system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When 2 rivers merge, what determines the name of the river below/above the confluence?

Why does the Ohio river have two rivers as its source instead of continuing to be one of the two rivers that flows down from beyond Pittsburgh? And why isn't the Ohio called the Mississippi (or vice versa), when it's actually the wider of the two rivers at their junction? The part of the Mississippi river that forms the border of Missouri and Illinios looks more like a tributary to me...
gpt
Tradition, mostly.  

There are two "official" naming conventions - the river system is supposed to be named after either the tributary with the largest water volume (under which the Mississippi should actually be called the Ohio River) or after the tributary with the longest course (under which the Mississippi should actually be called the Missouri River).

But those conventions were thought up by people trying to standardize everything within the last century or so.  In the meantime, the river had been known as the Mississippi for generations.  It had been on maps for hundreds of years, people had been sailing and steaming up and down it, singing songs about it, and on and on.  It'd be too much of a hassle to change the name just to suit some scientific convention.