FrancophonIA/Vikidia-EnFr huggingface dataset:
A tributary of a river is another river that flows into it. If one river flows into a second river, then the first river is a tributary of the second river. A tributary is a body of water that flows into another body of water.

The opposite of a tributary is a distributary. A distributary is a  river that branches off from the main flow of water, for example in a river delta.