Odai Irrigation Canal

What the Odai Irrigation Canal Is

The Odai Irrigation Canal (小田井灌漑用水路, Odai Kangai Yōsuiro) is an agricultural canal begun in 1707 that runs about 33 kilometers along the north bank of the Kinokawa River, from Hashimoto City through Katsuragi Town and Kinokawa City to Iwade City in Wakayama Prefecture, and four of its structures in Kinokawa and Katsuragi were registered as Registered Tangible Cultural Properties of Japan in 2006. Japanese government sources often shorten the name to Odai Yōsui (小田井用水).

The registered four are three aqueducts, called toi (渡井), that carry the canal over side streams, and one water gate where the canal dips beneath a stream. Each began as a timber structure in the Edo period and was rebuilt in brick between 1909 and 1919. They belong to the Odai Land Improvement District, the farmers' organization that runs the canal. The Odai Irrigation Canal still waters about 610 hectares of rice paddies, and all four structures remain part of the working channel.

Ōhata Saizō and a Canal on a Gentle Slope

According to Japan's Ministry of Agriculture, Forestry and Fisheries (MAFF), the Odai Irrigation Canal was dug by Ōhata Saizō (大畑才蔵) on the orders of Tokugawa Yoshimune, then lord of the Kishū domain. Saizō was born in 1642 to a farming family in Kamuro village, now part of Hashimoto, and rose to become a domain official in charge of irrigation works. He had already laid out the Fujisakii canal, about 24 kilometers long, which draws from the Kinokawa at Fujisaki in Kinokawa City on the same bank.

MAFF's Kinki regional office gives the canal's length as about 33 kilometers with a total fall of only 22 meters, a gradient of roughly 1 in 1,500, or about 67 centimeters per kilometer. Holding that slope meant tracing the contour lines of the river terraces, and the channel meets one small stream after another on its way west. Where they cross, the canal either rides over the stream on an aqueduct or passes under it through an inverted siphon (fusekoshi).

Without modern surveying instruments, Saizō is credited with devising a water-filled leveling device called a mizumoridai, calculating the earth to be moved and the labor required, and dividing the route into sections dug at the same time. A message from the governor of Wakayama Prefecture in November 2017 dates the start of work to 1707 and says the first 21 kilometers were finished in one year and eight months. MAFF's profile of Saizō, however, says the canal was not completed in his hands, so sources differ on when the full line opened. The finished canal supported the creation of about 1,000 hectares of new rice land.

From Timber Flumes to Brick on the Odai Irrigation Canal

The Edo-period aqueducts were wooden flumes, and MAFF's notes on each structure say heavy rain damaged them often. The Agency for Cultural Affairs records that an improvement program on the Odai Irrigation Canal began in the late Meiji era, and the registered four are the brick replacements built under it.

They were registered on March 2, 2006, as numbers 30-0085 to 30-0088, and MAFF describes them as the first civil engineering structures in Wakayama Prefecture to be registered. From upstream to downstream, with dates and dimensions from the Agency's database, they are:

• Nakatani River Water Gate (Nakaiburi, Katsuragi), 1912: a brick inverted siphon taking the canal under the Nakatani River, 13 meters long and 6.4 meters wide
• Koniwadani River Aqueduct (Kasedahigashi, Katsuragi), 1909: the earliest of the four, a single brick arch 9.3 meters long and 7.4 meters wide
• Tatsunotoi Aqueduct (on the line between Nishinoyama in Kinokawa and Takada in Katsuragi), 1919: the largest, crossing the valley of the Anabuse River, 21 meters long and 5.3 meters wide
• Kozumi River Aqueduct (Nishimitani in Kinokawa, on the boundary with Higashisakamoto in Iwade), 1914: the smallest, 6.0 meters long and 3.8 meters wide

Measured on a map, the upstream Nakatani River Water Gate and the downstream Kozumi River Aqueduct are about 19 kilometers apart in a straight line. The Koniwadani and Kozumi aqueducts were registered for contributing to the country's historic scenery, the Tatsunotoi Aqueduct and the Nakatani River Water Gate for being difficult to reproduce.

Brickwork Shared by the Odai Irrigation Canal Structures

All four stand on brick arches. The Agency's descriptions give the arch rings as four bricks thick at the Nakatani River Water Gate, the Koniwadani River Aqueduct and the Kozumi River Aqueduct, and seven bricks thick at the Tatsunotoi Aqueduct. MAFF classes the three aqueducts as filled-spandrel arch bridges, solid from the arch up to the channel.

The bond of the walls is where they differ. The Nakatani, Koniwadani and Tatsunotoi structures use Flemish bond, known in Japan as French bond (フランス積), with long faces and short ends alternating in every course; at Tatsunotoi only the wall rising above the stone-built spandrels is brick. The Kozumi River Aqueduct alone uses English bond, in which a course of long faces alternates with a course of ends. Look closely at the joints and the two patterns are easy to tell apart.

World Heritage Irrigation Structure Listing and Upkeep

On October 10, 2017, the Odai Irrigation Canal was added to the World Heritage Irrigation Structures list of the International Commission on Irrigation and Drainage (ICID). Wakayama Prefecture's newsletter Kenmin no Tomo also describes it as one of MAFF's 100 selected historic canals (Sosui Hyakusen).

The Odai Land Improvement District looks after the four structures. MAFF reports that in January 2006, after the recommendation to register them, the district and local officials held their first conservation day of grass cutting and litter picking. Routine cleaning by the district continues, along with visits from elementary school classes.

Visiting the Odai Irrigation Canal

All four structures of the Odai Irrigation Canal still carry irrigation water. There is no reception desk or admission fee, and they are viewed from the surrounding roads. Stay out of the channel and off its edges, and keep out of the fields. No official guidance on photography could be found (checked October 2026).

Each one lies near a station on the JR Wakayama Line, which runs parallel to the canal. MAFF gives walking times of about 15 minutes from Shimoisaka Station to the Kozumi River Aqueduct, about 10 minutes from Nishikaseda Station to the Tatsunotoi Aqueduct and about 10 minutes from Kaseda Station to the Koniwadani River Aqueduct. Its page on the Nakatani River Water Gate also says 10 minutes from Kaseda Station, but Nakaiburi lies about 5 kilometers east of Kaseda, and the nearest stop is Nakaiburi Station, about 250 meters away in a straight line. By car, MAFF allows 40 minutes to an hour from Wakayama Interchange on the Hanwa Expressway.

Riding the line west from Nakaiburi to Kaseda, Nishikaseda and Shimoisaka lets you see the four in the order the water reaches them.