Yokoyu River Stone Masonry Dams

What Are the Yokoyu River Stone Masonry Dams?

The Yokoyu River Stone Masonry Dams (横湯川石積堰堤群, Yokoyugawa Ishizumi Enteigun) are eight erosion-control dams built between 1924 and 1944 on the Yokoyu River where it runs through Shibu Onsen in Yamanouchi Town, Nagano Prefecture, and all eight were registered together as Registered Tangible Cultural Properties of Japan on October 14, 2021. They belong to Nagano Prefecture.

The Yokoyu River drops out of the mountains east of the Yudanaka-Shibu hot spring villages and joins the Yomase River just below the town. None of the eight structures is a reservoir dam. Each is a low sabo dam, 2 to 4 meters high, with a concrete gravity core faced in stone, built to catch gravel and slow the river rather than to hold water. Adding up the spacing given in the Agency for Cultural Affairs descriptions, the run from the lowest dam (No. 1) to the highest (No. 57) is roughly 1.2 kilometers, with the dams stepping up the riverbed like a staircase.

The collective name is the one used by Nagano Prefecture's Hokushin Construction Office, which looks after the dams. The national register lists each dam separately, by number.

How the Yokoyu River Stone Masonry Dams Came to Be

The Yokoyu flows through a steep, crumbling valley, and landslides and debris flows hit the villages along it long before the modern era. According to Nagano Prefecture's sabo records, the prefecture began erosion-control work on the river in 1906, placing stone dams near the sources of the sediment upstream. Floods at the end of the Meiji era swept most of that work away, and the prefecture abandoned the project.

Local residents pressed for help, and in 1918 the national Home Ministry took the work over directly. Prefectural records say the ministry planned 13 large boulder-masonry dams: four on the lower river, three in the middle reach and six upstream. Four of the eight registered dams, Nos. 39, 50, 51 and 57, date from this phase, between 1924 and 1927.

In 1932 the project passed back to Nagano Prefecture, and a Yomase River sabo office was set up in the village of Hirao, now part of Yamanouchi. The plan came from Akagi Masao, the engineer who returned from study in Europe in 1925 and went on to direct erosion control for the Home Ministry; prefectural documents credit him with planning a channel-works scheme for the Yokoyu and Kakuma rivers and supervising its construction. Work continued until the war halted it, and the Hokushin Construction Office refers to the dams of this period by the nickname "Akagi Sabo." Nos. 1, 2 and 3, and the unnumbered Yokoyu River Dam of 1944, belong to this prefectural phase.

The Eight Yokoyu River Stone Masonry Dams, Downstream to Upstream

The numbers do not follow either the building order or the river. The oldest dam is No. 39, finished in 1924; No. 1 came 14 years later, in 1938. Listed from the bottom of the river up, with the completion year and size recorded by the Agency for Cultural Affairs:

• No. 1 Dam: 1938, crest length 48 m, height 3.0 m. The lowest dam on the river
• No. 2 Dam: 1937, 45 m long, 3.0 m high. About 200 m above No. 1
• No. 3 Dam: 1936, 40 m long, 3.0 m high. About 200 m above No. 2, directly below the Kurokawa Bridge
• No. 51 Dam: 1925, altered 1932, 52 m long, 1.8 m high. About 170 m above No. 3
• Yokoyu River Dam: 1944, 39 m long, 3.8 m high. About 140 m above No. 51
• No. 39 Dam: 1924, 41 m long, 3.6 m high. About 120 m above the Yokoyu River Dam
• No. 50 Dam: 1925, 34 m long, 2.4 m high. About 160 m above No. 39, beside the Wago Bridge
• No. 57 Dam: 1927, 58 m long, 2.0 m high. About 230 m above No. 50

The four Home Ministry dams cluster upstream, the three prefectural dams downstream, and the wartime Yokoyu River Dam slots in between. Not one of them stands more than 4 meters high. The idea is easy to read in the sequence: instead of one big barrier, a chain of low weirs at short intervals flattens the river's gradient so that the water loses its force.

Each dam treats its stone differently. No. 2 uses rounded cobbles of even size, No. 51 rough fieldstone, and No. 50 squared stone laid in Sangi-zumi corners that rise steeply at both shoulders. The eight joined the national register as Registered Tangible Cultural Properties in the 100th round of registrations, with consecutive numbers 20-0591 to 20-0598, all under the criterion of contributing to the historic landscape of the country. Each dam's own page covers its construction and what to look for.

Visiting the Yokoyu River Stone Masonry Dams

The Yokoyu River Stone Masonry Dams are working flood-control structures in the riverbed, so there is no ticket office, no opening hours and no admission fee. You see them from the roads and bridges above the river. No route down to the riverbed is signposted, and the water rises quickly after rain and during the spring snowmelt, so keep away from the banks at those times.

By train, the starting point is Yudanaka Station, about 50 minutes from Nagano Station on the Nagano Electric Railway limited express. From Yudanaka, a bus bound for Kanbayashi reaches Shibu Onsen in about 7 minutes; get off at Shibu Wagobashi to arrive at the Wago Bridge in the middle of the village. No. 50 sits beside the bridge. Below it come No. 39, the Yokoyu River Dam and No. 51, then No. 3 under the Kurokawa Bridge, and finally Nos. 2 and 1 further down. No. 57 lies a little over 200 meters upstream of the Wago Bridge. By car, allow about 20 minutes from the Shinshu-Nakano interchange on the Joshin-etsu Expressway.

To see all eight, start at the Wago Bridge and go on foot. From No. 57 at the top to No. 1 at the bottom is roughly 1.2 kilometers of river.