The traditional Saga irrigation system in Saga city, Kyushu Region, Japan.
Kanako Inai
2025
The Tsukushi Plain stretches about 30 to 40 kilometers from the Ariake Sea, with an elevation of only 5 to 6 meters above sea level. This vast plain was formed by the tidal fluctuations of the Ariake Sea, the sediment carried by rivers from the surrounding mountains, and the land reclamation by humans. The Ariake Sea has the largest tidal range in Japan, with a difference between high and low tide that can exceed 6 meters.
Approximately 6,000 years ago, the coastline was about 20 kilometers further from its current location. Over time, rivers deposited sediment, raising the riverbeds, and flooding created new channels, gradually expanding the plain. Around the 1600s (the focus period of this report), artificial land reclamation began in earnest to expand agricultural land, further shaping the present-day Tsukushi Plain.
The water network within Saga City has been ingeniously designed and functions as a cohesive, integrated water management system that extends from the mountain base to the Ariake Sea.
Saga City lacked a large river capable of sufficiently supplying water. Around 1600, Naridomi Hyōgo Shigeyasu constructed the Ishii sluice on the Kase River to address this issue. It allowed drawing water to the castle town via the Tafuse River from the Kase River. By linking the Tafuse River and other secondary rivers sourced from the surrounding mountains to an intricate network of canals, he aimed to resolve its water shortages.
Circular Stories
The canals were multi-functional waterways closely connected to people’s daily lives. Essential to everyday life, these canals were managed collectively by the local community, with regular dredging and cleaning activities. Fish and aquatic plant seeds harvested from the canals were popular ingredients for cooking, while the dredged mud was spread over paddy fields as fertilizer. There was a sustainable cycle in which human maintenance and the natural environment interacted.
