Sayamaike Reservoir on Nishiyoke River serving the large-scale irrigation area of Osaka Plains is the oldest artificial agricultural reservoir in Japan dating back to 1,400 years and continues to be in use to this day.

When the reservoir was constructed, artificial twig-mattresses (shikisoda) were used to reinforce the banks. This technology, used widely in the East Asian region, was introduced to Japan through the Korean Peninsula. The 60 meter-long gutters, which drained the water from the pond, were made by hollowing out Japanese umbrella pines, and the gutter connections were designed to prevent the water from leaking. In addition, for the repairs made 800 years ago, it is thought that technologies for processing stone and constructing large structures were introduced from China. Thus it is an important heritage that proves the technological exchanges made with East Asia during those days.
During the repairs carried out 800 years ago, the size of the reservoir was not changed except for the banks, which were raised to approximately 10.2 meters. The gutters beneath the banks, however, formerly made of wood, were now made of stone to stabilize the water supply. The irrigated area now is double that of 1,400 years ago, increasing downstream crop production.
The current size of the Sayamaike Reservoir was attained during the repairs carried out about 400 years ago, when the water supply was increased to approximately 2.50 million m3. It was also decided to make provision for supplying water to the 80 or so downstream villages. The water supply system, management and organization established during these repairs are still in use today.
Repairs carried out within the past 100 years used concrete and heavy machinery to create a more stable supply of water. During the recent repairs, disaster prevention functions and measures were taken to make sure that the reservoir can be used in urban conditions.
With the rapid economic growth after the Second World War, large landslides flowed into the Sayamaike Reservoir due to the development of dormitory towns in the upstream areas of Osaka, now a metropolis. The surrounding areas have continued to be developed after the dam was constructed, and the Sayamaike Reservoir is now used as a recreational area for the citizens. The reservoir is cleaned every month by a civic organization and festivals are also held in the area.
The structure is an outstanding example of Operation and Management over a long period of time. The rotational irrigation water delivery system that began in the 1600s is still used today.