Omaru Water Supply: Centered on the History of the Edo Period

History of Omaru Water Supply

The Omaru irrigation canal takes water from the Tama River in Omaru, Inagi City, and flows to Noborito in Kawasaki City. Since the Edo period, it has been maintained and managed as a very important agricultural water source that nourishes the villages in the Inagi City area and downstream villages. There are no clear historical documents regarding the timing of the canal's excavation, but it is believed to have been created around the 17th century as part of a large-scale flood control and water utilization policy aimed at increasing tax revenue for the Edo Shogunate. In the surrounding areas, excavation work for canals such as the Nikaryo irrigation canal and the Fuchu irrigation canal was carried out in the early Edo period, indicating that large-scale canal excavations were conducted in various places in the Tama River basin during this time. Additionally, according to an ancient document from the 3rd year of Enkyo (1746) (Kawasaki City, Sahoda family documents), it is recorded that since the 12th year of Genroku (1699), the Omaru irrigation association had been responsible for the burden of repair materials, so it is certain that its establishment dates back at least to the 17th century.

Omaru Waterway

The outline of the irrigation waterway is as follows. The intake was located at "Ichinoyama-shita" in Omaru (slightly upstream of the current Nambu Line Tamagawa Bridge). A water intake dam approximately 100 ken (about 182 meters) long was built on the Tamagawa River, and the water dammed here was taken in through a water gate (a gate for drawing in irrigation water) that was 2 ken (about 3.6 meters) wide. The water taken in first flows through the inner moat towards the distribution gate. The distribution gate was installed to separate the irrigation water for Omaru Village from that for other villages, with the width of the moat divided in a ratio of 1 for Omaru Village to 2 for other villages. The irrigation water for Omaru Village that was divided here is called Ohori, and after nourishing the southern part of Omaru Village, it flows through Naganuma Village and Yanokuchi Village, heading further towards Kawasaki. On the other hand, the irrigation water for other villages splits into Sugahori and Shinbori in the eastern part of Omaru Village. Shinbori flows across the central part of Naganuma Village, while Sugahori flows in a way that bypasses the northern part of the village, then heads towards Oshitachi Village, where it further divides into three streams at a point called Kenkaguchi. In this way, the irrigation water divided into several streams further branched out in a net-like pattern, nourishing the rice fields of the villages in the Kawasaki area downstream from Yanokuchi.

Villages Utilizing Irrigation Water

The areas that used Omaru irrigation water during the Edo period included four villages in Inagi City (Omaru Village, Naganuma Village, Oshitate Village, and Yanokuchi Village) and five villages in Kawasaki City (Suga Village, Nakanojima Village, Sugo Village, Gotanda Village, and Noborito Village). These villages spanned two districts, Tachikawa and Tama, and had different ruling lords, but they were united in their use of Omaru irrigation water and organized the "Omaru Irrigation Nine Village Association." The most important task of this irrigation association was the maintenance and management of the irrigation water, which included repairs to the irrigation ditches and facilities such as weirs and sluices, with the materials for these repairs being borne by each village. Additionally, disputes over water usage frequently arose, often concerning the distribution of water during summer droughts or the establishment of weirs on the Tama River, leading the villages to petition the shogunate for arbitration each time.

Current Omaru Water

The Omaru irrigation system, which has been maintained and managed since the Edo period, is gradually losing its function as agricultural water in modern times. The current management is carried out by the "Omaru Irrigation Land Improvement District," established in August 1952, but the area of cultivated land utilizing the irrigation water has been decreasing year by year. As rice fields have decreased, unnecessary irrigation water has been filled in, changing the landscape. However, recently, it has been developed as a new waterfront park, continuing to exist as a valuable waterside area.

• Waterway Diagram of Omaru Irrigation (Created from the History of Inagi City Volume 1)
• Ancient Documents on Water Disputes (Sahoda Family Documents)
• Ancient Documents on Water Supply Repairs (Ashikawa Family Documents)