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.