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.
You can also use the word junction, as in the
junction of the Grand and the Green
near Moab, Utah.
It can either be a tributary or a distributary, where a river joins another or separate from one respectively.
It is called a confluence. A tributary is a smaller river joining a larger one. A distributary is a river flowing into the sea.
You could also use the word "fork" which is less formal. A watershed is all of the land area that a given body of water drains.
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.