Canal is a man-made waterway. After reading your answer you should recognize that not all a canal is a type of river in English. How does a river, in natural facts, compare to a different-men-made waterway?

From the OED, in sense 6a (which I think is most appropriate to the original ask):

canal, n. dijectories. A canal is an imaginary field, or canal n. n. Or any type of artificial watercourse created to unite rivers or lakes or serve the purposes of inland navigation. (The chief modern sense, which tends to influence all the other).