The Gatekeeper at the Head of the Main Waterway
The Heisakubara Power Station Main Waterway Settling Basin (平作原発電所幹線水路沈砂池) is a reinforced concrete settling basin built in 1928 about 270 metres downstream of the Onbara Reservoir Dam in Kamisaibara, Kagamino Town, Okayama Prefecture, and registered as a Registered Tangible Cultural Property on 29 November 2006.
Roughly rectangular, it covers 85 square metres. Water drawn from the reservoir passes through it soon after entering the main waterway, drops its sediment and then travels about 2.3 kilometres to the head tank.
Three settling basins at Heisakubara are registered. The two on the Endō and Endōbara rivers sit beside their intakes and deal with stream-borne silt. This one guards the main line instead, standing where reservoir water starts its long run toward the turbines.
Small, and Roofed Where It Matters
The Heisakubara Power Station Main Waterway Settling Basin was registered as a structure that contributes to the historic scenery of the country.
At 85 square metres it is far smaller than the Endō River basin (506 square metres) or the Endōbara River basin (370 square metres). Water that has already rested in a reservoir carries less silt than a mountain stream, which likely explains the modest size.
What the Agency for Cultural Affairs highlights is its cover. On the west and south, where the mountainside juts close, reinforced concrete slabs are cantilevered out over the basin to keep falling earth out.
Up the valley, the Endōbara basin solved the same problem by stacking stone walls on its hillside edge. Here the answer is a concrete slab reaching out like a canopy.
What to Look For
Look for those overhanging slabs. Only the edges that face the slope are covered; the rest stays open. The builders worked out which way trouble would come from and shielded just those sides.
Consider the location, too. Sitting a mere 270 metres below the dam, the basin shows that the dam and the main waterway were planned as a single piece of machinery.
Line up the three settling basins by area, 506, 370 and 85 square metres, and it becomes clear how the source of the water dictated each one's size and defenses.
Visiting the Main Waterway Settling Basin
The Heisakubara Power Station Main Waterway Settling Basin is a working facility run by Chugoku Electric Power, and no visitor facilities are advertised. The area around the basin and any gated or fenced zone are closed; only the exterior can be seen from places where access is permitted.
It lies on the downstream side of Onbara Dam, where the access road is closed, so do not try to force a closer look. The Onbara Plateau sits at about 700 metres, and winter snow can exceed 1 metre.
Chugoku Electric's page gives no restriction on photographing exteriors, but you cannot enter operational areas to take pictures.
- 📍 Plan Your Visit to Okayama
- Find tours, activities & transport passes near Heisakubara Power Station Main Waterway Settling Basin
- [ Kurashiki Bikan District & Korakuen Garden Tours ]
Q&A
- What is the Heisakubara Power Station Main Waterway Settling Basin?
- It is a reinforced concrete basin built in 1928 near the start of the main waterway to settle out sediment from reservoir water. It became a Registered Tangible Cultural Property in 2006.
- Where is the Heisakubara Power Station Main Waterway Settling Basin?
- It is in the Onbarayama area of Kamisaibara, Kagamino Town, Okayama Prefecture, about 270 metres downstream of the Onbara Reservoir Dam.
- What is distinctive about the Heisakubara Power Station Main Waterway Settling Basin?
- It is a roughly rectangular basin of 85 square metres whose west and south sides are covered by projecting concrete slabs that keep debris from the slope out.
- Can I visit the Heisakubara Power Station Main Waterway Settling Basin?
- Only the exterior, from places where access is allowed. It is part of a working plant and the operational areas are closed.
- Where does water go after the Heisakubara Power Station Main Waterway Settling Basin?
- It flows about 2.3 kilometres along the main waterway to the head tank, then drops through the penstock to the main building.
Basic Information
| Name | Heisakubara Power Station Main Waterway Settling Basin |
|---|---|
| Location | Onbarayama area, Kamisaibara, Kagamino Town, Tomata District, Okayama Prefecture |
| Designation | Registered Tangible Cultural Property (Structure) |
| Year Designated | Registered 2006/11/29 (Heisei 18) |
| Quantity | 1 site |
| Dimensions | Area 85 square metres |
| Owner | Chugoku Electric Power Co., Inc. |
| Public Access | Exterior viewing only; operational areas closed |
References
- Agency for Cultural Affairs, Database of National Cultural Properties: Heisakubara Power Station Main Waterway Settling Basin
- https://kunishitei.bunka.go.jp/bsys/maindetails/101/00006044
- Agency for Cultural Affairs, Database of National Cultural Properties: Heisakubara Power Station Onbara Reservoir Dam
- https://kunishitei.bunka.go.jp/bsys/maindetails/101/00005989
- Agency for Cultural Affairs, Database of National Cultural Properties: Heisakubara Power Station Head Tank
- https://kunishitei.bunka.go.jp/bsys/maindetails/101/00006050
- Chugoku Electric Power: History of Heisakubara Power Station (Japanese)
- https://www.energia.co.jp/isan/heisakubara_history.html
- Chugoku Electric Power: Heisakubara Power Station, Historic Electric Power Heritage (Japanese)
- https://www.energia.co.jp/isan/heisakubara.html
- Kagamino Town Tourism: Lake Onbara (Japanese)
- https://www.kagamino.holiday/spot/entry-322.html
Last verified: 2026.09.25
