VOL.204 JUNE 2025
JAPAN’S RELATIONSHIP WITH WATER
Honen-ike Dam, Japan’s Oldest and Notably Beautiful Stone Multiple-Arch Dam
Honen-ike Dam is Japan’s oldest stone dam.
Honen-ike Dam, located in Kan-onji, Kagawa Prefecture, is known as Japan’s oldest and only stone multiple-arch dam.1 Built as an agricultural reservoir, this dam is recognized for its historical and engineering significance and has been designated an Important Cultural Property of Japan.
Kan-onji, located in the southwestern part of Kagawa Prefecture in the Shikoku region, is a nature-rich area bordered to the west by Hiuchi-nada, a semi-enclosed bay of the Seto Inland Sea, and to the south by the Sanuki mountain range, which forms the boundary with Tokushima and Ehime prefectures.
Honen-ike Dam is located upstream on the Kunita River, which flows through the mountain valley. The stone arch dam has a majestic and dignified appearance, resembling a medieval European castle, and is a popular tourist spot. To learn more about the background of its construction, we spoke with OKADA Yosuke of the Cultural Promotion Division of Kan-onji City.
“Kagawa Prefecture, facing the Seto Inland Sea, has long struggled with water shortages due to its low rainfall and lack of large rivers. In particular, Onohara-cho (now part of Kan-onji City) suffered severe droughts, with dry plains so widespread that people said, ‘it even burns under the moonlight’,” explains OKADA.
“After two major droughts in the early 1920s, momentum grew to build a modern irrigation pond, which led to the planning of Honen-ike Dam. Construction began in 1926 as a prefectural government project under the guidance of SANO Tojiro,2 an engineering expert. With the cooperation of local farmers working in teams, the dam was completed in just three years and eight months. The completed dam stretches 145.5 meters long and stands 30.4 meters high.”
Even though Japan’s concrete dam construction technology was still in its early stages, the project brought together around 150,000 workers and was completed without a single accident or loss of life. It was highly praised as a public works achievement, with a construction method that was innovative for the period.
“At the time, concrete gravity dams—structures that resist the horizontal thrust of the retained water entirely by their own weight—were the norm. But the Honen-ike Dam was built as a stone masonry multiple-arch dam, combining arches with supporting walls. It also incorporated several innovative features for its time, such as a siphon-style spillway built into the wall to release floodwater, and concrete blocks used on the challenging outer walls. These advanced design and construction methods are still greatly regarded today.”
The construction of the Honen-ike Dam brought major changes to the surrounding communities. According to OKADA, “With the dam now able to store about 1.6 million tons of water, the water supply stabilized. This revitalized 530 hectares of farmland on the west side of the Kunita River in Kan-onji. Today, in addition to rice farming, vegetable cultivation is thriving, and the area has become one of Japan’s leading lettuce producers.”
Honen-ike also plays an important role beyond agriculture. By regulating water storage and release, it helps reduce flood damage in downstream urban areas, serving as an essential part of the region’s disaster prevention infrastructure and closely supporting urban functions.
At this time of year, the yuru-nuki (water release) event takes place at Honen-ike to supply water to the rice fields, attracting many visitors. “Yuru-nuki is a seasonal event where the dam’s intake valve is opened to let water flow into the paddies. To keep water levels balanced with other reservoirs downstream, it is held every year from late July to early August. With a thunderous roar, as much as four tons of water are released every second, creating a magnificent spectacle. We hope many people will come to enjoy this amazing sight.”
- 1. A dam structure that combines multiple arches supported by pillars called buttresses to distribute water pressure. Built using stone masonry, it balances strength and lightness. This type is rare in Japan and was constructed to secure water for agriculture.
- 2. A Doctor of Engineering who contributed to the development of modern civil engineering in Japan. Known as an expert in constructing agricultural irrigation ponds and dams.
By MOROHASHI Kumiko
Photo: Kan-onji City; PIXTA