsen from the former riverside site to the shores of Lake Akaya is one of the most visible examples of how the dam changed the local landscape and communities.
A Dam, Lake, and Hot Spring Destination
Aimata Dam plays an important role in flood control, hydroelectric power generation, and water-resource management for the Kanto region. At the same time, the reservoir created by the dam has become an attractive destination in its own right.
From Lake Akaya and Sarugakyo Onsen to the historic Mikuni Kaido and the surrounding mountains, the area offers opportunities to experience both the natural beauty and history of northern Gunma.
Visitors interested in dams, reservoirs, mountain landscapes, hot springs, and the history of Japan’s water infrastructure will find Aimata Dam and Lake Akaya an interesting part of a trip through Minakami.