Although Yanagawa is known for its canals, it is actually not all that rich in water. This is because the land here is much like a tidal flat. Digging wells will only yield salt water, and the current from the large river that flows nearby becomes salty due to the backflow of seawater at high tide. This was the impetus for the citizens of Yanagawa to make their living by storing water in the canals.

Pumping groundwater on tidal flat features can result in land subsidence. The canals also function to prevent subsidence by gradually saturating the ground with moisture.

Their most crucial function is to hold back water at high tide and allow it to flow freely. At the same time, it also prevents the water level from rising. The canals have many floodgates for water level control. During heavy rains the floodgates are closed, and when the upstream canal becomes too filled with water, the gates are opened to drain it downstream. In this way the canals control rising water levels via subsequent draining. Weirs are used in addition to these floodgates to control water flow by raising and lowering perforated plates.