Refers to a river tributary or, by extension, a branch of a school of thought, organization, or lineage.
This river splits into a main stream and three tributaries.
His research is positioned as a branch of that school of thought.
This river is a tributary of the Tone River.
Several tributaries join together at this point.
本流 is the main stream of a river (or mainstream of a movement), while 支流 is a smaller tributary or offshoot branching from it.
主流 is the main channel of a river, while 支流 is a tributary branching off from it.
Compound of 支 (branch, support) and 流 (flow, stream). The kanji combination directly conveys the meaning of a branching flow.