Kounomizo-Hyakutaroumizo Irrigation System was built around 300 years ago to develop new rice fields and it has greatly contributed to the agricultural development of this region. The overall length of Kounomizo is 15.4km and that of Hyakutaroumizo is 18.9km. Those facilities currently irrigate 2,822ha (about 40%) of the paddy fields in the region.
Kounomizo Old Weir was constructed across Kuma River, one of the three most rapid rivers in Japan. Its construction started in 1696 and took 9 years to complete. Even at the upper stream and undertaken during the relatively dry season, it is easy to assume the construction was extremely difficult taking in consideration the poor civil engineering technology of that time. Indeed, before its completion, the weir was flown away twice by floods. So to bear the water pressure and the impact of flowing rocks and tree trunks, unusual L-shaped structure was finally adopted.
Another distinctive feature of Kounomizo Irrigation System is its three irrigation tunnels with total length of 2,524m. It was the longest in Japan through the Edo period (1603-1867). The inside of the two tunnels were strengthened by A-frame structure called Gassho-zukuri with stone pillars of 110kg apiece. There exist only three such tunnels in Japan, and Kounomizo’s tunnel which adopted this method in 1729 is the oldest.
About Hyakutaroumizo, the most salient feature is that it was hand dug by farmers from children to the elderly through five construction periods over several hundred years. Unlike Kounomizo’s construction which was ordered by a feudal lord, there was no governmental support nor special instructors or leaders. Therefore, it can be said that Hyakutaroumizo is the product of blood, sweat and tears of farmers. The exact year of the first excavation is unknown due to the lack of the document, but it is believed that its construction already started sometime in Kamakura Period (1185-1133). The second construction began in 1677, and the fourth completed in 1710. The fifth construction started in 1740 but ended in failure without water flowing into the canal.
It is said that the excavation of Kounomizo was carried out while referring to the civil engineering methods of Hyakutaroumizo, which had been dug only by farmers over the years. The water of most branches of Kounomizo eventually flows into Hyakutaroumizo and water use of both canals is integrated.
In 2006, Kounomizo-Hyakutaroumizo Irrigation System was designated as one of the Top 100 Canals in Japan. In addition, both of their facilities appear in elementary school textbooks as agricultural heritage of the region.
Kounomizo-Hyakutaroumizo Irrigation System is still operating. Its management and maintenance are in charge of Land Improvement Districts.