In the early Edo period, about 360 years ago, people in Okayama had been suffering from food shortages due to population growth and frequent poor harvests. Mitsumasa Ikeda, the local feudal lord, decided to undertake a huge project for large-scale development of newly reclaimed fields in 1656, to improve crop productivity, offer stability, and raising the living standards of farmers. In 1657, a Bay Area Land Reclamation Plan was established for the area of Kojima Bay, which is located between the Yoshii River estuary and the Asahi River estuary. However, the project was temporarily abandoned due to difficulties in securing a sufficient amount of irrigation water for the newly reclaimed rice fields and controlling drainage from medium and small rivers in the area.

Nagatada Tsuda, who was a so-called “Gundai” (a type of magistrate in feudal Japan), embarked on a project for an irrigation canal which would be 19.9km in length, with a width of 4 to 7 meters. The purpose of this project was to secure an adequate water supply for the new fields from the Yoshii River. This canal would be called the Kurayasu River. This canal became the source of the water supply for the new land of the Kurata-Shinden district, which was 329ha in total, (reclaimed in 1679). The Kurayasu River also turned out to be a big help in supplying water to the new land of the Oki-Shinden district (built later,) along with other rivers, one being the Suna River. The canal also used to be quite convenient as a waterway, connecting the Yoshii River and Okayama Castle over the shortest span. On the Okayama plain, people had to rely on irrigation facilities, especially in the newly reclaimed areas. The Kurayasu River was an epoch-making project.

The Kurayasu River continues to be appreciated even in the present day, as its essential role in the lives of the people who live in the district remains unchanged. Yoshii Locks at the intake of the Kurayasu River, being a perpetually protected historic landmark, are the oldest such remains in Japan. Yoshii Locks consist of two stone sluices constructed of robust granite and one elliptical lock chamber. With advanced technologies, Yoshii Locks enabled the boats to pass.

In the present day, Yoshii Locks are designated as an Okayama Prefecture designated Historical Site.

At the same time, Nagatada Tsuda built 11.7km of sea dikes at the Hyakken River estuary. He then built a huge retarding basin on the inland side and also 5 drainage gates that were made of granite, completed in 1687. These were great assets to rice farming as they extracted residual water from the rice fields, which was harmful to crops. This was the most technologically innovative project of its time.

The Hyakken River as the main drainage canal has assisted in the development of Oki-Shinden land reclamation district, which was Japan's largest area of land development at that time. It was 1,918ha in total area when it was completed in 1694. Through the concept of combining the functions of flood control and land development, the Hyakken River, 12.9km in length from the point where it branches off from the Asahi River, has prevented or reduced the detrimental effects of floodwaters from the Asahi River.

The completion of these projects resulted in the sea-land reclamation of over 2,200ha in the area. Thus these projects brought about a large increase in food production and contributed to developing regional agriculture as well as improving the quality of farmers’ lives. Furthermore, the concepts and advanced technology applied in these projects have greatly influenced both the theory and practice in the development of similar facilities across Japan from that period on.

In the local area, many civil societies such as the Okayama Clan Nagatada Tsuda study group and Tomiyama Nature’s Club have been working on the preservation of these facilities, by engaging in clean-up activities and honouring ancestors, respecting these rivers as the “Mother River” of Okayama. In Okayama Prefecture, these projects are adopted as reading materials for school students, such as “Reclamation in Okayama” and “The Story of the Hyakken River”.

Water Heritage

The structure represents a milestone/turning points in the development of irrigated agriculture and bears exceptional testimony to the development of agriculture and an increase in food production along with the improvement of the economic condition of farmers;

This epoch-making project allowed the Kurayasu River to connect both the Yoshii River to the east and the Asahi River to the west to secure an adequate water supply as there was an inadequate amount of rainfall in the area and people had to rely on irrigation facilities.

Since the Hyakken River has a huge retarding basin and stone drainage gates at the estuary to control drainage and flooding, it is said that the river was the most innovative use of technology of its time.

