A basin built to make two flows into one
The Kamisaibara Power Station Confluence Basin is a reinforced concrete water structure of 1930 on the right bank of the Yoshii River in the Heisakubara district of Kamisaibara, Kagamino, Okayama Prefecture, and it was listed as a Registered Tangible Cultural Property on 29 November 2006. Its job is to bring together two supplies of water that arrive from different directions and hand them on as one steady flow.
One supply comes through the sluice gates of the intake weir a few steps upstream, fresh from the river. The other is the tailrace of the Heisakubara Power Station, water that has already turned a generator once. The basin receives both, slows them down and lets the sand drop out before the combined flow enters the long open channel towards the turbines.
The plan is close to the shape of a comb, with an overflow weir set along the boundary between river and channel, and the whole covers 694 square metres. A management bridge about 45 metres long runs along the top of the overflow weir, carried on piers spaced unusually close together. Walking the site, that row of stubby piers is the first thing the eye catches.
Why the basin was registered
Nine structures of this station were registered on 29 November 2006 under the criterion that they contribute to the historic landscape of the country. The Kamisaibara Power Station Confluence Basin holds registration number 33-0141, second in the run after the intake weir it sits beside.
Registration, created in 1996, is aimed at buildings and works that are still in service. It asks an owner to notify the authorities before major alteration rather than to preserve a structure unchanged, so a Registered Tangible Cultural Property normally keeps its original function. Okayama Prefecture lists all nine of these structures as facilities in current use.
What the basin records is an idea rather than a technique. Between 1920 and 1932 Chugoku Godo Denki, a predecessor of Chugoku Electric Power, put four run-of-river stations along the upper Yoshii, at Iri in 1920, Heisakubara in February 1928, Kamisaibara in July 1930 and Okutsu in February 1932. Each takes the water the one above it has finished with. The same river is worked repeatedly on its way down to the Tsuyama basin, and a structure of this kind is what makes the handover possible. A near twin, the confluence basin of the Okutsu station, was built in the same year further downstream.
What to look at
Begin with the piers of the management bridge. Their close spacing is not decorative. A bridge sitting on the crest of an overflow weir has to be shallow, because depth would obstruct the water passing beneath it, and a shallow deck needs frequent support. The design solves a hydraulic problem and produces a rhythm as a by-product.
Then the comb-shaped plan. Water entering a basin at speed will scour one part and silt another. Dividing the plan into bays spreads the incoming flow and gives the sand a chance to settle where the operator can reach it. The shape is the calculation made visible.
Finally, look at the point where the tailrace from the upstream station meets the water from the weir. Two flows of different temperature and different clarity join at the Kamisaibara Power Station Confluence Basin, and on a still day the line between them can be picked out for some distance.
Visiting
This is a working part of a generating scheme, with no admission charge, no ticket office and no published opening hours. The management bridge is an operator's walkway rather than a public path.
Okayama Prefecture directs anyone wishing to view the facilities to arrange it in advance with the Tsuyama Power Office of Chugoku Electric Power, on 0868-27-0275. Ask about photography when you telephone.
Heisakubara lies in the northern part of Kagamino near the Tottori border, on National Route 179 between Tsuyama and Ningyo Pass. Coming by public transport, take the JR Kishin Line to Tsuyama Station or Inno-sho Station and change to the Chutetsu Hokubu Bus service towards Okutsu Onsen and Ishigoshi; Kagamino town publishes the timetables. Snow lies deep here in winter, so late spring through autumn makes for an easier trip.
- 📍 Plan Your Visit to Okayama
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Q&A
- What does the Kamisaibara Power Station Confluence Basin actually do?
- It combines water admitted through the sluice gates of the Yoshii River intake weir with the tailrace water of the Heisakubara Power Station, reduces the flow speed so that sand settles out, and passes the combined supply into the channel leading to the turbines.
- How large is the Kamisaibara Power Station Confluence Basin?
- It covers 694 square metres and is built of reinforced concrete, with a tailrace and a management bridge about 45 metres long included in the registration.
- Can I walk across the management bridge?
- No. It is an access walkway for operating staff inside the fenced area. Viewing of the plant is arranged through the Tsuyama Power Office of Chugoku Electric Power on 0868-27-0275.
- When was the basin built?
- In 1930, with the rest of the Kamisaibara scheme, which began generating in July of that year.
- How do I get to Kamisaibara by train and bus?
- Take the JR Kishin Line to Tsuyama or Inno-sho, then a Chutetsu Hokubu Bus towards Okutsu Onsen and Ishigoshi. Timetables appear on the Kagamino town public transport page. Buses are infrequent, so plan the return trip first.
Basic Information
| Name | Kamisaibara Power Station Confluence Basin |
|---|---|
| Location | Heisakubara, Kamisaibara, Kagamino-cho, Tomata-gun, Okayama Prefecture |
| Designation | Registered Tangible Cultural Property (structure) |
| Year Designated | Registered 1930 structure on 2006-11-29 |
| Quantity | 1 site |
| Dimensions | Reinforced concrete, area 694 square metres; with tailrace and a management bridge about 45 metres long |
| Owner | Chugoku Electric Power Co., Inc. |
| Public Access | Operating facility. No public entry; viewing of the plant by prior arrangement with the Tsuyama Power Office, 0868-27-0275 |
References
- National Cultural Properties Database, Agency for Cultural Affairs: Kamisaibara Power Station Confluence Basin
- https://kunishitei.bunka.go.jp/heritage/detail/101/00006054
- Cultural Heritage Online, Agency for Cultural Affairs: Kamisaibara Power Station Confluence Basin
- https://online.bunka.go.jp/heritages/detail/197049
- Okayama Prefecture, Cultural Properties Division: Registered Cultural Properties in Okayama Prefecture
- https://www.pref.okayama.jp/page/detail-50777.html
- Okayama Prefecture: Kamisaibara Power Station Facilities (PDF)
- https://www.pref.okayama.jp/uploaded/attachment/260992.pdf
- Okayama Prefecture: Okutsu Power Station Confluence Basin (PDF)
- https://www.pref.okayama.jp/uploaded/attachment/260993.pdf
- Kagamino Town: Route buses and public transport
- https://www.town.kagamino.lg.jp/soshiki/22/2132.html
Last verified: 2026.09.08