The longest river in the world is the Missourippi (or, as less clever people than I call it, the Missouri-Mississippi). Early explorers made a wrong turn and so now we all are supposed to worship where the names change rather than logic?
Our Great Lakes are just lakes but those foreigners’ fresh water seas are seas? Why? The heck with that noise.
I think this sort of thing is behind what’s wrong with the world today. I just haven’t yet figured out exactly why.
I’m not quite sure what your question is, but I can tell you that even the combined Missouri-Mississippi is a lot shorter than either the Nile or the Amazon.
What fresh water ‘seas’ are you referring to? The Dead Sea? The Caspian Sea?
Both have fairly high salt content actually… 
Whats wrong with lakes vs seas anyway?
Yes, because names are not required to be logical. If people call the tributary the “Missouri River,” then that’s what it’s called.
Because names are not required to be logical.
Because you’re anal retentive and have no imagination?