A confluence is where two or more bodies of water meet together, usually referring to the joining of tributaries.
Reciprocal to a tributary is a distributary, a river that branches off of and flows away from the main stream.
The Anduin has many tributaries(about eight or nine known), being the river Entwash the greatest of them, and where the two rivers met, a mighty delta formed, measuring a hundred miles from the splitting of the Entwash to its final inflow into Anduin. This deltaformed the Mouths of Entwash.
'Right tributary' and 'left tributary' (or 'right-bank tributary' and 'left-bank tributary') are terms stating the position of the tributary relative to the flow of the mainstem river. These terms are defined from the perspective of looking downstream (in the direction the water current is going).