tributary

n. small stream flowing into a larger body of water; one who pays tribute, taxpayer

A tributary (or confluent/affluent) is a stream or river which flows into a mainstem (or parent) river, and which does not flow directly into a sea. In orography, tributaries are ordered from those nearest to the source of the river to those nearest to the mouth of the river. A confluence is where two or more tributaries or rivers flow together.
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Con·flu·ent adj. 1. Flowing together; blended into one. 2. Merging or running together so as to form a mass, as sores in a rash. [more]Tributary - Community and Resources

(n.)
A stream or river flowing into a larger river or into a lake; an affluent.
  

(n.)
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.
  

(a.)
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.
  

(a.)
Paying tribute to another, either from compulsion, as an acknowledgment of submission, or to secure protection, or for the purpose of purchasing peace.
  

(a.)
Paid in tribute.
  

(a.)
Hence, subject; subordinate; inferior.
  

A stream that supplies water to a larger stream.