Tributary

/ˈtribyəˌterē/

noun

1. a stream or river flowing into a larger river
He used often to say there was only one Road; that it was like a great river: its springs were at every doorstep, and every path was its tributary.
– from The Fellowship of the Ring, by J.R.R. Tolkien

2. a subsidiary, a person or state that pays tribute to another state or ruler
Chinese foreign policy in the nineteenth century can be understood only against its traditional Chinese background, the tributary system.
– Harvard Journal of Asiatic Studies