A tributary's a stream or river which flows into a mainstem (or parent) river. A tributary doesn't flow directly into a sea. Tributaries and the mainstem river serve to drain the surrounding drainage basin of its surface water and groundwater by leading the water out into an ocean or some other large body of water. A confluence's when two or more bodies of water meet together, usually referring to the action of tributaries. An affluent's synonymous to the word 'tributary', being defined as a st… (More on Tributary)