What Is the Yokoyu River No. 51 Dam?
The Yokoyu River No. 51 Dam (横湯川第五一号堰堤, Yokoyugawa Dai-gojūichigō Entei) is a gravity-type concrete erosion-control dam built by the Home Ministry in 1925 and altered in 1932 on the Yokoyu River in Yamanouchi Town, Nagano Prefecture, and registered as a Registered Tangible Cultural Property of Japan on October 14, 2021. It is owned by Nagano Prefecture.
It stands about 170 meters above No. 3, fourth from the bottom of the series. From here upstream, Home Ministry dams and the wartime Yokoyu River Dam alternate.
Long, Low and Rough-Faced
The Yokoyu River No. 51 Dam is 52 meters long but only 1.8 meters high. It is the lowest of the eight and the second longest after No. 57: a single shallow step drawn right across the river, with the water spreading thin as it spills over the crest.
According to the Agency for Cultural Affairs, the face is built of irregular fieldstone. Stones of every shape and size are fitted together, which gives it a very different texture from the matched cobbles of No. 2 downstream or the squared corner stones of No. 50 upstream. It keeps the rugged look typical of the Home Ministry dams.
One of Four Dams Guarding the Wago Bridge
The agency notes that in the Home Ministry's Yokoyu sabo works, the Yokoyu River No. 51 Dam was one of four dams built first, to protect the area around the Wago Bridge where houses were most densely packed. The Wago Bridge crosses the river at the heart of Shibu Onsen, with inns and homes crowded around it. The national project that started in 1918, in other words, began where people lived closest to the water.
Nagano Prefecture's records likewise say the ministry's plan put four large boulder-masonry dams on the lower river. No. 51 covers the stretch that the Taisho-era works treated as most urgent.
The alteration year, 1932, is also the year the project passed from the Home Ministry to the prefecture. What the alteration involved is not recorded in published sources.
Why the Yokoyu River No. 51 Dam Was Registered
The Yokoyu River No. 51 Dam (registration number 20-0594) was registered for its contribution to the historic landscape of the country. It preserves the character of the Home Ministry's direct works of the Taisho era and has protected the center of the hot spring town ever since.
Seeing the Yokoyu River No. 51 Dam
The Yokoyu River No. 51 Dam is an outdoor structure with no admission fee, viewed from the roads and bridges nearby. The official photographs show the front from the right bank and the back from the left bank. Because it is so low, the fitting of the fieldstone is easiest to see when the river is running light. About 140 meters upstream is the 1944 Yokoyu River Dam, so you can compare Taisho-era stonework with a wartime dam in a few minutes' 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 No. 51 Dam
- [ Matsumoto Castle & Kamikochi Hiking Tours ]
Q&A
- When was the Yokoyu River No. 51 Dam built?
- The Home Ministry built it in 1925, and it was altered in 1932.
- How big is the Yokoyu River No. 51 Dam?
- It is 52 meters long and 1.8 meters high, the lowest of the eight registered Yokoyu River dams.
- What is the face of the Yokoyu River No. 51 Dam made of?
- Irregular fieldstone, with stones of varied shape and size fitted together.
- Why was the Yokoyu River No. 51 Dam among the first to be built?
- It was one of four dams the Home Ministry built first to protect the area around the Wago Bridge, where houses were most densely clustered.
- Can I visit the Yokoyu River No. 51 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 No. 51 Dam (Yokoyugawa Dai-gojūichigō 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 52 m, height 1.8 m (gravity-type concrete, faced with fieldstone) |
| 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 No. 51 Dam
- https://kunishitei.bunka.go.jp/bsys/maindetails/101/00014007
- Cultural Heritage Online: Yokoyu River No. 51 Dam (Japanese)
- https://online.bunka.go.jp/heritages/detail/520357
- Nagano Prefecture: Overview of Erosion Control by River System (PDF, Japanese)
- https://www.pref.nagano.lg.jp/sabo/siryou/documents/04_4suikeisabo.pdf
Last verified: 2026.10.01
