Well, I speak French so let me help you.
Both riviere and fleuve translate into "river" in English, but they mean different things in French.
Riviere is a water course that flows into another water course (sorta like Ellicott Creek flowing into the Niagara River).
Fleuve is a water course that flows into an ocean, or sea, or a very large lake or bay.
"Belle fleuve" morphed into "Buffalo".
It's like ocean/sea, or river/creek/branch. Technically, they mean different things, but they are used interchangeably nowadays.
FYI, the Niagara "River" is technically a "strait".