User blog comment:Darknessabove/The Explorer, Chapter 5, Volume 1: Graceless Goodbye/@comment-98.30.55.33-20150225030618/@comment-98.30.55.33-20150225111904

The beginning of a river is it's mouth, this is pretty well-known terminology

The bottom of a river is known as the riverbed.

I probably should have clarified that first, I guess.