A Canal Crossing a Stream in the Air
The Okutsu Power Station No. 4 Waterway Bridge is a reinforced concrete aqueduct completed in 1932 in the Shimosaihara district of Kagamino, Okayama Prefecture, and it was placed on the national register of Registered Tangible Cultural Properties on 18 October 2006 under number 33-0121.
It carries no road. What crosses here is water: the headrace of the Okutsu Power Station, lifted over the Orodani River, a right-bank tributary of the Yoshii. The bridge measures 18 metres in length and is divided into three bays of roughly 6.1 metres each. It stands about 1,760 metres below the No. 3 Waterway Bridge, so the two crossings are far enough apart that no walk connects them casually.
Past this point the canal has one long stretch left. Roughly 4.2 kilometres further down it reaches the plant's regulating reservoir, and only then does the water fall to the turbines. Chugoku Electric Power gives the station's output as 7,500 kilowatts.
Look at the Legs, Not the Deck
The interesting part of the Okutsu Power Station No. 4 Waterway Bridge is underneath it.
The Agency for Cultural Affairs records the type as a three-span portal balanced rigid frame. In a rigid frame the deck and the supports are cast as one continuous body rather than a beam dropped onto separate columns, so bending in the deck is passed down into the legs and the whole thing behaves as a single member. The balanced arrangement distributes those forces across the three bays instead of concentrating them at the ends.
Then there are the piers. They widen as they descend, splaying outward toward the base rather than running straight down, and they sit not on individual pads but on a continuous footing that runs beneath the whole substructure. Both decisions point the same way. A stream bed of loose gravel and irregular rock gives poor and uneven bearing; spreading the load over a single connected foundation, and broadening each leg where it meets that foundation, reduces the risk that one support settles further than its neighbours and cracks the frame above. In a structure that must stay watertight for a century, differential settlement is the failure that matters.
What the Register Recognised
Japan's system of Registered Tangible Cultural Property began in 1996, and Okayama Prefecture's cultural properties division sets out three qualifying criteria. The Okutsu structures were taken up under the first of them, contribution to the historic character of the national landscape, and they went onto the list together in October 2006 with numbers running upward from 33-0115.
Reinforced concrete was still a relatively young material in Japanese civil engineering when this aqueduct was built, and much of what was cast in the 1920s and 1930s has since been demolished or rebuilt. Here the frame, the flared piers and the continuous footing survive doing the job they were designed for. The value the register recognises is not rarity of form but continuity of function: an early Showa concrete structure that has never stopped carrying water.
Seeing the Aqueduct
Shimosaihara lies in the upper Yoshii valley in northern Kagamino, wooded country with scattered settlement. Driving is the practical approach: about 25 minutes from the Innosho interchange of the Chugoku Expressway along Route 179 toward Kurayoshi, then on along the valley road. Buses from JR Tsuyama Station reach Okutsu Onsen in roughly an hour, but they run only a few times a day and leave a considerable distance to cover on foot.
The Okutsu Power Station No. 4 Waterway Bridge cannot be walked across. It carries a water channel, not a deck for people, and it belongs to a working plant owned by Chugoku Electric Power, with operational land around it. There is no car park, no signboard and no viewing point. Photography from public roads and from the stream bank is unrestricted. The best angle is from below and slightly to one side, where the three openings and the taper of the legs read clearly against the trees; winter, once the leaves are down, gives the clearest sightline, though snow can make the valley road difficult. These details were confirmed in September 2026.
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Q&A
- What does the Okutsu Power Station No. 4 Waterway Bridge carry?
- It carries the headrace canal of the power station over the Orodani River, a right-bank tributary of the Yoshii. It is an aqueduct, not a road bridge, so there is no deck for vehicles or pedestrians.
- How is the Okutsu Power Station No. 4 Waterway Bridge built?
- It is a reinforced concrete three-span portal balanced rigid frame, 18 metres long with bays of about 6.1 metres. The piers widen toward the base and rest on a single continuous footing rather than separate pads.
- Why do the piers of the Okutsu Power Station No. 4 Waterway Bridge splay outward at the bottom?
- Broadening each leg where it meets the foundation spreads the load over a wider area of an uneven stream bed. Combined with the continuous footing, it reduces the chance that one support settles more than the others and cracks the frame.
- Where does the Okutsu Power Station No. 4 Waterway Bridge sit in the canal system?
- About 1,760 metres downstream of the No. 3 Waterway Bridge, and roughly 4.2 kilometres upstream of the regulating reservoir that precedes the powerhouse.
- When was the Okutsu Power Station No. 4 Waterway Bridge registered, and can it be visited?
- It was registered as a Registered Tangible Cultural Property (Structure) on 18 October 2006, number 33-0121. There is no public entry or crossing; viewing and photography from public ground are unrestricted.
Basic Information
| Name | Okutsu Power Station No. 4 Waterway Bridge (奥津発電所四号水路橋) |
|---|---|
| Location | Kamiharada, Shimosaihara, Kagamino, Tomata District, Okayama Prefecture |
| Designation | Registered Tangible Cultural Property (Structure), register no. 33-0121 |
| Year Designated | Registered 18 October 2006 (Heisei 18) |
| Quantity | 1 structure |
| Dimensions | Length approx. 18 m, 3 spans of approx. 6.1 m; reinforced concrete rigid frame |
| Owner | The Chugoku Electric Power Co., Inc. |
| Public Access | Operating hydroelectric infrastructure. No entry or crossing. Viewing from public ground only. |
References
- National Database of Cultural Properties, Agency for Cultural Affairs: Okutsu Power Station No. 4 Waterway Bridge
- https://kunishitei.bunka.go.jp/heritage/detail/101/00005911
- National Database of Cultural Properties, Agency for Cultural Affairs: Okutsu Power Station Regulating Reservoir
- https://kunishitei.bunka.go.jp/heritage/detail/101/00005913
- Okayama Prefecture, Cultural Properties Division: the cultural property registration system
- https://www.pref.okayama.jp/page/505223.html
- The Chugoku Electric Power Co., Inc.: list of hydroelectric stations
- https://www.energia.co.jp/energy/general/water/water_all.html
- Kagamino Town official website
- https://www.town.kagamino.lg.jp/
Last verified: 2026.09.09