How is the size of the river determined?
The size of a river may be said to be the sum of the area where water flows from upstream to downstream of the river. However, the amount of water flowing in a river changes with the seasons and weather. The area over which the water flows also changes, so the calculation is not easy.
When describing the size of a river, the "watershed area" is often used. This is not the total area of the area where water flows. The Ministry of Land, Infrastructure, Transport and Tourism, which manages Japan's rivers, explains that a river is a natural waterway that collects rainfall and snowmelt on the ground, and that the basin area represents the size of the area where the water collects. The Ministry of Land, Infrastructure, Transport and Tourism, which manages Japan's rivers, explains that the watershed area includes not only the area where water flows, but also the forests and fields where rainfall pours into the river.
Therefore, the watershed area is much larger than the area where the water actually flows. For example, the area of Yamagata Prefecture as a whole is 9323 square kilometers, but the watershed area of the Mogami River, which represents the prefecture, is 7040 square kilometers, making up approximately 76% of the prefecture's total area.
The Amazon River, which has the world's largest river basin, covers an area of 7.05 million square kilometers, about 19 times the total area of Japan.
Let us introduce another term related to the width of a river, "river width. The Ministry of Land, Infrastructure, Transport and Tourism defines the width of a river as "the distance between the levee and the bank on the opposite side of the river. The Arakawa River, which runs between Konosu City and Yoshimi Town in Saitama Prefecture, is the widest river in Japan at 2,537 m. The width of the river where water normally flows is said to be a few tens of meters.
