General revenue of the Mamigasaki River and Irrigation Canals Coordination Council and constituent organizations; Subsidies from the national, prefectural and municipal governments The Yamagata-goseki Irrigation System is a collective name of five irrigation systems, Sasazeki, Gotenzeki, Hakkagozeki, Miyamachizeki, and Sotsukizeki Canals. They are located on the Mamigasaki River alluvial fan in the southeastern Yamagata Basin and flow in a network that stretches approximately 115 km from east to west through the center of Yamagata City, the capital of Yamagata Prefecture.

The steep topography of the alluvial fan (2.2% gradient), which was formed by igneous rocks, was naturally prone to flooding. The great flood of October 1623 caused extensive damage to the urban area of Yamagata, centered on the alluvial fan; hence, the following year, Torii Tadamasa, the then domain lord, altered the flow of the Mamigasaki River to the north of the urban area. Upon request of the surrounding villages that had traditional water rights at that time, the Yamagata-goseki Irrigation System was created by constructing five water intakes.

The system had an unique water-diversion rule. This rule determined the volume of water for each water intake by calculating the total irrigation water, including the inflow of stream and the return water from agricultural drainage, as 120% of the distribution of the Mamigasaki River. After almost three and a half centuries of operation, the rule was abolished when integrating the five water intakes into the Mamigasaki River Headwork in a national irrigation project implemented from 1972 to 1986.

Thus, for approximately 350 years, the Yamagata-goseki Irrigation System has contributed significantly to the development of agriculture in Yamagata City. It is only in the past 50 years that the irrigation area has dramatically decreased from approximately 2,400 to 120 ha owing to urbanization and new irrigation from new other water sources with the national project; nevertheless, it remains as an important site for the region’s history.

Yamagata City has two large rivers:, the Mamigasaki and Sukawa Rivers. The Sukawa River is unsuitable for agricultural use because strongly acidic water flows into it from its tributaries. Therefore, irrigation water from the Mamigasaki River was essential for farmers living around Yamagata City. An example of its importance is described in the "Mamigasaki River Agricultural Irrigation History." The Gotenzeki Canal, used to irrigate farmland west of Yamagata Castle, passes through the castle moat in the center of Yamagata City. In 1889, the government planned to sell the moat on national land to former castle lords, the Mizuno family. In response to this plan, representatives of villages west of the castle repeatedly submitted petitions to the government to rather sell the moat to the villages. The representatives claimed that the privatization plan to the Mizuno family would threaten the availability of drinking water and of irrigation water for approximately 1,000 ha of farmland in some villages. The strength of this reaction by the village representatives indicated the importance of the water through Gotenzeki canal to the survival of the villages.

The irrigation area was approximately 2,400 ha when the canals were completed. However, the Mamigasaki River has a steeply sloping alluvial fan, a small catchment area, and an annual precipitation of approximately 1,200 mm, which often led to a shortage of irrigation water.

In general, the water diversion rate is proportional to the irrigation area. For approximately 350 years, the Yamagata-goseki Irrigation System had a unique rule to divert irrigation water evenly and efficiently by increasing the irrigation water from the inflow of stream water and also capturing the return flow of agricultural drainage. Specifically, the water intake for each canal was determined by calculating the additional irrigation water as 20% from return water from agricultural drainage, with the following percentages for each canal: 50% for Sasazeki, 20% for Gotenzeki, 20% for Hakkagozeki, 15% for Miyamachizeki, and 15% for Sotsukizeki.

Following these rules, the villagers approved the domain-built temporary water intakes on the Mamigasaki River instead of installing permanent water intakes because frequent flooding resulting in changes to its course and made it technically difficult to install permanent water intakes. Specifically, the water intake upstream of the Sasazeki Canal was composed of large rocks with diameters of 70– 80 cm. The water intakes downstream of the Gotenzeki and Hakkagozeki canals comprised smaller rocks and stones. Further downstream, the water intakes of the Miyamachizeki and Sotsukizeki Canals were constructed using straw rice bags, stone sand, and soil planted with turfgrass. During irrigation periods, only approved villagers were allowed to go onto the floodplain to manage water distribution.

This traditional practice of water diversion continued until 1986 when the five irrigation water intakes were integrated into the Mamigasaki River Headwork. Although the origin of this method is not clear, it is a practice that was developed during repeated water conflicts over a long period, demonstrating a history of solving numerous water problems.

The Yamagata-goseki Irrigation System, which flowed in a network around Yamagata Castle, were used for daily life and for fire prevention around the castle. Because the Gotenzeki Canal led water to the castle moat, it was necessary to maintain the moat. In this way, the canals played an essential role in the lives of inhabitants in this castle town.

The canals were used as irrigation water in the vast farmland west of Yamagata Castle and other areas, contributing significantly to the development of agriculture in Yamagata City. After 1870, many flour and rice watermills were established alongside the canals, including 20 watermills on the Hakkagozeki Canal in 1888, 23 watermills on the Gotenzeki Canal in 1904, and 3 watermills on the Sotsukizeki Canal in 1927. In addition, water from canals was used for various industries, such as paper manufacturing, dyeing, and carp farming. For about ten years since 1878, Yamagata Prefecture leased the Yamagata Castle site from the national government to raise dairy cows, with the water being used as drinking water for the animals.

The Yamagata-goseki Irrigation System was used to irrigate approximately 2,400 ha of farmland. Approximately 1,700 ha of farmland was newly irrigated from the Mogami River instead of the canals through the Mogami River Midstream Irrigation Project, which was implemented from 1972 to 1986. Consequently, the irrigation area of the canals has decreased to less than 700 ha. Since then, owing to urbanization, the current irrigation area of the canals has been significantly decreased to approximately 120 ha.

The five former water intakes were integrated into the Mamigasaki River Headworks in 1986. As a result, during the current irrigation period of 128 days, from May 6 to September 10, a maximum of 2.121 m3/sec is collectively withdrawn, of which a maximum of 0.460 m3/sec is distributed into the canals.

The total length of the Yamagata-goseki Irrigation System, because of urbanization, has decreased to approximately 115 km due to the decrease in the amount of farmland requiring irrigation: the Sasazeki Canal is 47.5 km, the Gotenzeki Canal, 17.5 km, the Hakkagozeki Canal, 16 km, the Miyamachizeki Canal, 15 km, and the Sotsukizeki Canal is 19 km.

Because of the emphasis on the convenience and durability of the canals, many historically valuable stone masonry canals were converted into concrete canals; only about 8 km of the traditional stone masonry canals remain. From 1999 to 2006, a 1.5 km length of stone masonry canals was restored as part of Yamagata City's central city revitalization project; the intention was to contribute to developing a waterfront space for citizens to relax and enjoy the central city area. Since 2020, in conjunction with further redevelopment of this central city area, Yamagata City is implementing a project to restore the Gotenzeki Canal.

From the 1960s to the 1980s, the water quality of the canals deteriorated rapidly owing to the inflow of domestic and industrial wastewater. After the 1990s, with the introduction of sewage treatment systems, water quality temporarily improved to the extent that Ranunculus nipponicus var. submersus, a water plant endemic to Japan which can only survive in clear streams, reappeared in the waterways.

Currently, each canal is maintained and managed by farmers' associations. Yamagata City supports the mowing, dredging, and repairing of canals through these associations. In addition, with the cooperation of neighborhood associations and businesses, the city promotes the creation of a maintenance and management system in which citizens can participate. Every November, the Mamigasaki River and Five Irrigation Canals Coordination Council hold an event to pick up trash and clean up the canals in cooperation with residents. In October, public relations activities are undertaken to promote residents’ understanding of the canals.