Yokoyugawa Dam: A Wartime Concrete Sabo Structure Above Yudanaka-Shibu Onsen
Walk upstream along the Yokoyu River from the Wagō Bridge in Shibu Onsen and the valley narrows quickly. Stone-faced dams step the riverbed at intervals — some heavily mossed, others showing crisp cut stone — and among them stands a low, grey concrete wall, thirty-nine meters across and just three point eight meters tall. This is Yokoyugawa Dam (横湯川堰堤), completed in 1944 and registered by Japan's Agency for Cultural Affairs as a Tangible Cultural Property in October 2021.
It belongs to a larger network: the Yokoyu River Stone-Masonry Dam Group, a chain of erosion-control structures (砂防, sabo) built between 1918 and 1945 to defend the villages and hot-spring resorts downstream. Yokoyugawa Dam itself sits about 140 meters upstream of the No. 51 Dam, slotted into a gap between two earlier structures built by the prewar Home Ministry.
A river that needed taming
The Yokoyu is a steep mountain stream flowing out of the Shiga Highlands. It joins the Yomase River, which in turn empties into the Shinano — Japan's longest. For centuries this watershed had a grim reputation. Debris flows during summer rains and snowmelt repeatedly tore through the riverside settlements, destroying farmland and threatening the bathhouses that had drawn pilgrims and travelers since before the Edo period.
Erosion-control work began in earnest during the Meiji era, but the great floods of 1910 and 1911 swept away nearly everything Nagano Prefecture had built. The prefecture suspended its sabo program, and the river was left to its own devices for several years. Residents pleaded for help. In 1918 the Home Ministry took direct charge of the Yokoyu, and over the next sixteen years constructed a series of stone-faced dams along the lower and middle reaches.
From 1932 the prefecture resumed its own work upstream, continuing until the program was suspended in 1945. This second prefectural phase is locally known as Akagi sabo, after the engineer who designed it.
Akagi Masao and the meaning of "sabo"
Dr. Akagi Masao (赤木正雄, 1887–1972) is remembered as the father of Japanese erosion control. A graduate of the forestry program at Tokyo Imperial University, he joined the Home Ministry in 1914 and worked on the rivers of Shiga and Nara before traveling at his own expense to Vienna in 1923. Two years at the University of Natural Resources convinced him that Japan's mountain rivers needed a different approach: instead of holding back sediment far downstream, engineers should stabilize the streambed in successive low steps from headwaters down.
The Yokoyu dam group is one of the clearest surviving illustrations of this idea in practice. Each structure traps a small amount of gravel behind it, which raises the upstream bed and gentles the gradient — so the next dam, a hundred or so meters higher, has an easier job. The sequence functions less like a single barrier than like a long staircase, allowing the river to move without ever moving fast enough to do damage.
The Japanese term Akagi championed, sabo, was so closely associated with this layered method that it entered international engineering vocabulary in 1951; the word now appears unchanged in technical papers worldwide.
Reading the dam
Yokoyugawa Dam is a gravity-type structure cast in concrete — its weight alone holds back the water. The face is plain compared with some of its older stone-built neighbors, but a few details reward close attention. On the right bank, an opening has been cut through the abutment to carry off irrigation water. This double duty, flood defense and water supply combined, was characteristic of wartime sabo planning, when materials were short and every structure was expected to earn its keep.
Notice also how the dam fits into the gap between two earlier Home Ministry dams. Surveyors of the early 1940s evidently judged that the riverbed between those structures was beginning to scour out, threatening their foundations. Yokoyugawa Dam was inserted to lock the bed in place — a small but essential repair to a system already a quarter-century old.
The crest is straight, the wings short, the apron unembellished. There is no decorative cut-stone shoulder here as on the more photogenic No. 50 Dam down in the village. Yet the modesty is part of the point: a working piece of infrastructure that has done its job through eight decades of typhoons and snowmelt floods.
Why it was registered
When the Agency for Cultural Affairs registered the Yokoyu group in 2021, the citation emphasized three points. First, the dams form an unusually complete record of prewar Japanese sabo: a continuous sequence from the early Home Ministry projects through the Akagi-era prefectural work. Second, they are still functional today, having continued to absorb and slow the river's debris flows over more than eight decades. Third, the dams have become part of the landscape that visitors come to see, framing the Yokoyu River as it runs through the historic onsen towns.
A commemorative monument and signboard were unveiled at Yamabiko Plaza in Yamanouchi on September 12, 2024, with about thirty guests in attendance, marking the public recognition of the group.
Visiting the dam
Yokoyugawa Dam sits in the Andai (安代) area, one of the nine hot-spring districts that together make up Yudanaka-Shibu Onsen-kyo. The riverbank can be glimpsed from National Route 292 as it climbs toward Shiga Kogen, but the best vantage is from the road or pedestrian paths in Andai itself. There is no formal observation point at this particular dam — it is, after all, a piece of working infrastructure rather than a tourist attraction — and approaching the channel directly is not recommended, especially after rain.
