Channel, canal, conduit, duct, aqueduct all mean something through which a fluid (as water) is led or flows.
Channel implies the natural bed of a stream of running or moving waters; the term is also applied to a deep portion of a stream or body of water either where the main current flows or where a good passage for boats exists.
Channel often applies also to a natural or an artificial passageway (as a tube, a gutter, a ditch, or a trough) through which something (as waste) flows or (as chain or wire) runs.
Canal is used for an artificial waterway which connects two bodies of water. It is also used in designations of various anatomical grooves or tubular channels (as for the containing of some structure or the passage of some substance).
Conduit may be applied to an artificial or natural passageway that serves to convey or transmit a fluid, but the term is more often used specifically for a large heavy pipe which conveys water from a reservoir to a point where it is distributed or for a pipe that carries the wires or cables of an electric system.
Duct has specific application to one of the small anatomical tubes through which a secretion is conveyed to where it is needed or is excreted from the organism. The term is also used in reference to any of the pipes of a furnace or an air-conditioning system through which air is taken in, circulated, or discharged.
Aqueduct is applied to an artificial channel for water (as a conduit) and especially to an artificial structure, in appearance like a bridge, for carrying water over a river, or over a gorge or gap between elevations.