A 122-Metre Concrete Channel Sheathed in Stone
Follow the headrace canal of the Okutsu Power Station downstream from where it crosses the Kirotani-gawa, and the open water channel disappears under a lid. What continues is the No. 1 Covered Conduit (Ichigo Gaikyo), a straight reinforced-concrete box roughly 122 metres long that carries the plant's working water past a stretch of unstable slope. Its walls do not look like ordinary concrete. Coursed stone, laid in both mortared and diagonal patterns, wraps the outside of the concrete trough, so that from the path the structure reads as a long masonry wall rather than a modern conduit.
The conduit belongs to a run-of-river hydroelectric scheme on the upper Yoshii River, in the Kamisaihara district of Kagamino, northern Okayama. Water diverted far upstream is led along a gently graded canal to a point where it can drop onto the turbines; the covered conduit is one link in that long, almost level watercourse. It was built in 1932 (Showa 7) by Chugoku Godo Denki, a regional utility that later became part of today's Chugoku Electric Power.
Why a Culvert Became a Cultural Property
In October 2006 the conduit was entered on the national register of Tangible Cultural Properties as registration No. 33-0117, together with a large group of dams, aqueducts and other components of the same power station. The criterion applied was that the structure contributes to the historic character of the national landscape. That phrasing matters: these are working pieces of infrastructure valued not as monuments but as an intact record of how a mountain river was engineered for electricity in the early Showa period.
The interest of the covered conduit lies in its hybrid construction. A rectangular concrete section provides the watertight channel, while the stone facing reinforces and protects the side walls where the ground is uneven. Builders of the 1930s combined new reinforced concrete with older masonry craft, and the No. 1 Covered Conduit shows that pairing across an unusual length. Because it survives essentially unaltered, it documents both the material choices and the workmanship of the period.
Reading the Structure on Foot
The conduit sits immediately downstream of the No. 1 Aqueduct, so the two are best seen as a sequence: water crosses a stream on the bridge, then vanishes into the covered channel. Look at how the stones are set. Some courses are laid flat in horizontal bands, while others are pitched on the diagonal in the pattern Japanese masons call tanizumi, a technique that packs stones tightly and sheds water. The join between crisp concrete edges and hand-fitted stone is the detail worth pausing over.
This is a remote, forested part of the Yoshii headwaters near Okutsu Onsen, a hot-spring village. The power-station structures are scattered along the river and its side valleys rather than gathered in one place, and several, including this conduit, stand on or beside private operational land owned by the utility. There is no visitor car park or signage at the conduit itself. Anyone tracing the group should keep to public paths, view the structures from outside, and treat them as the live equipment they still are.
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Q&A
- What exactly is a "covered conduit"?
- It is a section of the canal that is roofed over rather than left open. Here a reinforced-concrete box carries the water, and its side walls are reinforced on the outside with coursed stone. The lid protects the flow and the channel where the hillside above is unstable.
- How long is it, and when was it built?
- It runs about 122 metres in a straight line and dates to 1932. It was one of the components registered as a Tangible Cultural Property in 2006 under No. 33-0117.
- Is the Okutsu Power Station still operating?
- Yes. The scheme remains a working run-of-river plant managed by Chugoku Electric Power, which is why its channels still carry water and are not open for public entry.
- Can I go inside or walk on it?
- No. The conduit is live infrastructure carrying flowing water, and access is restricted. It can be appreciated from adjacent public ground, ideally alongside the neighbouring No. 1 Aqueduct.
- How does it relate to the other Okutsu structures?
- It is one of roughly a dozen registered components strung along the same watercourse, including intake weirs, aqueduct bridges and a confluence basin. Together they form a continuous early-Showa hydroelectric system.
Basic Information
| Name | Okutsu Power Station No. 1 Covered Conduit (Okutsu Hatsudensho Ichigo Gaikyo) |
|---|---|
| Location | Kamisaihara, Kagamino, Tomata District, Okayama Prefecture |
| Designation | Registered Tangible Cultural Property (Structure), No. 33-0117 |
| Registered | 18 October 2006 |
| Built | 1932 (Showa 7) |
| Quantity / Scale | 1 structure; stone and reinforced concrete, length approx. 122 m |
| Owner | The Chugoku Electric Power Co., Inc. |
| Access | Working infrastructure; no public entry. View from adjacent public paths only. |
References
- National Database of Cultural Properties (ACA) — Okutsu Power Station No. 1 Covered Conduit
- https://kunishitei.bunka.go.jp/bsys/maindetails/101/00005907
- Cultural Heritage Online — Okutsu Power Station No. 1 Covered Conduit
- https://bunka.nii.ac.jp/heritages/detail/171212
- Chugoku Electric Power — Hydroelectric Stations
- https://www.energia.co.jp/energy/general/water/water_all.html
- Kagamino Town official site
- https://www.town.kagamino.lg.jp/
Last verified: 2026.07.04