Guess it depends on the source. Where I looked it up, it said second behind the Finke.
https://www.oldest.org/nature/rivers/
As I understand it the Appalachians are so old, parts are in Morocco, parts in Scotland, and the rest in North America - they predate the Atlantic Ocean. No wonder they have some of the oldest rivers in the world!
And once upon a time, they were among the highest ever, too.
-jk