Retaining Wall of the Power Station's Iron Pipe Line and

Iri Power Station Penstock Retaining Wall and Main Building Retaining Wall: A Testament to Taishō-Era Hydroelectric Engineering

Deep in the mountainous terrain of Kagamino Town in northern Okayama Prefecture, where the Yoshii River carves its way through forested valleys, stands one of Japan's most remarkable surviving examples of early 20th-century hydroelectric infrastructure. The Iri Power Station Penstock Retaining Wall and Main Building Retaining Wall (入発電所鉄管路擁壁及び本館擁壁) is a Registered Tangible Cultural Property that has silently supported the operations of Okayama's oldest surviving hydroelectric power station for over a century.

Constructed in 1920 (Taishō 9) by the Bisaku Electric Company (備作電気株式会社), these retaining walls are integral components of a water-conduit style power station that harnessed the natural drop of the Yoshii River to generate electricity for the rapidly industrializing southern region of Okayama Prefecture. Today, they stand as enduring monuments to the ingenuity of Japan's early hydroelectric pioneers.

History and Background

Northern Okayama Prefecture, blessed with steep terrain and abundant water resources from the Chūgoku Mountains, proved ideal for hydroelectric development. From the late Meiji era through the early Shōwa period, numerous water-powered generating stations were built along the Yoshii River system. Among these, the Iri Power Station began operations in May 1920, following the earlier Isaka and Hade power stations. However, since neither of those predecessors survives today, the Iri Power Station holds the distinction of being the oldest existing hydroelectric facility on the Yoshii River — and indeed the oldest surviving hydroelectric power station in all of Okayama Prefecture.

The station was originally built by the Bisaku Electric Company to supply the growing electricity demands of Okayama's southern industrial and urban areas. Following Japan's postwar electric utility reorganization in 1951, ownership transferred to Chugoku Electric Power Company, which continues to operate the station today with an output of 1,600 kilowatts. Remarkably, no staff are permanently stationed at the facility — it is remotely controlled from the Tsuyama Control Center, with maintenance crews visiting twice monthly.

Why It Was Designated a Cultural Property

In October 2006, three components of the Iri Power Station complex were registered as national Tangible Cultural Properties (Registration No. 33-0129 for the retaining walls): the Main Building, the Water Tank, and the Penstock Retaining Wall and Main Building Retaining Wall. These structures were recognized as valuable heritage that contributes to the historical landscape of Japan and provides important evidence for understanding the history of hydroelectric power generation in the country.

The retaining walls specifically demonstrate the civil engineering techniques employed during the Taishō era for constructing the supporting infrastructure essential to water-conduit power stations. Built using a combination of large and small cobblestones laid in cemented masonry (練積, nerizumi), they represent a construction method that was both practical and aesthetically harmonious with the natural mountain landscape.

Architectural Features and Highlights

The cultural property consists of two distinct sections of retaining wall. The first runs along both sides of the penstock route — the steep path where the iron water pipe descends from the water tank to the power station building — extending approximately 36 meters in length. The second section wraps around the north and east sides of the main power station building in an angled configuration, stretching about 31 meters.

Both sections are constructed from carefully assembled cobblestones of varying sizes, laid in cemented masonry. This technique, using locally sourced river stones, creates walls that are both structurally robust and visually striking. The rounded surfaces of the cobblestones give the walls a distinctive texture that contrasts beautifully with the surrounding greenery of the mountain valley. After more than a century of service, these walls continue to fulfill their structural role while also bearing witness to the craftsmanship of early 20th-century civil engineering.

The main building itself, a single-story wooden structure with a gabled copper-plate roof and mortar-finished exterior measuring approximately 15 by 12 meters, features a king-post truss roof framework. It is recognized as the oldest surviving main building among the hydroelectric power stations on the Yoshii River system.

Understanding the Water-Conduit Power Station

The Iri Power Station operates as a water-conduit style (水路式) hydroelectric facility. In this system, water is diverted from the river at an upstream intake weir and channeled through a gently sloping conduit extending over 2,000 meters. At the end of this conduit, the water collects in a tank before dropping through a steel penstock pipe to drive turbines in the power station below. The retaining walls play a crucial role in stabilizing the steep terrain along this penstock route and around the main building, preventing erosion and landslides that could compromise the facility's operation.

Originally, the station drew water from an intake weir directly on the Yoshii River. Following the construction of the Tomata Dam in the area, the water source was modified so that the station now receives water from the discharge channel of the Tomata Power Station.

Visiting Iri Power Station

The Iri Power Station is located in a quiet mountain setting within Kagamino Town. As an active power generation facility owned by Chugoku Electric Power Company, the interior is not open to general visitors. However, the retaining walls and exterior of the main building can be appreciated from nearby public areas, offering a unique glimpse into Japan's industrial heritage in a remarkably pristine natural environment.

The best time to visit is during autumn, when the surrounding mountain forests transform into brilliant shades of red, orange, and gold, providing a spectacular backdrop for the historic stone walls. Spring also offers lovely scenery with fresh green foliage contrasting against the weathered cobblestone surfaces.

Nearby Attractions

Kagamino Town and its surroundings offer a wealth of attractions that can be combined with a visit to the Iri Power Station for a rewarding day trip or weekend getaway.

Okutsu Onsen, one of the three great hot springs of Mimasaka (美作三湯), is a historic spa town known for its alkaline waters with remarkable skin-beautifying properties. The tradition of "foot-stamping laundry" (足踏み洗濯), where women wash clothes by treading on them in the hot spring water along the riverbank, is a unique cultural experience demonstrated for visitors from late March through early December.

Okutsu Gorge (奥津渓) is a nationally designated Place of Scenic Beauty, famous for its crystal-clear waters, dramatic rock formations, and potholes (甌穴) carved over millennia. The gorge is particularly celebrated for its autumn foliage, widely regarded as among the most beautiful in the region.

Other notable destinations include Iwai Falls (岩井滝), a waterfall unique for flowing over an overhanging cliff that allows visitors to walk behind the curtain of water; the Fairy Forest Glass Museum (妖精の森ガラス美術館), which showcases exquisite uranium glass art; and Lake Okutsu (奥津湖), where kayaking, SUP, and cycling activities are available along the scenic lakeshore.