Ashizawa Stone Masonry Dams

Four Meiji-Era Stone Dams on the Ashizawa Stream

The Ashizawa Stone Masonry Dams (芦澤石積堰堤, Ashizawa ishizumi entei) are four dry-laid stone sabo dams built by Japan's Home Ministry in 1887 on the Ashizawa stream in Asa, Omi Village, Nagano Prefecture, and all four were entered on the national register as Registered Tangible Cultural Properties on October 7, 2014. Sabo is the Japanese term for erosion and sediment control, and a sabo dam is a low barrier that holds back gravel and boulders rather than water. The dams are numbered from the bottom of the valley upward, and each is registered separately.

The Ashizawa is a small tributary in the basin of the Omi River, part of the Shinano River system, and it runs through the Neo district of the village. No. 2 sits 90 metres upstream of No. 1, No. 3 lies a further 190 metres up, and No. 4 another 300 metres beyond, so the whole sequence stretches roughly 580 metres along the stream.

The Agency for Cultural Affairs records the four as follows.

• Ashizawa No. 1 Stone Masonry Dam: crest length 10 metres, height 4.2 metres
• Ashizawa No. 2 Stone Masonry Dam: crest length 12 metres, height 3.8 metres
• Ashizawa No. 3 Stone Masonry Dam: crest length 16 metres, height 4.0 metres
• Ashizawa No. 4 Stone Masonry Dam: crest length 15 metres, height 6.8 metres

Only No. 4, at the top, rises to 6.8 metres. The lower three are held to around 4 metres.

The Home Ministry's War on Mountain Erosion

The Ashizawa Stone Masonry Dams belong to an early national campaign to control erosion in the headwaters of the Shinano, Japan's longest river. According to Nagano Prefecture's Saigawa Sabo Office, the Home Ministry began direct sabo works in 1881, choosing 12 tributaries of the Chikuma and Sai rivers that were shedding the most sediment downstream. Within the office's present district, work began on one stream in 1883 and at Ashizawa in 1887, combining stone dams with fences and planted terraces on the bare hillsides above.

The Ashizawa works appear in a 1911 Home Ministry record of mountain sabo projects, which dates them to the months of April through December 1887. In its note on No. 4, the Agency for Cultural Affairs calls the dams a valuable survival from the very first public works carried out by the ministry after its regional civil engineering supervisory offices were set up.

The ground here needs the help. Omi Village sits at the northern end of the Chikuhoku Basin on soft, crumbly Neogene strata. More than 130 years after they were built, the four dams are still working sabo structures, protecting homes and fields downstream.

Mutsumaki Dry-Stone Work and a Modern Profile

Each of the Ashizawa Stone Masonry Dams is built from undressed fieldstones, mostly rounded andesite boulders 50 to 120 centimetres across, laid without mortar in a dry-stone technique known as mutsumaki. The stones lock together by shape and weight alone. Nagano Prefecture notes that the method demands a high level of skill and would be hard to reproduce today. The stone came from within Omi Village.

The design is cleaner than the rough material suggests. A lower spillway notch in the centre is set off from the raised wing sections on either side by crisp straight lines, an elevation the Agency describes as modern and common to all four dams. In plan, the crest is laid on a curve, apparently to gain some of the strength of an arch.

The layout carries an idea too. In its note on No. 3, the Agency explains that low, gently sloped stone dams placed one after another along the stream steadied the channel bed and helped vegetation return to the slopes. Instead of one tall wall to catch everything, a staircase of modest barriers flattens the gradient. All four were registered under the criterion of contributing to the historic landscape of the country, with consecutive registration numbers 20-0455 to 20-0458.

Keeping the Old Dams While Adding New Protection

Registration did not settle every question. The Ashizawa Stone Masonry Dams fall short of current design standards for sabo structures, and the valley below holds houses, an evacuation site and facilities on a designated emergency transport route. Nagano Prefecture launched a new sabo project here in fiscal 2014. Between October 2016 and January 2017, a committee of officials, heritage and sabo specialists and local residents met three times to plan works that would leave the historic dams untouched.

The core of the plan was to rebuild two concrete check dams from 1962 and 1974. To avoid disturbing the stone dams upstream, engineers thickened them on the downstream side instead of raising them, turning them into debris-flow barriers between fiscal 2019 and fiscal 2022. Their faces were clad in stone quarried in Omi Village so that they sit comfortably beside the Meiji masonry. A new open-type sabo dam followed further downstream in fiscal 2023 and fiscal 2024, also finished in local stone.

The work was completed in fiscal 2025. The streamside path now has a gravel surface, log steps and boardwalks lead close to the stone dams, 7 interpretive signs explain each structure, and a car park has been added. On one short walk you can compare dry-stone dams from 1887 with their reinforced modern neighbours.

Visiting the Ashizawa Dams

The Ashizawa Stone Masonry Dams are open-air structures with no admission fee and no booking. Nagano Prefecture's sabo division lists no visiting conditions and recommends spring through autumn. A walking path of about 1 kilometre follows the stream, and a prefectural blog reports that a round trip from the car park at the Omi Village sports ground to No. 4 at the top takes about 1 hour on foot. Heading upstream, you pass No. 1, No. 2, No. 3 and the reinforced dams before reaching No. 4.

The nearest station is Hijiri-Kogen on the JR Shinonoi Line, and the dams lie roughly 1 kilometre west of it. By car, the Saigawa Sabo Office directs visitors from the sports ground entrance west along Prefectural Route 12 (the Maruko-Shinshu-Shinsen road) for about 400 metres, then north at the corner by the Omi Shinrin Kensetsu company building. Its 2021 directions put a gravel car park beside the road about 600 metres after the turn, with a walk of around 10 minutes from there. Signs now guide drivers from the Omi interchange on the Nagano Expressway.

The prefectural sheet does not tick the box for toilets, so do not count on finding one. The prefecture warns of uneven footing and falling stones and branches, and advises that, for safety, people generally should not approach sabo dams. Stay on the path and boardwalks rather than climbing onto the dam faces or into the spillways. The prefectural materials state no restrictions on photography.