No one has mentioned my favourite yet in this thread, Spider John Koerner, a guy who takes a traditional song, makes it totally his own, and still leaves it trad. To me, that's a folk-singer!
Are Canadians Americans? To make the claim is a typically Canadian passive-aggressive ploy, humbly drawing attention to our adversary's arrogance. George Bowering, recently Canada's poet laureate, deals with the ambiguity by referring to the you-alls as USAmericans.
Is Joni Mitchell Canadian? Absolutely. Is she a folksinger? Sure, but if she is a Canadian folksinger then so is her old drinking buddy Neil Young, e. g. Helpless, e. g. Long May You Run.