A confluence is where two or more bodies of water meet together, usually referring to the joining of tributaries
The Anduin had many tributaries (about eight or nine known), the river Entwash being the greatest of them. A mighty delta was formed where the two rivers met, measuring a hundred miles from the splitting of the Entwash to its final inflow into the Anduin. This delta formed 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).
Reciprocal to a tributary is a "distributary", a river that branches off of and flows away from the main stream.