Tone River Weir is a large-scale weir constructed across the main channel of the Tone River on the border between Chiyoda Town in Gunma Prefecture and Gyoda City in Saitama Prefecture. Located approximately 154 kilometers upstream from the mouth of the Tone River, it is one of Japan’s major water resource facilities.
The weir was completed in 1968 to help address water shortages in the Tokyo metropolitan area. Today, it supplies water for domestic and industrial use to areas including Tokyo and Saitama, while also providing agricultural water to extensive farmland in Gunma and Saitama. It plays an essential role in supporting the daily lives, agriculture, and industries of the metropolitan region.
Tone River Weir is also more than an important water infrastructure facility. The surrounding area is popular for nature observation, fishing, and water sports. Visitors can learn about the river environment while enjoying the scenery and recreational opportunities along the Tone River.
The construction of Tone River Weir was prompted by the rapid increase in water demand in Tokyo during Japan’s period of rapid economic growth in the 1960s. As the population grew and urbanization progressed, Tokyo faced increasingly serious water shortages.
To address this problem, Japan’s Ministry of Construction, now the Ministry of Land, Infrastructure, Transport and Tourism, formulated the Tone River Water Diversion Project in 1963. The project aimed to develop a large-scale water resource system that would transport water from the Tone River to the Tokyo metropolitan area.
In 1964, just before the Tokyo Olympics, Tokyo experienced a serious drought. The severe water shortage became widely known as the “Tokyo Desert”, highlighting the urgent need to secure a stable water supply for the rapidly growing city.
In response, canals and water intake facilities were rapidly developed to divert water from the Tone River toward the Arakawa River. In 1968, Tone River Weir was completed as a key facility for ensuring a stable supply of water from the Tone River.
The weir uses large gates that can be raised and lowered to regulate the water level. Water is then diverted toward the Arakawa River through the Musashi Canal. This system has helped provide a stable supply of water for households, agriculture, and other uses throughout the metropolitan region.
Tone River Weir is not simply a structure that holds back water. It functions as a multipurpose water intake facility, supplying water for a variety of purposes. Water collected here is distributed through canals and other water infrastructure, supporting people’s daily lives and regional industries.
Water taken from the Tone River at the weir is used as tap water in Tokyo, Saitama, and Gunma. The system currently provides a water source supporting the daily lives of approximately 16.7 million people.
Approximately 40 percent of Tokyo’s tap water and around 70 percent of Saitama’s tap water depend on water from the Tone River system, illustrating the river’s importance to the metropolitan water supply.
Water is also supplied to factories and businesses in Saitama Prefecture. Approximately 100 businesses use this industrial water, which is essential for manufacturing and other industrial activities.
Water from Tone River Weir is also distributed to extensive agricultural areas in Gunma and Saitama. The agricultural water supply covers approximately 23,300 hectares of farmland across 28 municipalities.
This water supports a wide range of agricultural activities, including rice cultivation and vegetable production.
Water from the Tone River has also contributed to improving the water quality of the Sumida River, where pollution had once been a serious concern.
By releasing surplus water and maintaining river flow, the water management system helps improve the aquatic environment and contributes to maintaining healthier river conditions.
Since beginning operation in 1968, Tone River Weir has supplied water continuously for more than half a century. On June 20, 2024, its cumulative water intake reached an extraordinary 100 billion tons.
This milestone reflects the facility’s long-standing role in supporting the lives and industries of the Tokyo metropolitan area. Today, water management continues around the clock, with operators adjusting water intake and distribution in real time according to demand and river conditions.
The Tone River is known as the southernmost major river on the Pacific side of Japan where salmon naturally migrate upstream. However, the number of salmon returning upstream declined from the 1970s onward due to changes in the river environment and other factors.
In response, the Japan Water Agency and local organizations began conservation efforts. Research and salmon release programs were carried out from the 1980s, and the fishways that allow fish to migrate upstream were extensively renovated between 1995 and 1997.
Following these efforts, the number of salmon migrating upstream gradually recovered. In 2005, more than 2,000 salmon were observed near Tone River Weir. The number of ayu sweetfish migrating upstream has also increased, indicating gradual improvements in the river environment.
Tone River Weir has a facility called the “Ozeki Nature Observation Room,” where visitors can observe fish migrating upstream at close range. The observation room is located beside a fishway and features windows that allow visitors to see what is happening underwater.
In particular, visitors may be able to watch ayu sweetfish migrating upstream in early summer. In autumn, there may also be opportunities to observe salmon making their way upstream.
The facility provides a rare opportunity to observe the natural life of river fish at close range. It is suitable for both children and adults and offers an enjoyable way to learn about the Tone River ecosystem.
The observation room is open free of charge from the morning until the afternoon and can be visited on weekends and public holidays. Nearby, the riverside area known as “Uruoi no Kishibe” and an information plaza provide additional places where visitors can walk and learn about the river environment.
Upstream from Tone River Weir, a broad stretch of open water provides an attractive setting for water sports such as windsurfing and jet skiing. On holidays, many enthusiasts visit the area to enjoy the expansive water surface and refreshing riverside environment.
The areas upstream and downstream of the weir also contain numerous fishing spots. Anglers come to the Tone River to target fish such as ayu sweetfish and black bass, with different types of fishing available depending on the season.
However, fishing is prohibited within designated areas extending approximately 160 meters upstream and 200 meters downstream of the weir. These restrictions are intended to protect fishery resources and maintain the river environment.
A road used for management and transportation runs across the top of Tone River Weir. Known as Musashi Ohashi Bridge, it is approximately 687 meters long and was completed in 1968.
The bridge carries Prefectural Route Ashikaga-Ora-Gyoda, an important regional road connecting Tochigi, Gunma, and Saitama prefectures. Because there are relatively few bridges crossing the Tone River in this area, Musashi Ohashi serves as an important transportation link for local communities.
The area around Tone River Weir has been developed not only as an important water resource facility but also as a place for environmental education and nature observation.
At the Tone Water Conveyance Comprehensive Management Office, visitors can learn about how the Tone River water diversion system works and why water resources are important. Exhibits provide an opportunity to better understand the infrastructure that supports the lives of millions of people.
Visitors can also enjoy views of the broad Tone River, observe migrating fish, and experience the riverside natural environment. The area is particularly suitable for educational visits, family outings, and leisurely walks.
Tone River Weir is both a massive piece of infrastructure supporting everyday life in the Tokyo metropolitan area and a place where visitors can discover the relationship between water resources, people, and nature. Its combination of engineering, environmental conservation, and riverside scenery makes it an interesting destination when exploring the Tone River region.