What Is the Yokoyu River Dam?
The Yokoyu River Dam (横湯川堰堤, Yokoyugawa Entei) is a gravity-type concrete erosion-control dam built in 1944 on the Yokoyu River where it runs through Shibu Onsen in Yamanouchi Town, Nagano Prefecture, and registered as a Registered Tangible Cultural Property of Japan on October 14, 2021. It is 39 meters long and 3.8 meters high and is owned by Nagano Prefecture.
It is the only one of the eight registered Yokoyu River dams without a number. It stands about 140 meters above No. 51 and about 120 meters below No. 39, wedged between two dams the Home Ministry built in the Taisho era. It is also the youngest and the tallest of the eight.
A Dam Added Between Older Ones
The Agency for Cultural Affairs explains that the Yokoyu River Dam was placed between dams built by the Home Ministry in order to strengthen the stability of the riverbed. No. 51 below it dates from 1925 and No. 39 above it from 1924; the Yokoyu River Dam came about 20 years later to fill the gap.
On a river controlled by a chain of low weirs, the further apart two weirs sit, the more easily the bed between them is generally scoured. Adding a dam in the roughly 260 meters separating No. 51 and No. 39 broke that stretch into shorter steps. At 3.8 meters it is the highest of the eight, a little above the 3.6-meter No. 39 just upstream.
The project had passed from the Home Ministry to Nagano Prefecture in 1932, so this is a prefectural dam. Its completion in 1944 fell in the middle of the war.
A Water Channel Through the Wing
The other thing that sets the Yokoyu River Dam apart is a water channel cut through the wing on its right bank. The wings are the ends of the dam body that tie into the banks on either side of the spillway notch. The agency describes the dam as a disaster-prevention structure that also serves a water-supply purpose.
A sabo dam exists to catch sediment and steady the riverbed, not to store or deliver water. The Yokoyu River Dam builds a water route into its body as well, combining flood protection and water use in one structure. Of the agency's descriptions of the eight dams, only this one mentions a water channel.
Why the Yokoyu River Dam Was Registered
The Yokoyu River Dam (registration number 20-0595) was registered for its contribution to the historic landscape of the country, together with the other seven dams in the series. It records how the Taisho-era Home Ministry dams were later reinforced, it keeps its wing channel, and it represents the final stage of the prefectural works.
Seeing the Yokoyu River Dam
The Yokoyu River Dam is an outdoor structure with no admission fee, seen from the roads and bridges nearby. Both official photographs, front and back, were taken from the right bank, the side where the water channel runs through the wing. No. 51 downstream and No. 39 upstream are each only a few minutes away on foot, so you can compare the Taisho-era stone dams with this 1944 one in a single short walk. There is no signposted route to the riverbed; keep away from the water when it is high. Directions to Shibu Onsen are on the Yokoyu River Stone Masonry Dams page.
- 📍 Plan Your Visit to Nagano
- Find tours, activities & transport passes near Yokoyu River Dam
- [ Matsumoto Castle & Kamikochi Hiking Tours ]
Q&A
- When was the Yokoyu River Dam built?
- In 1944. It is the youngest of the eight registered Yokoyu River dams.
- Why does the Yokoyu River Dam have no number?
- No published source explains why. It was added later between two Home Ministry dams (Nos. 51 and 39), and its registered name is simply Yokoyu River Dam.
- How big is the Yokoyu River Dam?
- It is 39 meters long and 3.8 meters high, the tallest of the eight.
- What is the water channel in the Yokoyu River Dam?
- A channel cut through the right-bank wing of the dam, which gives the structure a water-supply role in addition to erosion control. The Agency for Cultural Affairs descriptions mention no such channel for the other seven dams.
- Can I visit the Yokoyu River Dam?
- Yes. It can be seen from the roads and bridges nearby at no charge. Keep away from the river when the water is high.
Basic Information
| Name | Yokoyu River Dam (Yokoyugawa Entei) |
|---|---|
| Location | Andai to Muranishi, Hirao, Yamanouchi Town, Shimotakai District, Nagano Prefecture |
| Designation | Registered Tangible Cultural Property (Structure) |
| Year Designated | 2021 (registered) |
| Quantity | 1 dam |
| Dimensions | Crest length 39 m, height 3.8 m (gravity-type concrete, water channel in the right-bank wing) |
| Owner | Nagano Prefecture |
| Public Access | Outdoor erosion-control structure; viewable from roads and bridges (no admission fee) |
References
- Database of National Cultural Properties (Agency for Cultural Affairs): Yokoyu River Dam
- https://kunishitei.bunka.go.jp/bsys/maindetails/101/00014008
- Cultural Heritage Online: Yokoyu River Dam (Japanese)
- https://online.bunka.go.jp/heritages/detail/543048
- Nagano Prefecture: Overview of Erosion Control by River System (PDF, Japanese)
- https://www.pref.nagano.lg.jp/sabo/siryou/documents/04_4suikeisabo.pdf
- Nagano Prefecture blog: Ceremony marking the registration of the Yokoyu River Stone Masonry Dams (Japanese)
- https://blog.nagano-ken.jp/hokushin/n012/27539.html
Last verified: 2026.10.01
