The Collaborative International Dictionary of English v.0.48:

Tributary \Trib"u*ta*ry\, a. [OE. tributaire, F. tributaire, L.
   tributarius. See Tribute.]
   1. Paying tribute to another, either from compulsion, as an
      acknowledgment of submission, or to secure protection, or
      for the purpose of purchasing peace.
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            [Julius] unto Rome made them tributary. --Chaucer.
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   2. Hence, subject; subordinate; inferior.
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            He to grace his tributary gods.       --Milton.
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   3. Paid in tribute. "Tributary tears." --Shak.
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   4. Yielding supplies of any kind; serving to form or make up,
      a greater object of the same kind, as a part, branch,
      etc.; contributing; as, the Ohio has many tributary
      streams, and is itself tributary to the Mississippi.
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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.
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   2. A stream or river flowing into a larger river or into a
      lake; an affluent.
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WordNet (r) 3.0 (2006):

tributary
    adj 1: (of a stream) flowing into a larger stream
    2: paying tribute; "a tributary colony"
    3: tending to bring about; being partly responsible for;
       "working conditions are not conducive to productivity"; "the
       seaport was a contributing factor in the growth of the city";
       "a contributory factor" [syn: conducive, contributing(a),
       contributive, contributory, tributary]
    n 1: a branch that flows into the main stream [syn: feeder,
         tributary, confluent, affluent] [ant: distributary]