Izawa River No. 3 Stone Check Dam

Thirty-Five Metres Across a Small Stream

Izawagawa Dai-sango Ishi-entei (荏沢川第三号石堰堤, also written Izawa-gawa Dai-sangō Ishi-entei, the No. 3 Stone Check Dam on the Izawa River) is a dry-stone erosion-control dam in Kuwabara, Chikuma City, Nagano Prefecture, built around 1884 in the Meiji era and listed as a Registered Tangible Cultural Property of Japan on 8 January 2009.

The proportions are the first surprise. The dam is 35 metres long and only 3.7 metres high, a ratio of nearly ten to one. Nagano Prefecture calls it the largest of the four stone dams surviving on this stream, and in surface area it plainly is, though two of its neighbours stand taller. It lies roughly 100 metres downstream of the No. 2 dam, on a stretch where the valley floor opens out and the channel loses its confinement.

That widening explains the length. Where a narrow gorge lets a builder anchor a dam into rock on both sides within a few metres, an open reach offers no such convenience. The Meiji engineers responded by extending the structure sideways until it reached solid ground, producing wing sections, sodebu in Japanese, that exceed ten metres on each side of the central spillway. The Agency for Cultural Affairs records that these wings were built as a single integrated body with the spillway rather than added as separate abutments.

Reading the Stonework

Stand in front of the dam and the masonry sorts itself into two grammars.

In the central spillway section, the mizutoshibu, the blocks are large. Water concentrates here and carries whatever the slopes above have shed, so mass was the answer: heavy stones that a flood cannot dislodge. Out along the wings, where flow reaches the face only during high water, the stones are noticeably smaller. Nagano Prefecture points to this graded use of material as one of the defining habits of early Meiji stone dam construction, and once noticed it is difficult to unsee.

The crest of the spillway is not level. It dips in a shallow curve that Japanese engineers call nawadarumi, the sag of a slack rope, which draws the flow toward the centre and away from the wings. The prefecture describes the resulting profile as an arch, and the word fits the visual effect even though the structure carries no arch action in the engineering sense.

Below the body, an apron 5.2 metres long continues downstream from the toe, paved so that falling water strikes dressed stone rather than gravel. Every course is set dry. There is no mortar anywhere in the structure, and the joints are packed with small stones driven in by hand, a technique that leaves the wall permeable and lets pore pressure escape rather than build up behind it.

The dam was registered under the criterion of contribution to the historical landscape of the nation, with registration number 20-0338. Prefectural records give the construction period for the group as 1882 to 1884, which places the work under the direct administration of the Home Ministry and well before the Sabo Act of 1897.

Visiting

A footpath of about one kilometre runs along the valley and passes all four dams, with parking and an explanatory board at the entrance. Access is free and unrestricted, with no fixed hours and no limits on photography. Nagano Prefecture recommends spring through autumn and notes that no toilets are provided on site. The drive from Chikuma City Hall takes roughly twenty minutes, following the prefectural Komine-Inariyama road into the Kuwabara district.

No. 3 is the second dam encountered when walking upstream from the trailhead, and it is the one most likely to stop a visitor. Its width fills the field of view in a way the others do not. Photographing it whole takes a step back and a wide lens; the wings run out of frame quickly.

Come after rain if the schedule allows. A thin sheet of water spreading across the spillway lip makes the sag of the crest legible, and the difference between the wet centre and the dry wings maps out exactly how the designers expected the stream to behave.

Residents of the Kuwabara district cut the grass along the path each year, working alongside the prefecture. The site is close to houses and the walking is undemanding, which makes it an easy addition to a day spent at the Inariyama merchant quarter, an Important Preservation District for Groups of Traditional Buildings since 2014, or the terraced fields of Obasute.