The Collaborative International Dictionary of English v.0.48:

Tributary \Trib"u*ta*ry\, n.; pl. Tributaries.
   1. A ruler or state that pays tribute, or a stated sum, to a
      conquering power, for the purpose of securing peace and
      protection, or as an acknowledgment of submission, or for
      the purchase of security.
      [1913 Webster]

   2. A stream or river flowing into a larger river or into a
      lake; an affluent.
      [1913 Webster]