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]

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "tributary":
   accessory, affluent, bayou, beck, billabong, branch, brook, burn,
   client, collateral, confluent, confluent stream, conquered, creek,
   dendritic drainage pattern, dependent, effluent, feeder, feudal,
   feudatory, fork, inferior, kill, liege, minor, offshoot, prong,
   rill, rivulet, run, runlet, runnel, satellite, secondary, servile,
   streamlet, sub, subdued, subject, subjugated, subordinate,
   subservient, under, vanquished, vassal