What is Tributary ?
Tributary is (noun) 1. a stream or river which flows into a larger river The Mole is one of the tributaries of the Thames. 2. a person who pays tribute to a ruler Tributaries came to the emperor with their gifts. (NOTE: The plural is tributaries.) (adj) who pays tribute to a ruler (formal.) Envoys from tributary states came to the emperor with their gifts.
source: Easier English, Student Dictionary Upper Intermediate Level