Inari River Sabo Dams

What Are the Inari River Sabo Dams?

The Inari River Sabo Dams (稲荷川砂防堰堤群, Inarigawa Sabō Enteigun) are seven erosion-control dams built between 1920 and 1933 on the Inari River in Nikko City, Tochigi Prefecture, which were registered together as Registered Tangible Cultural Properties (Structures) of Japan on 21 August 2002. They belong to the national government (Ministry of Land, Infrastructure, Transport and Tourism) and are maintained by the Nikko Sabo Office of the Kanto Regional Development Bureau.

The seven are the Second, Third, Fourth, Sixth and Tenth Inari River dams, plus two dams farther upstream in the Kamatsusawa section: the Kamatsusawa Downstream Sabo Dam and the Kamatsusawa Sabo Dam. All are gravity dams of concrete faced with stone, the kind of structure known in Japan as a Erosion-control dam. "Sabo" is the Japanese word for erosion and debris-flow control, and it is used untranslated in international engineering. Their registration numbers run without a break from 09-0054 to 09-0060.

The collective name is the one the Nikko Sabo Office uses when introducing the site. The Agency for Cultural Affairs registers each dam on its own, under a number or a local place name.

A Violent River above a Shrine Town

The Inari River rises on Mount Nyoho and Mount Akanagi at the eastern edge of the Nikko volcanic group, runs down past the eastern side of Toshogu, Futarasan Shrine and Rinnoji, and joins the Daiya River about 300 meters east of the Shinkyo Bridge. According to the Japan Society of Civil Engineers (JSCE), it drains just 12.4 square kilometers over a course of 9.8 kilometers, with an average gradient of roughly 1 in 10. Its valley walls are scored by deep landslide scars, locally called nagi, some of them about 100 meters deep.

The Nikko Sabo Office describes the Inari as a "violent river." In 1662 a flood swept away Inari-machi, a quarter of the town on the east side of the Nikko sanctuary, killing 140 people; two memorial stupas were raised for them the following year. The Agency for Cultural Affairs also records a great flood in 1902 that cut the upper valley sharply downward.

Tochigi Prefecture began surveying for erosion-control work around 1916. Because of the river's importance the project was taken over by the national Home Ministry in 1918, and on 10 July that year the ministry opened an Inari River works office. The plan called for 17 dams and 8 groundsills. By 1955 the project had completed 20 dams and 15 groundsills.

The very first structure, the First Dam, was finished in the summer of 1919 and was washed away by a debris flow in a storm on 15 September the same year. It had been laid as Karazumi, dry stone with nothing binding the blocks, and that was judged to be its weakness. Every dam that followed was built as Nerizumi, with the joints filled with concrete. The Second Dam, completed in August 1920, was the first built on this principle, and its success in a major flood that same year opened the way for the rest of the program.

How the Inari River Sabo Dams Are Arranged

The numbers do not follow the dams' order along the river. Using the distances given in the Agency for Cultural Affairs descriptions, the seven run from downstream to upstream as follows (completion year and size from the national database).

• Inari River Sixth Sabo Dam: 1922; 69 meters long, 7.3 meters high. About 2.8 kilometers above the Daiya River confluence, the lowest of the seven
• Inari River Fourth Sabo Dam: 1921; 69 meters long, 3.6 meters high. 323 meters above the Sixth
• Inari River Second Sabo Dam: 1920; 40 meters long, 7.0 meters high. 110 meters above the Fourth, and the oldest of the seven
• Inari River Third Sabo Dam: 1921; 76 meters long, 5.8 meters high. 84 meters above the Second
• Inari River Tenth Sabo Dam: 1923, enlarged in the Showa era; 45 meters long, 11 meters high. About 600 meters above the Third
• Kamatsusawa Downstream Sabo Dam: 1932; 25 meters long, 17 meters high. 366 meters above the Tenth
• Kamatsusawa Sabo Dam: 1933; 64 meters long, 20 meters high. 225 meters above the Kamatsusawa Downstream dam

Added together, those distances put about 1.7 kilometers between the Sixth Dam and the Kamatsusawa Sabo Dam. The lower four sit close together; farther up, the valley narrows and the dams become shorter and much taller.

What the Inari River Sabo Dams Share

All seven Inari River Sabo Dams are mortared masonry dams whose faces are laid in Tanizumi, with the stones set diagonally so that the face reads as a lattice rather than as horizontal courses. The Nikko Sabo Office notes that prewar work was carried out directly by the Home Ministry, with materials hauled and foundations dug by people and draft animals, rubble concrete in the core and cut stone on the face. Contracting out began only in 1960.

The designer usually named is Kaba Makoto (蒲孚), a Home Ministry engineer. The Agency for Cultural Affairs credits him with the Second Dam and identifies a wide, round-shouldered trapezoidal Spillway notch as the hallmark of the dams built under this project. The JSCE calls the shape the "Kaba type" and notes that nothing quite like it exists elsewhere in Japan.

In 2014 the JSCE selected 16 dams in the basin, including eight registered ones, as Civil Engineering Heritage under the title "Sabo Dams of the Inari River Basin, Nikko." The seven registered dams are still working erosion-control structures. Each dam's construction and points of interest are covered on its own page.

Visiting the Inari River Sabo Dams

The Inari River Sabo Dams are outdoor flood-control works, so there is no reception, no opening hours and no admission fee. Because they are still in service, you view them from the path along the river and may not climb onto them. The Nikko Sabo Office has put up signposts and information boards along the dam section, and the stream can be followed on foot from one dam to the next.

By rail, start at JR Nikko Station or Tobu-Nikko Station. According to the Nikko City Tourism Association, the World Heritage loop bus takes about 7 minutes to the Shinkyo stop, and from there Takinoo Shrine, behind the shrine-and-temple complex, is about 35 minutes on foot. The dams lie north of the shrine, up the Inari valley, and the walk from the Sixth Dam to the Tenth climbs steadily. By car, the Tochigi Film Commission gives about 10 minutes from Nikko Interchange on the Nikko-Utsunomiya Road, but there is no parking set aside for the dams.

A round trip on foot from the station takes about half a day. After rain the stream can rise quickly, so keep away from the banks, and expect icy paths in winter. For background on erosion control in Nikko, the Nikko Sabo Office runs a SABO Information Corner inside the Nikko Kyodo Center in town.