The Lowest of the Four Ashizawa Dams
The Ashizawa No. 1 Stone Masonry Dam (芦澤第一号石積堰堤, Ashizawa dai-ichigō ishizumi entei) is a dry-laid stone sabo dam built in 1887 across the Ashizawa stream in Asa, Omi Village, Higashichikuma District, Nagano Prefecture, and it was entered on the national register as a Registered Tangible Cultural Property on October 7, 2014. Its registration number is 20-0455, and the dam belongs to Nagano Prefecture.
Of the four Meiji-era stone dams that survive on the stream, No. 1 sits furthest downstream. No. 2 stands 90 metres above it, and the numbers climb as you walk up the valley.
The Ashizawa No. 1 Stone Masonry Dam is 10 metres long at the crest and 4.2 metres high, the shortest crest of the four but the second tallest after No. 4. It is built of fieldstones up to about 1 metre across, laid back at a gentle batter of 1 to 1.5, meaning the face recedes 1.5 units horizontally for every unit of height. In cross-section it is less a wall than a steep bank of stone.
Why It Was Registered
The dam was registered for contributing to the historic landscape of the country. The Agency for Cultural Affairs places it in the Omi River basin of the Shinano River system and notes that it keeps its original form well, adding to the historic landscape of erosion control in the Shinano headwaters.
That point about original form matters. More than 130 years on, the stones still hold without mortar, set in a dry-stone technique called mutsumaki, in which each boulder is wedged so that the whole mass stands by itself. Nagano Prefecture's Saigawa Sabo Office says the method took a high level of skill and would be difficult to repeat today.
The Agency also points to a profile shared by all four Ashizawa dams: a central spillway notch set off from the wing sections by clean straight lines. Because Ashizawa No. 1 is the first of the series that walkers reach, it is where most people meet that modern outline.
What to Look For
Start with the stones. The largest at the Ashizawa No. 1 Stone Masonry Dam reach about 1 metre across, rounded boulders left in their natural shape and fitted together, with smaller stones packed into the gaps. Step back and the jumble resolves into a single sloping surface.
Then look at the dam side-on. A batter of 1 to 1.5 is gentler than the 1 to 1.3 recorded for No. 3, which is why a structure 4.2 metres high reads less as a barrier to look up at than as a ramp of stone laid across the stream.
Finally, follow the top edge. The spillway in the middle is cut lower than the wings, and the change of level comes at sharp, straight breaks. The crest is also laid on a curve, so shift your position along the path to see how the line bends.
Visiting No. 1
The Ashizawa No. 1 Stone Masonry Dam is an open-air structure with no fee and no booking. Walking up the streamside path from the bottom, it is the first of the four stone dams you come to. A Nagano Prefecture blog notes that turning north off Prefectural Route 12 at the Omi Shinrin Kensetsu company building brings a car close to No. 1, but there is nowhere to park beside the dam.
View it from the path rather than climbing onto the face or into the spillway. The prefectural materials state no restrictions on photography. Directions to the valley, parking and the layout of the full walk are covered on the Ashizawa Stone Masonry Dams page.
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Q&A
- When was the Ashizawa No. 1 Stone Masonry Dam built and registered?
- It was built in 1887 as part of a Home Ministry sabo project and registered as a Registered Tangible Cultural Property on October 7, 2014, under registration number 20-0455.
- How big is the Ashizawa No. 1 Stone Masonry Dam?
- The Agency for Cultural Affairs records it as 10 metres long and 4.2 metres high, built of fieldstones up to about 1 metre across laid at a gentle batter of 1 to 1.5.
- Where is the Ashizawa No. 1 Stone Masonry Dam among the four dams?
- It is the lowest of the four on the Ashizawa stream. No. 2 stands 90 metres upstream.
- Can I visit the Ashizawa No. 1 Stone Masonry Dam freely?
- Yes. It is an open-air sabo structure with no fee or booking. Nagano Prefecture lists no visiting conditions and recommends spring to autumn. For safety, view it from the path.
- How was the Ashizawa No. 1 Stone Masonry Dam built?
- Undressed fieldstones were laid without mortar in a dry-stone technique called mutsumaki. The Saigawa Sabo Office says it took a high level of skill and would be hard to repeat today.
Basic Information
| Name | Ashizawa No. 1 Stone Masonry Dam (芦澤第一号石積堰堤) |
|---|---|
| Location | Aza Ashizawa, Asa, Omi Village, Higashichikuma District, Nagano Prefecture |
| Designation | Registered Tangible Cultural Property (Structure) |
| Year Designated | Registered 2014/10/7 (Heisei 26), registration no. 20-0455 |
| Quantity | 1 structure |
| Dimensions | Crest length 10 metres, height 4.2 metres |
| Owner | Nagano Prefecture |
| Public Access | Open-air sabo structure; free access; recommended season spring to autumn (Nagano Prefecture sabo division) |
References
- Ashizawa No. 1 Stone Masonry Dam (Database of National Cultural Properties, Agency for Cultural Affairs)
- https://kunishitei.bunka.go.jp/bsys/maindetails/101/00010145
- Ashizawa No. 1 Stone Masonry Dam (Cultural Heritage Online)
- https://online.bunka.go.jp/heritages/detail/232758
- Ashizawa No. 2 Stone Masonry Dam (Database of National Cultural Properties, Agency for Cultural Affairs)
- https://kunishitei.bunka.go.jp/bsys/maindetails/101/00010146
- Ashizawa No. 3 Stone Masonry Dam (Database of National Cultural Properties, Agency for Cultural Affairs)
- https://kunishitei.bunka.go.jp/bsys/maindetails/101/00010147
- Ashizawa stone masonry dams, Omi Village (Nagano Prefecture Saigawa Sabo Office)
- https://www.pref.nagano.lg.jp/saisabo/jigyo/ashizawa201403.html
- Modern heritage card No. 2: Ashizawa stone masonry dams (Nagano Prefecture blog, Matsumoto region)
- https://blog.nagano-ken.jp/matsuchi/recommend/viewspots/39197.html
- Civil engineering treasures: sabo facilities (Nagano Prefecture Sabo Division)
- https://www.pref.nagano.lg.jp/sabo/siryou/documents/sono9.pdf
Last verified: 2026.10.06
