Marunuma Dam on August 27. (©Sankei by Takumi Kamoshida)
Walking along a path overgrown with trees and plants, a massive wall suddenly emerges, silent and imposing like a fortress. This is Marunuma Dam, located in Katashina Village, Gunma Prefecture, one of the gateways to the high-altitude marshlands of Ozegahara.
Completed in 1931, it is still in operation today. The dam was built between Lake Ojiri and Lake Marunuma as part of a reservoir project undertaken by Jomo Electric at the time.
The dam features a reinforced concrete cutoff wall that holds back water, supported by buttresses to resist pressure. This design is known as the buttress style. Eight dams of this type were built in Japan, six of which still exist. Marunuma Dam is the largest, standing 32.1 meters tall. In 2003, it became the first hydroelectric dam to be designated an Important Cultural Property of Japan.
Heart of the Community
In recent years, its unusual design and rarity have attracted visitors from outside the prefecture. A local volunteer group, Marunuma wo Aisuru Kai, literally the "Society of Those Who Love Marunuma," even built a raft called the "Tom Sawyer," allowing people to view the dam directly from the water.
"The dam is the heart of Marunuma's development," says 74-year-old Shigeru Kurita, the association's chairman. While it originally served as vital infrastructure supporting the region, it now also helps promote tourism.
Frost Damage
Maintaining the dam, however, comes with challenges — especially aging and frost damage. Katashina Village is the only officially designated special heavy-snow area in the Kanto region.
A representative of TEPCO Renewable Power, which manages the facility, explains, "Winter is a constant battle with frost damage. We want to ensure this historically valuable dam is preserved for future generations."
Spanning the Showa (1926–1989), Heisei (1989–2019), and Reiwa eras, Marunuma Dam continues to endure, kept alive by the dedication and support of many people.
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(Read the article in Japanese.)
Author: Takumi Kamoshida, The Sankei Shimbun
