tributary; branch of a river
Literal geographical use: a smaller stream that flows into a larger river.
This river has several tributaries.
主流 refers to the dominant mainstream faction or trend, while 傍流 refers to a minor, non-mainstream branch or faction.
Compound of 傍 (side, nearby) and 流 (flow, current). The literal meaning 'side stream' extended to figurative uses for non-mainstream groups or ideas.