main current; main stream
Refers to the primary flow of a river, or metaphorically to the main trend or direction of something.
The main current of this river is very fast.
Let's not stray from the main stream of the discussion.
Compound of 幹 (kan, 'trunk' or 'main part') and 流 (ryū, 'flow' or 'current'). The exact historical derivation is uncertain, but the spelling is conventionally associated with the meaning 'main current'.