For example, the Nile flows into the Mediterranean. The Mediterranean is therefore the Nile's ____.
Edit: I'm looking for a way to name the Mediterranean in relation to the Nile. The Nile is one of the Mediterranean's affluents, so the Mediterranean is the Nile's ____.
This is a decent list of geological terms, and it doesn't have any word to specifically identify the relation of a river to its final outflow point (where it discharges). Which might not be into an ocean or sea - sometimes it's a desert basin.
I think mouth does well enough for the actual place where the river ends (assuming it doesn't just peter out in the desert), but that doesn't really embrace the whole body of water. I wouldn't object to outlet or outflow either, but I doubt they're commonly used in this exact context.
In short, I don't think there's a "real" technical term - but I'm not a geologist, so I don't know. The best I can come up with is terminus. For example, the Awash River drains into Lake Abbe, which is described as its terminus. Also the Black Sea is the terminus for a number of large rivers.
The accepted name for the endpoint of a river is either mouth if it goes right into a big body of water, estuary if it goes into a smaller semi-enclosed coastal body of water connected to a larger body, or confluence if it goes into another river. In the case of the Nile, it has an estuary. However, if a river does have a "mouth", the body of water the river empties into is not called "the mouth". Instead, it is just the body of water containing the mouth.
The other end of the river is called its source or headwaters.
The entire area drained by the river is its watershed, or drainage basin.
How about:
The Mediterranean is the Nile's debouchment?
Other options along the lines of affluents would be to call the Mediterranean the Nile's, egress, exhaust, discharge, etc.
The most common term for this is simply the lake or river’s outlet. However, that does not say that is the end-point for that body of water.
For that, you may prefer endorheic basin, which per Wikipedia is:
An endorheic basin (from the Ancient Greek: ἔνδον, éndon, “within” and ῥεῖν, rheîn, “to flow”), also called a terminal or closed basin, is a closed drainage basin that retains water and allows no outflow to other external bodies of water, such as rivers or oceans, but converges instead into lakes or swamps, permanent or seasonal, that equilibrate through evaporation.
If so, you may wish to use the simpler terminal basin or closed basin, so people do not have to look up the fancy Greek term.
For example, although many rivers flow into Lake Tahoe, it has a unique outlet: the Truckee River, which flows into Pyramid Lake where it dead-ends. There is no outlet. Pyramid Lake is the terminal basin for the Truckee River, and ultimately also for Lake Tahoe whose outlet it is.
A term that is commonly used to describe a river system that flows into an ocean, sea, lake or reservoir, is the mouth of the river. In the case of the Nile, it is referred to as a delta.
A tributary or Affluent flows to the Parent River or Stem also called Trunk (in reference with hydrology).
An Estuary is a partly enclosed coastal body of water with one or more streams flowing into it. An estuary has a free connection to open sea.
