Old Iwabuchi Water Gate

The Red Gate Where the Sumida River Begins

The Former Iwabuchi Water Gate (旧岩淵水門, also written Kyū-Iwabuchi Suimon) is a reinforced concrete sluice gate in Shimo 5-chōme, Kita Ward, Tokyo, completed in 1924 and designated an Important Cultural Property of Japan on 15 August 2024. It stands at the point where the Arakawa splits from the Sumida River, and locals have called it the Red Gate for decades.

Most Important Cultural Properties in Tokyo are temples, shrines or houses. This one is a piece of flood control infrastructure, and it was designated on two grounds at once: technical excellence and historical value. Very few civil engineering works carry both.

The reason goes back to August 1910, when weeks of rain sent the Arakawa over its banks and flooded a large part of low-lying Tokyo. The government's answer was drastic. Rather than reinforce the existing channel, engineers cut an entirely new one, twenty-two kilometres of artificial river running from Kita Ward to Tokyo Bay. Work on the Arakawa Floodway began in 1911 and continued for over two decades. Water first ran through it on 12 October 1924.

A floodway of that scale needs a control point where it separates from the old river. That control point is this gate. Construction started in 1916 and finished in 1924, eight years for a single structure.

Aoyama Akira and the Concrete Beneath the Waterline

Design responsibility fell to Aoyama Akira of the Home Ministry's Tokyo Civil Engineering Branch Office. Aoyama had spent years in Panama, working on the canal, and returned to Japan with experience in large-scale concrete construction that almost nobody else in the country possessed. He is generally described as the only Japanese engineer to have taken part in that project.

What earned the gate its technical citation is largely invisible from the bank. Reinforced concrete was still new in Japanese river works in the 1910s, and the whole body of this structure was built in it. The foundation combines caisson-type cylindrical footings, sunk and filled, with a thick slab, a belt-and-braces approach that produced something very difficult to shift.

It held. The Great Kantō Earthquake struck in September 1923, while the gate was still under construction, and the structure came through. It then went on working for roughly sixty years, holding back Arakawa floodwater from the Sumida and the dense city downstream, until a larger successor took over.

The measurements are precise in the designation: a reinforced concrete water gate 61.8 metres long, with flanking wing walls and the parapet of the right-bank revetment upstream included in the listing. Five steel lift gates open through the body, one of them a navigation passage for boats. A set of forty-eight construction drawings for the works is recorded as attached property. Its designation number is 02773.

Duty passed to the present Iwabuchi Water Gate, finished in 1982 and painted blue, which is why Tokyoites now speak of a Red Gate and a Blue Gate standing a short distance apart. The red paint dates from repair work in the late 1950s or early 1960s, according to Kita Ward.

Visiting the Gate

There is no admission charge, no gate hours and no ticket office. The structure sits in open riverside parkland and can be seen at any time. The bridge alongside it brings you close enough to read the rivet lines on the steel, and crossing over takes you onto Nakanoshima, an island of green officially called Arakawa Akasuimon Ryokuchi. From there the current Blue Gate is in plain view downstream, which makes the comparison easy: 1924 against 1982, five nine-metre openings against a handful of far wider ones.

Photography is unrestricted. The best light for the red steelwork comes late in the afternoon, when the sun is low over the floodway and the paint reads warm rather than orange. Kita Ward and event listings note that the two gates are lit up on the twelfth of each month, a nod to the date the floodway first carried water.

Getting There

The nearest stations are Akabane-Iwabuchi and Shimo on the Tokyo Metro Namboku Line, both roughly fifteen minutes on foot. From JR Akabane Station's east exit the walk takes about twenty minutes. Toei buses bound for Toshima 5-chōme Danchi stop at Iwabuchichō and Shimo 2-chōme, ten minutes away. There is no dedicated car park.

Combine It With the Arakawa Museum of Aqua

The Arakawa Museum of Aqua, known as amoa, sits close by at Shimo 5-41-1 inside the Arakawa Downstream River Office grounds. Admission is free. It covers the flood history of the river, the engineering of the floodway and the local ecology across three floors, and it includes a working flow model with gates visitors can raise and lower themselves. Opening hours are 9:30 to 17:00 on weekdays and 10:00 to 17:00 at weekends and public holidays, closing thirty minutes earlier from November to February, with last entry half an hour before closing. It shuts on Mondays, on the following weekday when a Monday is a public holiday, and over the New Year period. Half an hour there before walking to the gate changes what you see when you get there.

One practical warning: there are no shops or restaurants at the riverside or in the museum, so bring water in summer. The embankment is exposed and offers almost no shade.