In the end, the Kurayasu River and the Hyakken River joined forces to realize a massive land development of more than 2,200ha which would be called Kurata-Shinden and Oki-Shinden land reclamation districts. Bringing abundant crops greatly helped the improvement of rural areas, and promoted farmer independence.

The structure was ahead of its times in terms of project formulation, engineering design, construction techniques, dimensions of the structure itself, quantum of water diverted, and size of the command;

The construction of the Kurayasu River started in February 1679 and completed in August 1679. Just conceiving the idea of connecting both the Yoshii River and the Asahi River is astounding, but what is even more incredible is the construction itself, which only took 6 months to complete.

This is a testament to excellent planning, the use of existing small and medium-sized rivers, water channels, and wetlands for most of the connection process. At the same time, the Kurayasu River featured small water channels which were crossed with stone siphons at 23 crossing points. Also, Yoshii Locks, which serve as the intake of the Kurayasu River, were built with robust granite and were placed on top of a rocky hill that is solid and stable. Furthermore, the Hyakken River had a drainage system that combined 5 stone drainage gates at the estuary, and a retarding basin on the inland side. This was to reduce the detrimental effects caused by water by utilizing high and low tides.

In this way, the concepts and advanced technology from this period have had a powerful influence on technological innovation and future generations.

The structure made outstanding contribution to enhancing food production, livelihood opportunities, rural prosperity, and poverty alleviation in a region;

The Kurayasu and the Hyakken River helped complete land development by increasing the crop productivity of the existing farmland. That is how the developers reduced the shortage of land, strengthened the system of crop productivity, and contributed to poverty reduction, the development of stability, and quality of farmers’ lives. As a result of this process, each farmer was provided land per the size of his family.

The structure was innovative in its ideas at the time of its construction;

Using innovative technology, the Kurayasu River brought abundant water to the Asahi River from the Yoshii River, which was 15km away from Kurata-Shinden district, allowing the 2 rivers to be connected. Finally, they designed it to be used as both an irrigation canal and as a waterway for boats. Furthermore, they built another river called the Hyakken River, installing a drainage system that combined a retarding basin, and many sluices made of stones at the estuary. These are the innovative ideas that Nagatada Tsuda made a reality.

The structure was an example of engineering marvel or excellence at the time of its construction;

Regarding the Hyakken River, the precise design and the techniques that combined a huge retarding basin and drainage gates were adopted in other land development projects which were completed in 1819 and 1852 in what is now Kumamoto Prefecture. Regarding Yoshii Locks, the Fukuoka feudal clan also adopted the techniques related to the placement of the lock, the structure of double sluices, and its granite stone pillar in the building of their sluices, called “Nakama-Karato”, completed in 1736. Furthermore, the Kurayasu River later became a model for “The Aiba River” which was built by the Hagi feudal clan in what now is Yamaguchi Prefecture, completed in 1744. These are prime examples of the definition of outstanding technology, as these were developed over 330 years ago.

The structure contributed to the evolution of efficient and contemporary engineering theories and practices;

For over 270 years, the stone drainage gates at the Hyakken River and the sea dikes by the estuary played complementary leadership roles in water transportation for flood and tidal defence. Although the structure of the river locks has been changed gradually, as in the case of those which have been renovated several times, the system has greatly influenced both the theory and practice in the development of drainage systems.

The creation of the irrigation system also established the use of stone foundations and associated techniques, allowing for elaborate structures.

The structure is an example of attention to environmental aspects in its design and construction;

The excavated sections of the Kurayasu River are over 4.3km in length, which is 22% of 19.9km of the total length, were built on the site of existing small and medium-sized rivers, water channels and wetlands. This was done to preserve the environment. The Hyakken River ecosystem includes many wild species of plants and animals, and it has been designated as a natural treasure of Japan.

The structure was unique in some positive and constructive way;

To construct the Kurayasu River irrigation canal, the Hyakken River and many other water area reclamations, a financial fund called “Shasoumai” was established. Also, it played a role in providing famine relief by offering funds at low interest.

