A 4.4-Metre Wall of Masonry Across a Side Stream
The Okutsu Power Station Ningyosentani River Water Intake Weir is a 1932 diversion weir on a tributary stream in the Kamisaihara district of Kagamino, Okayama Prefecture, and it entered the national register of Registered Tangible Cultural Properties on 18 October 2006 under number 33-0119.
The crest measures 4.4 metres and the height 1.5 metres. That is the smallest set of dimensions in the whole Okutsu group, closer to a garden wall than to anything most people would call a dam, and it is thrown across the Ningyosentani River, a right-bank tributary that comes down off the ridge separating Okayama from Tottori.
What it captures does not go straight to a turbine. The weir feeds a branch channel that runs down to meet the main headrace canal roughly 500 metres below the plant's No. 1 Covered Conduit. Okutsu is a canal-type station rather than a reservoir plant: Chugoku Electric Power rates it at 7,500 kilowatts, and that figure is reached by adding together every stream the canal can be persuaded to touch.
Two Faces, Two Slopes
The most informative thing about this weir is not its size. It is the difference between its two sides.
The upstream face is set at a batter of 1 vertical to 0.1 horizontal, which is close enough to vertical that the eye reads it as a flat wall. The downstream face is laid back much further, at 1 to 1.3, so that it slopes away at roughly the angle of a steep staircase. Water is meant to spill over the top rather than be retained, and those two profiles are the reason it can do so safely: the near-vertical upstream face gives the pooled water a clean surface to rise against, while the long downstream slope carries the falling sheet down the structure instead of letting it drop free onto the foundation and dig a hole there.
The construction is a gravity weir of mortared masonry, meaning it resists the push of the water by sheer weight rather than by anchorage. At this scale the weight is modest, and the shaping of the two faces does much of the work that mass alone would not.
Why Something This Small Is on the Register
The register of Registered Tangible Cultural Property was created in 1996 for structures over fifty years old that are still doing their job, and Okayama Prefecture's cultural properties division lists three qualifying criteria. The one applied to the Okutsu structures is contribution to the historic character of the national landscape.
That phrase is easier to justify for a powerhouse than for a weir of this size. The Okutsu Power Station Ningyosentani River Water Intake Weir earns its place by belonging to something larger. The Agency for Cultural Affairs describes it as one of the plant's facilities that make use of the side valleys, and side valleys are the point. A 1930s engineer working in this terrain could not build one big dam; the geology and the river would not allow it. Instead the design collects small volumes from many places and lets the canal do the adding.
The register captures that logic by listing the small components alongside the large ones. Remove the tributary weirs from the list and the surviving record would describe a headrace and a powerhouse without explaining where the water came from.
Finding It, and What to Expect
Kamisaihara is the northernmost part of Kagamino, up against the prefectural boundary, and reaching it without a car is awkward. Buses from JR Tsuyama Station run to Okutsu Onsen in about an hour but only a few times daily, and the weir lies well beyond the hot spring village. Most visitors drive: about 25 minutes from the Innosho interchange of the Chugoku Expressway along Route 179, then north up the valley into Kamisaihara.
Expect very little in the way of visitor provision. The Okutsu Power Station Ningyosentani River Water Intake Weir belongs to a working plant owned by Chugoku Electric Power; there is no parking area, no explanatory board and no path to the structure, and the surrounding ground is operational. Photographing it from public roads and riverbanks is unrestricted, but entry is not permitted. Because it is small and set in wooded ground, it is easy to pass without noticing, and a low-water spell in late autumn gives the clearest view of the masonry. Snow closes off casual visits in midwinter. These access details were checked in September 2026.
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Q&A
- How big is the Okutsu Power Station Ningyosentani River Water Intake Weir?
- The crest is 4.4 metres across and the structure stands 1.5 metres high, which makes it the smallest of the registered Okutsu components.
- Why do the two faces of the Okutsu Power Station Ningyosentani River Water Intake Weir have different slopes?
- The upstream face is battered at 1 to 0.1, almost vertical, so that impounded water rises against a flat surface. The downstream face is laid back at 1 to 1.3 so that water spilling over the crest runs down the structure rather than falling free and scouring its base.
- Where does water taken by the Okutsu Power Station Ningyosentani River Water Intake Weir go?
- Into a branch channel that joins the main headrace canal of the power station about 500 metres downstream of the No. 1 Covered Conduit. From there it travels south with the rest of the collected flow.
- When was the Okutsu Power Station Ningyosentani River Water Intake Weir registered, and under what category?
- It was registered as a Registered Tangible Cultural Property (Structure) on 18 October 2006, register number 33-0119. The structure itself dates from 1932.
- Can visitors approach the Okutsu Power Station Ningyosentani River Water Intake Weir?
- No. It stands on operational land belonging to a working power station, with no parking, signage or footpath. Viewing and photography from public ground are unrestricted.
Basic Information
| Name | Okutsu Power Station Ningyosentani River Water Intake Weir (奥津発電所人形仙谷川取水堰堤) |
|---|---|
| Location | Dotoriba, Kamisaihara, Kagamino, Tomata District, Okayama Prefecture |
| Designation | Registered Tangible Cultural Property (Structure), register no. 33-0119 |
| Year Designated | Registered 18 October 2006 (Heisei 18) |
| Quantity | 1 structure |
| Dimensions | Crest length 4.4 m, height 1.5 m; upstream batter 1 to 0.1, downstream batter 1 to 1.3 |
| Owner | The Chugoku Electric Power Co., Inc. |
| Public Access | Operating hydroelectric infrastructure. No entry. Viewing from public ground only. |
References
- National Database of Cultural Properties, Agency for Cultural Affairs: Okutsu Power Station Ningyosentani River Water Intake Weir
- https://kunishitei.bunka.go.jp/heritage/detail/101/00005909
- 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