Hakone Waterway Completion Day

What Is Hakone Waterway Completion Day?

Every February 25th is "Hakone Waterway Completion Day" (箱根用水完成の日, Hakone Yōsui Kansei no Hi) in Japan. On February 25, 1670, the Hakone Yōsui (箱根用水, Hakone Waterway) was completed — an irrigation tunnel that channeled water from Lake Ashi (芦ノ湖, Ashinoko) in the Hakone mountains through a 1,280-meter-long tunnel bored through the mountain ridge to irrigate the farms of the Suruga region (present-day Shizuoka Prefecture). The project, completed in the early Edo period, was a remarkable feat of pre-modern engineering that transformed agricultural productivity in the region.

Engineering in the Edo Period

The project was conceived by Ōba Gennojō, headman of Fukara Village, and carried out by Edo merchants including Tomono Yoemon under contract with the local domains. It was designed to solve a persistent problem: the Hakone region had abundant water in Lake Ashi, but the mountains between the lake and the farming areas of Suruga Province prevented the water from reaching the fields where it was needed. The solution — boring a tunnel through the outer rim of the Hakone volcano near Kojiri Pass to connect the lake to channels flowing down to the farming areas — took about four years of hand digging from both ends, from 1666 to 1670.

Historical Significance of Water Control

The Hakone Yōsui represents the Edo period's sophisticated approach to hydraulic engineering for agricultural development. Throughout Japan, Edo-period engineers constructed irrigation channels, reservoirs, river embankments, and land reclamation projects that dramatically expanded Japan's agricultural productive capacity. The rice fields and settlements made possible by these water control projects formed the foundation of Edo-period rural prosperity and population growth.

Hakone Yōsui Today

The Hakone Yōsui — now more than 350 years old — continues to supply water for irrigation and hydroelectric power in Susono and neighboring areas of Shizuoka Prefecture, irrigating about 530 hectares. The waterway was registered as a World Heritage Irrigation Structure in 2014 and selected as a Civil Engineering Heritage site by the Japan Society of Civil Engineers in 2023, preserving this remarkable example of Edo-period hydraulic engineering as a historical monument.

Fun Facts

• Lake Ashi: Lake Ashi (芦ノ湖, Ashinoko) — the source of the Hakone Yōsui — is a caldera lake formed about 3,000 years ago by volcanic activity of Mt. Hakone. Today it is best known as one of Japan's most scenic lakes, famous for its views of Mt. Fuji reflected in its waters and as the setting of torii gates appearing to float on the lake surface from the Hakone shrine.
• Hakone's volcanic heritage: The Hakone area remains volcanically active — phreatic eruptions at Mt. Ōwakudani (大涌谷) have periodically restricted access to parts of Hakone. The volcanic heat that makes Hakone's famous hot springs (onsen) is the same geological force that originally created Lake Ashi and inspired the ambitious waterway engineering it commemorates.