Furthermore, there were many individuals and masonry technocrats who put forth great effort to complete all the projects. These projects were realized as a result of the efforts of all those involved, exemplary techniques and the availability of funds. The fact that this undertaking has stood the test of time for almost 300 years points to the special nature of the construction.

The structure bears the stamp of a cultural tradition or a civilization of past;

The components of the system under the proposal are making the impossible possible through a collection of sites that each represent a part of the agricultural heritage of the region. Stone locks, stone seawalls and stone drainage canals are the basis of the technology. The leaders of the Okayama clan must have had a strong will and a solid philosophical grounding which allowed them to complete such a formidable task.

This irrigation and drainage system was built with such great attention to spirituality that it reflects traces of tradition, culture and civilization. This system is not only one of the most precious assets of the region but also serves as a model for humanity in tackling agricultural crises throughout the world.

Engineering utility vis-à-vis designed utility

The Kurayasu River connected the Yoshii River and the Asahi Rivers for more than 300 years since it was built in 1679, however, it was divided by a major renovation of the Hyakken River in 1985. Since then the Asahi River has become a source of water for the west side of the Hyakken River, while the Yoshii River has remained the same, providing water to the east side of the Hyakken River.

Both sides of the Kurayasu River have been well protected and have maintained their original appearance, while there have been several rivers related improvements, they have continued to be an essential part of the irrigation system in the area. Also, the Hyakken River has undergone several improvements since it was first constructed, yet it has maintained its function as a flood control and drainage system.

Assessing the values of cultural heritage, Yoshii Locks have been selected as an Okayama Prefecture designated Historic Site, even though they no longer serve as intake gates due to the construction of the Sakane Weir in 1980. However, they remain the oldest existing lock gates (flood gates) in Japan and have maintained their original appearance for 340 years.

Present State of Conservation

From ancient times to the present day, Okayama has always been a leader in rice farming. Several extraordinary ideas and methods were created here and adopted in other areas across Japan.

And, people in the area always think about the long history of agricultural development with a sense of appreciation. So, they have been protecting the irrigation and drainage facilities carefully.

The Kurayasu River has a stable water flow which is provided by the Sakane Weir and is managed by the Yoshiigawa-Karyu Land Improvement District and Okayama City. There is a water-use adjustment organization comprised of local farmers to make sure the irrigation system is in the best condition and works with good water efficiency. The revetment is a two-sided waterway with all sections consisting of natural stones and blocks, but in some areas, some river spaces are formed with double-sided natural stones by revetment improvement. The Kurayasu River is protected by local people carrying out regular clean-up events out of respect for their “Mother River” that has allowed rice growing and protected their lives for over 3 centuries. Some communities have formed organizations like the “Tomiyama Nature’s Club”, which share the historical and cultural background of the Kurayasu River by holding events such as historical walking tours.

As the Hyakken River has undergone several major renovations since it was selected as a designated first-class river in 1966, its capacity for flood control and drainage has been strengthened even more. For example, as you may recall, during the deadliest floods in the recent history of West Japan (in July 2018), the Hyakken River fulfilled the functions it was designed to, even 330 years after its completion. This is proof that the Edo era was a period of advanced technical prowess. This fact was widely reported, and people in Okayama Prefecture recognized again the extremely important role that the Hyakken River played. At the same time, the renovations have created space for holding events and festivals for locals to visit and enjoy. The Hyakken River has nurtured the natural environment of the districts through which it flows. Furthermore, it has been designated as a natural treasure of Japan by the Forest Culture Association.

Yoshii Locks, being a perpetually protected historic landmark, are the oldest such remains in Japan, and Yoshii Locks are well maintained and preserved by the Yoshii Locks Conservation Society, which is supervised by the Okayama City Board of Education. Yoshii Locks are designated as an Okayama Prefecture designated Historic Site where a multitude of people come to see the incredible technology that was put in place 340 years ago. Residents and children also participate in learning workshop events to see the solid and stable stone foundation of the locks.

In 2013, Emperor Naruhito visited Yoshii Locks for an inspection and to learn about the history of irrigation and water transport.