Tributary
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A tributary, or an affluent, is a stream that typically flows into a larger stream (main stem or parent) or lake, or rarely into a smaller stream, e.g., the Esla tributary to the Douro. A tributary does not flow directly into a sea or ocean. Tributaries and the main stem into which they flow drain the surrounding drainage basin of its surface water and groundwater, leading the water out into an ocean, another stream, or into an endorheic basin.