Pair the visit with a walk along the Yokoyu. The downstream No. 50 Dam, with its cut-stone shoulders and the Wagō Bridge (和合橋) at the center of Shibu Onsen, is more visually striking and the easiest of the group to photograph. Further upstream, in the Jigokudani area, the river meets the famous Snow Monkey Park; the trail to the park crosses several smaller sabo structures along the way.
What else to see nearby
Andai is sandwiched between Yudanaka and Shibu, the two most historic of the nine onsen districts. Shibu is famous for its kuyu-meguri, a circuit of nine public baths reserved mostly for guests staying overnight in the village — local custom holds that completing all nine washes away the "nine sufferings" (ku, a pun on both "nine" and "hardship"). The wooden Kanaguya inn, with its 1936 four-story bathhouse wing, is itself a registered cultural property and worth a look from the street even if you are not staying there.
Twenty minutes' walk further up the valley brings you to Jigokudani Yaen-koen, where Japanese macaques have been bathing in a hot spring since the 1960s. In winter the snow-monkeys are the area's signature image. From the same trailhead, the old hiking paths through Shiga Highlands branch off, opening up an entire upland of beech forest, ski runs, and small mountain lakes.
For something quieter, the Old Yudanaka Station building (1927), a registered cultural property in its own right, sits twenty minutes' walk away in central Yudanaka and now serves as a small community space and footbath.
- 📍 Plan Your Visit to Nagano
- Find tours, activities & transport passes near Yokoyugawa Dam
- [ Matsumoto Castle & Kamikochi Hiking Tours ]
Q&A
- What exactly is a "sabo" dam?
- A sabo dam is a small barrier designed not to store water but to trap and slow the gravel and debris that mountain rivers carry. The word entered international use in 1951 and is now standard in engineering English.
- Can I walk up to the dam?
- The dam can be viewed from public roads and footpaths in the Andai area. There is no dedicated viewing platform, and entering the river channel itself is not safe, particularly during or after heavy rain.
- How do I reach Yamanouchi from Tokyo?
- Take the Hokuriku Shinkansen to Nagano (about 1 hour 20 minutes), then change to the Nagano Dentetsu line for Yudanaka, the terminus (about 50 minutes by limited express). From Yudanaka, buses bound for Kanbayashi or Shiga Kogen stop in Andai and Shibu.
- When is the best time of year to visit?
- Late October to early November offers the most dramatic combination — autumn foliage along the Yokoyu, steam rising from the bathhouses, and snow already on the higher peaks. Winter brings deep snow and the famous monkeys; summer is cool and green.
- Are the other dams in the group accessible too?
- Several are visible from the road. The No. 50 Dam at the Wagō Bridge in Shibu is the most photographed, with cut-stone wings flanking the spillway. The No. 1 Dam, at the downstream end of the group, can be seen from the bridges below Shibu.
Basic Information
| Name | Yokoyugawa Dam (横湯川堰堤, Yokoyugawa Entei) |
|---|---|
| Designation | Registered Tangible Cultural Property (Structure), Agency for Cultural Affairs |
| Registration Date | October 14, 2021 (Reiwa 3) |
| Year of Construction | 1944 (Shōwa 19) |
| Structure | Gravity-type concrete sabo dam |
| Dimensions | Length 39 m, Height 3.8 m |
| Count | 1 structure |
| Location | Aza Andai to Aza Murayanishi, Ōaza Hibon, Yamanouchi Town, Shimotakai District, Nagano Prefecture |
| Owner | Nagano Prefecture |
| Group Affiliation | One of the Yokoyu River Stone-Masonry Dam Group (横湯川石積堰堤群) |
| Access | About 15 minutes by car from the Shinshū Nakano IC on the Jōshin'etsu Expressway; or a short ride from Yudanaka Station (Nagano Dentetsu line) |
| Viewing | Visible from public roads in the Andai area; no formal observation facility |
References
- Yokoyugawa Dam — Cultural Heritage Online (Agency for Cultural Affairs)
- https://bunka.nii.ac.jp/heritages/detail/543048
- National Database of Designated Cultural Properties — Yokoyugawa Dam entry
- https://kunishitei.bunka.go.jp/bsys/maindetails/101/00014008
- Yokoyu River Stone-Masonry Dam Group Registration Ceremony — Nagano Prefecture Hokushin Construction Office Blog
- https://blog.nagano-ken.jp/hokushin/n012/27539.html
- Cultural Properties of Yamanouchi Town — Yamanouchi Town Official Site
- https://www.town.yamanouchi.nagano.jp/soshiki/bunkasozo/gyomu/rekishi_bunkazai/bunkazai/909.html
- Akagi Masao — National Sabo Association
- https://www.sabo.or.jp/kyo-annnai/akagimasao.htm
- Yudanaka-Shibu Onsen-kyo Area — Snow Monkey Town Yamanouchi Official Site
- https://www.info-yamanouchi.net/area/yuda/
Last verified: 2026.05.16