A canal was an artificial channel built for the purpose of delivering water for irrigation.
In 2266, on Bootes III, the brothers of Eve McHuron would walk home with their boots caked heavily in mud from a canal. (TOS: "Mudd's Women")
In 2268, during his time on Amerind as Kirok, James T. Kirk proposed to Miramanee the construction of a canal. This canal would bring water from the lake to the lodge. From there, they could expand the system, irrigating the fields and doubling the food supply. (TOS: "The Paradise Syndrome")