A bounded way along which something travels. A channel shapes what passes through it as much as it carries it.
Old French chanel, from Latin canalis (a canal, a pipe, a groove), from canna (a reed, a cane) — the same root behind 'canal.'
The banks decide the flow as much as the water does — a channel narrower than what wants to move through it backs up, and one wider than needed loses the current's force, so choosing a channel already chooses what kind of passage the thing will get.