Yamanokami Entei: A Living Monument to Early Showa-Era Disaster Prevention
Standing quietly in the forested gorge of the Mizunashi River, Yamanokami Entei (山ノ神堰堤) is a gravity-type sabo dam that has protected the residents of Hadano City for over nine decades. Built in 1932 as part of a national response to devastating earthquake-triggered landslides, this Registered Tangible Cultural Property represents a remarkable fusion of civil engineering ingenuity and harmony with the natural landscape. Located in the scenic foothills of the Tanzawa Mountains in Kanagawa Prefecture, the dam continues to serve its original function while also standing as a testament to Japan's long history of living alongside—and engineering solutions for—the forces of nature.
Historical Background: From Earthquake Devastation to Engineering Achievement
The story of Yamanokami Entei begins with a series of natural disasters that reshaped the Tanzawa region. In 1923, the Great Kanto Earthquake caused the collapse of approximately 6,000 hectares—roughly 20% of the surface area of the Tanzawa Mountains. Seven years later, in November 1930, the North Izu Earthquake (Kita-Izu Jishin), a magnitude 7.3 event centered near the Tanna Fault in the northern Izu Peninsula, triggered further landslides throughout the Tanzawa mountain system.
The Mizunashi River, which originates near the summit of Mount To (Tonodake, 1,491 m), had long been prone to catastrophic flooding. In ordinary conditions, the river's flow would disappear underground as it crossed the Hadano Basin—hence the name "Mizunashi" (waterless). However, during heavy rains, it would transform into a torrent carrying enormous volumes of rock and sediment, sometimes flooding an area over 200 meters wide and destroying farmland and homes along its course.
In response to these compounding disasters, the Japanese Ministry of Home Affairs (Naimusho) launched a direct national project to construct a series of sabo (erosion control) dams along the Mizunashi River. Yamanokami Entei was completed in 1932 (Showa 7) as one of the key structures in this comprehensive flood and sediment control system, alongside the nearby Saruwatari Entei upstream and the Togawa Entei downstream.
Why Was It Designated as a Cultural Property?
Yamanokami Entei was registered as a National Registered Tangible Cultural Property (Building) on March 18, 2003 (Heisei 15). The registration was granted under the criterion that the structure is "difficult to reproduce" (再現することが容易でない), recognizing the exceptional craftsmanship and historical engineering techniques embodied in its construction.
Several factors contributed to its cultural significance. The dam is a gravity-type masonry structure (重力式練積堰堤) with a dam length of 32 meters and a height of 13 meters, featuring an upstream slope ratio of 1:0.5 and a downstream slope ratio of 1:0.2. What makes it particularly noteworthy is the ingenious irrigation water conduit built into its right bank. Rather than simply blocking the river's flow, the engineers incorporated a cylindrical protrusion that follows the natural contours of the existing bedrock to channel irrigation water from the spillway's right bank edge around and down the front face of the dam. This practical and creative solution demonstrates the sophisticated approach of early Showa-era engineers who designed structures to serve multiple purposes—disaster prevention and agricultural support—simultaneously.
The dam also includes a secondary sub-dam (副堰堤) and upstream right bank revetment, forming a complete erosion control system. The stonework of the dam body blends harmoniously with the surrounding rock faces and forest, exemplifying a design philosophy that respects the natural environment even while fundamentally altering it for safety.
Architectural and Engineering Highlights
Yamanokami Entei is a gravity-type concrete dam built using rubble masonry construction techniques. The dam relies on its own mass and the weight of the materials to resist the pressure of water and sediment—a proven approach for mountain sabo facilities.
The most distinctive engineering feature is the cylindrical water conduit on the right bank side. At the right edge of the spillway, the engineers constructed a tube-shaped extension that protrudes from the dam's front face. This conduit was carefully shaped to conform to the natural rock formations at the site, allowing irrigation water to be directed from behind the dam around its face and downstream to agricultural fields. This kind of dual-purpose design—integrating flood control with agricultural infrastructure—was characteristic of the pragmatic engineering philosophy of the era.
The dam sits in a narrow gorge along the middle reaches of the Mizunashi River, taking advantage of the natural constriction in the valley to maximize its effectiveness. The stone and concrete materials have aged gracefully over the decades, with moss and vegetation softening the structure's appearance and integrating it seamlessly into the forest landscape.
Visitor Information and How to Experience the Dam
Yamanokami Entei is located upstream from Kanagawa Prefectural Hadano Togawa Park (県立秦野戸川公園), one of the most popular recreational areas in western Kanagawa. While the dam itself is situated in the mountain forest above the park, it can be viewed from the park area and accessed via the forest road that continues upstream along the Mizunashi River toward Mount To.
The surrounding Hadano Togawa Park offers an excellent setting for a day trip. The park's iconic landmark is the Kaze no Tsuribashi (Wind Suspension Bridge), a stunning 267-meter-long, 35-meter-high pedestrian bridge spanning the Mizunashi River with panoramic views of the Tanzawa Mountains. The park features six themed areas including family recreation, riverside play, cultural arts, and forest nature observation. Visitors can enjoy barbecuing, nature walks, farming experiences, and relaxation at the traditional tea house "Osumi Sankyo."
