I get that your argument is basically "You don't realize how many people 8 billion people is". My argument is "You don't realize how big the mississippi is".

Bringing up 20ft implies you are including the headwaters which is not my argument. I'm referring to downstream.

Even if in theory you managed to dam it - the Mississippi is so large and fast moving it would EASILY find another route. Like there would not be a "bay of Mississippi" or whatever.