A geography word [closed]
I think the term you are looking for is debouch or debouche. Wikipedia says
In the geography of rivers, streams, and glaciers, a debouch, or debouche, is a place where runoff from a small, confined space emerges into a larger, broader space.
and this
Some examples are: where a river or stream emerges from a narrow constraining landform, such as a defile, into open country or a wider space.