Yoke Dam

A granite wall across the upper Shigenobu

The Yoke Dam is a stone and concrete erosion control dam on the upper Shigenobu River at Yamanouchi in Toon City, Ehime Prefecture, completed in 1935 and entered on the national register as a Registered Tangible Cultural Property in 2001. The main structure spans 115 meters of the valley floor and stands 12 meters high, with a second, lower dam 92 meters wide set downstream of it.

Ehime Prefecture ran the project directly rather than through a contractor. Work began in October 1932 and the two dams were finished 901 days later. Both faces are dressed in granite quarried on the islands of the Seto Inland Sea and carried inland; the blocks set into them number some 17000. The design was taken as the model for the sediment control works built afterwards along the rest of the Shigenobu.

The purpose was blunt enough. Erosion in the headwaters was feeding gravel and sand into a river that runs short and steep before reaching the Dogo plain, and the Yoke Dam was built to hold that material high in the valley. Ninety years on it is still doing so.

Why the Yoke Dam is on the register

The Agency for Cultural Affairs registered the dam on 28 August 2001 as a single structure, under the criterion for works that contribute to the historic landscape of the country. The reasoning turns on condition rather than rarity: the masonry has come through in good order, and what survives shows the standard of workmanship that went into it. Registration as a Registered Tangible Cultural Property applies to civil engineering as readily as to houses and temples, though structures of this kind make up only a small part of the Ehime list.

Reading the Yoke Dam as a piece of construction history is straightforward once you know how it was built. Nothing about a stone facing was cheap or quick in 1932. Granite had to be shipped from the islands, hauled up the valley and set block by block, which is why later dams on other rivers were poured in plain concrete instead. The two Shigenobu dams belong to a short window when public works still expected stone to be laid by hand.

What to look at

The face of the Yoke Dam is the reason to come. The blocks were set course by course with joints tight enough that from the far bank the wall reads as one continuous surface. Follow a single course with your eye, though, and the stones turn out not to be identical: sizes shift along the run, and each block was worked to fit its neighbour rather than to a standard. In ordinary weather water slides over the crest in a thin, even sheet, drops to the apron and moves on to the lower dam.

Spring is when most people come. Cherry trees line the banks, and towards the end of April local volunteers rig ropes right across the channel and hang carp streamers from them, an event known as the koinobori crossing of the Shigenobu. The streamers usually stay up into early May, and the sight of them filling the gap between the two banks gives a fair sense of how wide the river is here.

Visiting the Yoke Dam

There is no gate and no charge. The Yoke Dam sits in open riverbed at Ohata-otsu 62 in Yamanouchi and can be seen from the bank at any hour; a car park and toilets are provided nearby. Photography from the bank is unrestricted. This is a working prefectural facility, so keep off the crest and the apron, and take particular care after rain, when the flow rises quickly and without warning.

The nearest railway station is Yokogawara, at the end of the Iyotetsu Yokogawara Line out of Matsuyama, and the drive from there up the valley takes about 15 minutes. Buses into the upper valley are infrequent, so a car or a taxi is the practical choice. Anyone coming for the carp streamers should aim for late April or the first days of May; the streamers are hung by residents, so the exact dates shift a little from year to year.