Official
Tamagawajosui and Hamura Intake Weir
Tamagawajosui has moistened farmland throughout Edo and along the river since the middle of the 17th century. On a sunny early morning drive, I went to Hamura Intake Weir on the Tama River. Tamagawajosui extends 43 km from Hamura to Yotsuya, and its altitude difference is 92 m. In other words, it is a gradient of about 470m going down by 1m. It is said that it was completed in just eight months even though it was difficult to control the slope and there were no heavy equipment. It's amazing that it's still being used in active 360 years later. By the way, 360 years from now is 2379. Can you imagine it? ？
There is also a fishway.
At the weir, there is a water level difference upstream and downstream, and the water surface does not continue, so fish cannot go up. To reduce this effect, a "fish path" has been set up. The fish path of the Hamura Intake Weir is called "half cone type", and it is devised so that a variety of water depths can be secured even if the amount of water changes.