A Bridge Built to Carry Water, Not Traffic
Where the headrace canal of the Okutsu Power Station reaches the Kirotani-gawa, a small stream that joins the Yoshii River, it does not stop. It crosses on its own bridge. The No. 1 Aqueduct (Ichigo Suirokyo) is an 18-metre reinforced-concrete rigid-frame span, built in 1932, that carries the plant's working water over the valley instead of any road. Its piers rise about 8 metres and splay outward toward the base like the foot of a wine glass; horizontal beams tie them together to steady the frame against the load overhead.
The aqueduct stands roughly 370 metres downstream of the intake weir on the Yoshii River itself, where water first enters the system. From there the canal runs at a gentle grade toward the powerhouse, and this bridge is the point where it steps over a side valley. The channel, or flume, that holds the water is cast as one piece with the bridge, so structure and watercourse are a single object rather than a trough resting on a separate deck.
An Engineer's Bridge of the Early Showa Period
The No. 1 Aqueduct was registered as a Tangible Cultural Property in October 2006 under No. 33-0116, one of a large group of Okutsu Power Station structures listed together. The register recognised it for contributing to the historic character of the national landscape, the standard applied across the whole scheme. What sets this piece apart within the group is its form: a portal-type balanced rigid-frame bridge, a design in which the deck and legs act as one continuous frame rather than a beam simply set on posts.
That choice reflects the confidence of 1930s concrete engineering. The builder, Chugoku Godo Denki, was extending electricity through the mountains of northern Okayama, and every stream crossing had to be solved economically and durably. The rigid frame let the designers use slender members while keeping the flume rigid enough to hold water without leaking. The outward-splaying piers and their tie-beams are the visible logic of that calculation, still legible nearly a century on.
What to Look for at the Crossing
Approach the aqueduct from below, from the bed of the Kirotani-gawa, and the proportions make sense: the wide, splayed feet grip the banks while the thin frame lifts the water across. Follow the line of the flume and you can trace where the water arrives, crosses, and continues into the covered channel just downstream. The two structures were designed as a sequence, and reading them together is more rewarding than seeing either alone.
The setting is the forested upper Yoshii valley near Okutsu Onsen, a quiet hot-spring district well north of the city of Okayama. There is no dedicated parking or interpretive board at the bridge. The structures belong to a working power plant and often sit on operational land, so views are best taken from public roads and the streambed, without climbing onto or into any part of the system. Water still runs through the flume, and the bridge is doing the same job it was cast for in 1932.
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Q&A
- What is an "aqueduct" in this context?
- It is a bridge that carries a water channel across a valley or stream, rather than carrying people or vehicles. Here the concrete flume that holds the canal water is built as one unit with the bridge itself.
- What does "rigid-frame" mean here?
- It describes a bridge where the deck and its legs form one continuous, rigidly joined frame. This lets slender members carry the load, and it is the form the ACA record notes for this crossing, a portal-type balanced rigid frame.
- How big is it?
- The bridge spans about 18 metres with a clear span of roughly 6.1 metres, and its piers stand about 8 metres tall. It dates to 1932.
- Where is it in relation to the rest of the plant?
- It lies about 370 metres downstream of the Yoshii River intake weir, and the No. 1 Covered Conduit continues directly downstream of it. Both are part of the same headrace canal.
- Can visitors cross it?
- No. It carries flowing water and is not a public crossing. It should be viewed from surrounding public ground rather than approached for access.
Basic Information
| Name | Okutsu Power Station No. 1 Aqueduct (Okutsu Hatsudensho Ichigo Suirokyo) |
|---|---|
| Location | Kamisaihara, Kagamino, Tomata District, Okayama Prefecture |
| Designation | Registered Tangible Cultural Property (Structure), No. 33-0116 |
| Registered | 18 October 2006 |
| Built | 1932 (Showa 7) |
| Quantity / Scale | 1 structure; reinforced-concrete rigid-frame bridge, length approx. 18 m (span approx. 6.1 m, piers approx. 8.0 m) |
| Owner | The Chugoku Electric Power Co., Inc. |
| Access | Working infrastructure; no public crossing. View from adjacent public ground only. |
References
- National Database of Cultural Properties (ACA) — Okutsu Power Station No. 1 Aqueduct
- https://kunishitei.bunka.go.jp/bsys/maindetails/101/00005906
- 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