For hikers, the area serves as the Okura trailhead, one of the most popular starting points for climbing Mount Tonodake and exploring the Tanzawa range. The Saruwatari Entei, another registered cultural property from the same construction project, can be seen along the hiking trail leading up to Mount To.
Surrounding Attractions
The Hadano area offers a wealth of experiences beyond the dam itself. Three sabo dams along the Mizunashi River—Yamanokami Entei, Togawa Entei, and Saruwatari Entei—are all registered cultural properties and together tell the story of Japan's sabo engineering heritage. Each dam showcases different engineering approaches adapted to its specific location in the river system.
Hadano City is also renowned as one of Japan's finest groundwater cities, with the famous Hadano Meisui (Hadano Famous Water) recognized as one of Japan's Top 100 Waters. The city's sake breweries benefit from this pristine water source. The Hadano History Museum (formerly Sakuradote Kofun Exhibition Hall) provides context on the region's long history from the Jomon period onward.
For those seeking more mountain adventure, the Tanzawa-Oyama Quasi-National Park offers extensive hiking trails through forests of beech and cypress, with views extending to Mount Fuji on clear days. The Shibusawa Hills area to the south offers gentler walks through terraced landscapes and seasonal flower gardens.
- 📍 Plan Your Visit to Kanagawa
- Find tours, activities & transport passes near Yamanokami Dam
- [ Kamakura Buddha Hakone Onsen & Yokohama Tours ]
Q＆A
- What is a sabo dam, and how is Yamanokami Entei different from a regular dam?
- A sabo dam (砂防堰堤) is specifically designed to trap sediment and control debris flows in mountain rivers, rather than to store water for supply or hydroelectric power. Yamanokami Entei is notable for integrating an irrigation water conduit into its structure, serving both disaster prevention and agricultural purposes simultaneously.
- Can visitors walk up to the dam directly?
- The dam is located upstream from Hadano Togawa Park in a forested mountain area. It can be viewed from below via the unpaved forest road that continues upstream along the Mizunashi River. The path passes through rugged terrain, so sturdy footwear is recommended. The downstream Togawa Entei within the park itself is more easily accessible.
- Is there an admission fee to see the dam or enter Hadano Togawa Park?
- Hadano Togawa Park is free to enter and open year-round. The dam is on public land and there is no admission fee. Parking at the park is available for a small fee.
- How do I get to Yamanokami Entei by public transportation?
- Take the Odakyu Odawara Line to Shibusawa Station, then board the Kanachu bus bound for Okura (about 15 minutes). From the Okura bus stop, Hadano Togawa Park is a short walk, and the dam is located further upstream along the Mizunashi River. By car, it is about 10 minutes from the Hadano-Tanzawa Smart IC on the Shin-Tomei Expressway.
- Are there other registered cultural properties nearby?
- Yes. The Togawa Entei (downstream) and Saruwatari Entei (upstream) were built as part of the same 1932 sabo project and are both National Registered Tangible Cultural Properties. Hadano City also has several other registered cultural properties, including historical merchant houses and the Soyanosuidou waterworks.
Basic Information
| Name | Yamanokami Entei (山ノ神堰堤) |
|---|---|
| Type | Gravity-type rubble masonry sabo dam (重力式練積堰堤) |
| Completed | 1932 (Showa 7) |
| Dimensions | Dam length: 32 m, Dam height: 13 m; with secondary sub-dam and upstream right bank revetment |
| Constructed By | Ministry of Home Affairs (内務省), national direct project |
| Owner | Kanagawa Prefecture |
| Cultural Property Status | National Registered Tangible Cultural Property (Building), registered March 18, 2003 |
| Location | Horiyamashita and Togawa, Hadano City, Kanagawa Prefecture |
| Access | Odakyu Line Shibusawa Station → Kanachu Bus to Okura (approx. 15 min) → upstream on foot along the Mizunashi River |
References
- 山ノ神堰堤 — 秦野市公式ホームページ
- https://www.city.hadano.kanagawa.jp/soshiki/4/1019/8/5/3/2/632.html
- 山ノ神堰堤 — 文化遺産オンライン
- https://bunka.nii.ac.jp/heritages/detail/179426
- 水無川の砂防遺産 — 神奈川県ホームページ
- https://www.pref.kanagawa.jp/docs/rt3/cnt/f645/p746199.html
- 水無川 (神奈川県) — Wikipedia
- https://ja.wikipedia.org/wiki/%E6%B0%B4%E7%84%A1%E5%B7%9D_(%E7%A5%9E%E5%A5%88%E5%B7%9D%E7%9C%8C)
- 秦野戸川公園 — 神奈川県ホームページ
- https://www.pref.kanagawa.jp/docs/tu5/cnt/f6599/p19718.html
Last verified: 2026.03.28