I've googled for a while and on some sites I've found the word "watershed" as the proposed word. Is it the word that best suits it?
In geography, a confluence is the meeting of two or more bodies of water. It usually refers to the point where two streams flow together, merging into a single stream.
It could also be an "estuary":
[A] partly enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the open sea.