What Is the Yokoyu River No. 1 Dam?
The Yokoyu River No. 1 Dam (横湯川第一号堰堤, Yokoyugawa Dai-ichigō Entei) is a gravity-type concrete erosion-control dam built in 1938 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 48 meters long and 3.0 meters high, and it belongs to Nagano Prefecture.
Of the eight registered dams on this stretch of the river, No. 1 is the one furthest downstream. Below it the Yokoyu runs into the Yomase River and on into the Shinano River system, so this is the last barrier the water meets after leaving the Shibu Onsen valley.
A Concrete Core with a Stone Face
The Agency for Cultural Affairs describes the Yokoyu River No. 1 Dam as a gravity dam of concrete with mortared stone masonry. "Gravity" means the structure resists the push of gravel and water behind it by sheer weight. The core is concrete; the visible faces are stones set in mortar, a technique known in Japanese as Nerizumi.
Pairing a concrete core with a mortared stone skin is typical of the prefectural works of the early Showa era. The Home Ministry dams further up the river still read as heaps of big boulders, while No. 1 presents a tidier, more even wall of stone across its full 48 meters. At 3.0 meters it is not built to block the river. Its job is to hold one step in the riverbed.
Despite the number, it was not the first dam built. No. 3 upstream was finished in 1936 and No. 2 in 1937; No. 1 followed a year later. In other words, the three prefectural dams were added one after another working down the river.
Why the Yokoyu River No. 1 Dam Was Registered
The Yokoyu River No. 1 Dam (registration number 20-0591) was listed under the criterion of contributing to the historic landscape of the country. The agency's description presents it as an erosion-control work built by Nagano Prefecture, carrying on from the Home Ministry's project, to protect the riverside houses and hot spring village from the debris flows that struck them again and again.
After the national government handed the work to the prefecture in 1932, the prefecture concentrated on securing the lower river. No. 1 was the last and lowest of those prefectural dams, and it closes off the bottom end of the series.
Seeing the Yokoyu River No. 1 Dam
The Yokoyu River No. 1 Dam is an outdoor flood-control structure with no interior and no admission charge; you view it from the roads and bridges nearby. The official registration photographs show the front from the left bank and the back from the right bank. No. 2 stands about 200 meters upstream, and looking at the two in turn is the easiest way to see how differently their stones are laid. No path down to the riverbed is signposted, so stay away from the water after heavy rain. Directions to Shibu Onsen and a suggested walking order for all eight dams are on the Yokoyu River Stone Masonry Dams page.
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- Find tours, activities & transport passes near Yokoyu River No. 1 Dam
- [ Matsumoto Castle & Kamikochi Hiking Tours ]
Q&A
- When was the Yokoyu River No. 1 Dam built?
- In 1938, during the period when Nagano Prefecture ran the project. Although it is numbered first, it was completed after No. 3 (1936) and No. 2 (1937) upstream.
- Where is the Yokoyu River No. 1 Dam?
- On the lowest reach of the Yokoyu River in Yamanouchi, Nagano. It is the furthest downstream of the eight registered dams, just above the point where the Yokoyu joins the Yomase River.
- How big is the Yokoyu River No. 1 Dam and how is it built?
- It is 48 meters long and 3.0 meters high, a gravity-type concrete dam faced with mortared stone masonry.
- Why is the Yokoyu River No. 1 Dam a Registered Tangible Cultural Property?
- It is a prefectural erosion-control work that protected the village and hot spring town from debris flows, and it was registered on October 14, 2021, for its contribution to the historic landscape.
- Can I visit the Yokoyu River No. 1 Dam?
- Yes. You can see it from the roads and bridges nearby at no charge. There is no signposted route to the riverbed, so keep away from the water when the river is high.
Basic Information
| Name | Yokoyu River No. 1 Dam (Yokoyugawa Dai-ichigō Entei) |
|---|---|
| Location | Murakami-Nakamichi-Minami to Kurokawa, Hirao, Yamanouchi Town, Shimotakai District, Nagano Prefecture |
| Designation | Registered Tangible Cultural Property (Structure) |
| Year Designated | 2021 (registered) |
| Quantity | 1 dam |
| Dimensions | Crest length 48 m, height 3.0 m (gravity-type concrete, mortared stone masonry) |
| 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. 1 Dam
- https://kunishitei.bunka.go.jp/bsys/maindetails/101/00014004
- Cultural Heritage Online: Yokoyu River No. 1 Dam (Japanese)
- https://online.bunka.go.jp/heritages/detail/520154
- 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
