Miyakaido Dam

An Arch of Stone in a Narrow Gorge

Miyakaido Dam (宮海道堰堤, Miyakaido Entei) is an arch-form stone erosion control dam on the upper Susuki River at Iriyamabe in Matsumoto City, Nagano Prefecture, completed in 1939 in the early Showa period, and entered on Japan's national register of Tangible Cultural Properties on 3 December 2024.

It is 34 metres long across the crest and 15 metres high, and it curves upstream on a circular arc of 30 metres radius. That curve is the reason the structure exists in this form. Most Japanese sabo dams, the check dams built to hold back sediment in mountain torrents, are straight gravity walls that resist water by sheer mass. An arch works differently: it carries the load sideways into the rock of both banks, so it can be thinner for the same height, provided the site gives it something solid to push against.

This site does. The Susuki River narrows sharply here and bedrock is exposed on both walls of the valley, and the engineers of the late 1930s read that condition correctly.

Why a Prefecture Built an Arch in 1939

The Susuki rises on Mitsumineyama, an 1,887-metre peak in the Chikuma hills east of Matsumoto, and runs west across the city before joining the Ta River at Nakajo. Its bed is fed by geology that weathers easily, which is a polite way of saying the river has always moved a great deal of gravel. It flooded repeatedly, and the torrential rain of the 1896 seasonal rain front caused particularly severe damage downstream. Sediment-laden floods kept reaching the town after that, and pressure grew for upstream works that would show results quickly.

What went up in 1938 and 1939 was built as a Nagano prefectural project. The masonry is neriishizumi, mortared stone, laid on the downstream face in the diagonal coursing called tanizumi, with cyclopean concrete packing the interior. At the crest the pattern changes: around the mizutoshi, the notch through which water passes, the ends of each course are laid flat in horizontal hirazumi while the stretch between them stays diagonal. Matsumoto City's own account of the structure calls that combination rare, and it is the detail most worth looking for. The lip of the notch is rounded rather than squared, softening the fall of water over the crest.

The chronology matters for how the dam is understood. In 1936 the Home Ministry began the Kamagafuchi sabo dam project on the upper Azusa River at Kamikochi, in the same basin. Miyakaido was built at roughly the same time and finished earlier. Many arch sabo dams followed after the war, but among works carried out by a prefecture rather than the national government, Matsumoto City describes Miyakaido as the earliest arch sabo dam in the country. The Agency for Cultural Affairs places it among the earliest arch sabo dams in Japan without the prefectural qualifier.

The criterion used at registration was that the structure is a model of form, which is a different and rarer ground than the landscape criterion applied to most registered buildings. It reflects the design rather than the setting.

Still Doing Its Job

More than eighty years on, the dam has not been retired into decoration. Sediment trapped behind it now extends some 1,200 metres upstream, because the valley widens above the gorge and the deposited material has spread out along it. That accumulated wedge is the structure working: every cubic metre of gravel held there is gravel that did not reach Matsumoto. The dam continues to function on the middle course of the Susuki, and Nagano Prefecture remains its owner.

Visiting takes some planning, and honesty is more useful here than encouragement. This is a working river control structure in a mountain valley, not a site with a car park, a path and an information board. Approach it from Matsumoto by heading east and following the Susuki upstream into the Iriyamabe district, roughly thirty minutes by car from the station area; there is no bus stop at the dam and no signed walking route to the base. Anyone intending to go should contact the Cultural Properties Division of Matsumoto City on 0263-34-3292 beforehand, both to confirm what access is currently appropriate and because river conditions change. Avoid the site during and after heavy rain, and stay out of the channel.

The Agency for Cultural Affairs has published a segment on the dam on its official YouTube channel, in the series titled Toroku e Ikou, which gives a look at the masonry and the setting for anyone who cannot make the trip. Photography from public ground is unrestricted; there is no admission fee, because there is nothing to admit anyone to.

Two other Matsumoto buildings were recommended for registration in the same July 2024 round, a former kimono shop and a former clinic, both in the city centre. Pairing a morning among the town's registered buildings with an afternoon drive up the Susuki shows how wide the register runs here: shopfronts and a clinic on the streets below, and a stone arch in a gorge above them.