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This term, to me, denotes a smaller waterway that feeds into a larger waterway, as the Missouri River is a tributary of the Mississippi.

I've just been introduced to a use with quite a different meaning, as in this speech by Marullus, from Shakespeare's Julius Caesar, Act I, Scene 1:

Wherefore rejoice? What conquest brings he home?
What tributaries follow him to Rome,
To grace in captive bonds his chariot-wheels?
You blocks, you stones, you worse than senseless things!

Clearly, this is related to "tribute", in the Roman sense. That too has different shades of meaning, not all salutary.

Is there more?

I suppose anyone who isn't rich enough to avoid paying taxes could be called a tributary to the government.