When your boat passes under a bridge, pay attention to the bridge’s piers. Some of them are V-shaped. What could be the reason for that? That shape shows where the canal width narrows when the water level is low. When that happens, water flowing on the even surface gains momentum. Conversely, when the water level is high, the canal grows wider, allowing it to drain lots of water while controlling the power of the stream.

These water-retention structures are called “motase” or “keepers”; the floodgates and weirs also form part of this system. The canals are composed of a complex assortment of these elements. However, take a moment to remember: the many rice paddies of Japan are also designed to retain floodwater. The crops need water precisely during the rainiest seasons. This water-retention feature subsequently helps prevent nature and river flooding. Thus did the ancient humans coexist with rivers