I never thought I'd say this, because I recall when 70 cents of ours would buy one of their dollars, but the Canadians haven't been keeping their presses going in overdrive. Their ccurrency is resource backed and they'll be selling their stuff to the Pacific Rim and India before it's over.
The US currency (well, