Myojin Himon Gate

Myojin Sluice Gate: An 1901 Waterwork Built of Artificial Stone

In the city of Obu, on the eastern edge of Aichi Prefecture, two small streams meet at a crossing that solves a problem most rivers never face: how to let one channel of water pass another without the two mixing. At this spot the Gokamura Canal slips beneath the Myojin River through a low stone tunnel. That tunnel is the Myojin Sluice Gate (Myojin Himon), finished in 1901 and still carrying water through its original arches today.

The structure is modest in scale. Its water passage runs about 21 metres and is formed by two stone arches set side by side, each built two stones thick. The national register describes the arches as segmental, meaning they rise less than a full half-circle, though the city's own descriptions call them semicircular. Above the arched passage sits a thicker wall, and it is this upper wall that gives the gate its real significance, because it was built using a now-rare nineteenth-century technique called artificial stone.

To understand why a small flood-control structure earned national recognition, it helps to know what kind of recognition it received, and to meet the engineer whose name is bound up with the material it is made from.

A Registered Property, Not a Designated One

On 4 February 2021 the Myojin Sluice Gate was entered onto Japan's roster of Registered Tangible Cultural Properties as a civil-engineering structure. The word registered matters here. Japan runs two parallel systems for protecting cultural property. The older one, designation, applies the strongest controls and produces the famous tiers of National Treasure and Important Cultural Property. The newer one, registration, was introduced in 1996 to protect a wider and more everyday range of buildings and structures, especially those from the Meiji period onward, with lighter regulation aimed at keeping them in active use rather than freezing them in place.

The Myojin Sluice Gate fits that purpose exactly. It was registered under the criterion of being difficult to recreate, and more than a century after it was built it is still functioning as a working sluice gate. The companion structure beside it, the Myojin-gawa Reverse-flow Sluice Gate, was registered on the same day. The pair brought the number of nationally registered properties in Obu to four.

The land around here was farmland from the Edo period, watered and drained by channels the villagers themselves had cut. The Gokamura Canal takes its name from the five villages that dug it together as a drainage course. When the Nobi Earthquake of 1891 and the flooding that followed wrecked the older wooden gates, the canal needed a sturdier replacement, and the material chosen for the job is what sets this structure apart.

Hattori Choshichi and the Method Called Artificial Stone

Artificial stone, or jinzoseki, was the work of Hattori Choshichi (1840–1919), an engineer born in the Mikawa region of what is now Aichi. In 1876, while working on an underground passage for a merchant family in Tokyo, he noticed that a soft, wet form of the traditional rammed-earth flooring known as tataki would set hard even underwater. He refined the discovery into a building material strong enough to rival concrete.

The recipe was simple and cheap. Decomposed granite soil, plentiful across this part of central Japan, was mixed with slaked lime and water, packed in place, and tamped until the surface began to sweat moisture. The lime and soil bound together as they cured, and unlike the imported cement of the day, the mixture hardened reliably in wet conditions and cost a fraction as much. Hattori first called it Choshichi tataki; after a foreign adviser at an 1881 exhibition asked what the material was, the name jinzoseki, artificial stone, took hold.

Over the following decades the method was used in roughly 350 projects, among them the breakwater at Yokkaichi, the harbour works at Ujina in Hiroshima, the port of Nagoya, and the headworks of the Meiji Irrigation system. Cheaper, more uniform cement gradually displaced it, and by the early 1920s the technique had all but vanished. The Myojin Sluice Gate is one of the earlier surviving artificial-stone sluice gates in Aichi, and a structure for which Hattori's involvement is documented rather than merely supposed.

There is a quiet sequel. The knowledge of how to work artificial stone had lapsed after the war, but in 2017 the gate was repaired using the original method. It is regarded as the first postwar restoration carried out in genuine artificial stone, which means the gate now records both a Meiji technique and the modern effort to recover it.

What to Look For at the Crossing

The first thing to register on the ground is the geometry. The Gokamura Canal runs roughly north to south and dives beneath the Myojin River, which crosses above it. Stand at the right angle and you can read the whole logic of the place: water flowing over water, kept apart by a structure low enough to step past without noticing, until you know what it is.

Look closely at the two materials working together. The arched passage below is cut stone, laid in the double thickness that lets it carry the load of the river crossing. The wall above is artificial stone, and where it has weathered you can see the difference in surface between the quarried blocks and the tamped, lime-bound mass poured around them. The 2017 repair used the same approach, so the structure reads as a single technique rather than a patchwork of eras.

A few steps away is the second gate, the Myojin-gawa Reverse-flow Sluice Gate of 1916, which carries the Myojin River across and once held a door to stop water backing up from the Sakaigawa, the larger river that historically marked the boundary between the old provinces of Owari and Mikawa. Seeing the two gates together makes the engineering legible in a way neither does alone. After passing the crossing, the canal continues south, joins the Ishigase River, and empties into Kinuura Bay on the Mikawa Sea.

Visiting and the Surrounding Area

The gate sits beside a working watercourse rather than inside a managed visitor site, so there is no gate, ticket, or opening hour. It can be viewed from the public riverbank, and visitors are asked to treat it as the live flood-control structure it still is and to keep to the public paths.

For the background that brings the gate to life, the Obu City History and Folklore Museum is the place to start. It stands about a ten-minute walk from JR Obu Station, beside Okura Park, and admission is free. The museum holds local material on the region's waterways and a three-dimensional relief map of the city's cultural properties, useful for placing the gate within its landscape before you go to see it. The museum closes on Mondays, on the last Friday of each month, and over the New Year period, so it is worth checking the calendar in advance.

Okura Park itself, next to the museum, is pleasant to walk and holds another of Obu's registered properties, the thatched gate of the former Okura villa. Together they make a half-day that runs from a merchant family's garden to a piece of working Meiji infrastructure, all within easy reach of a single station.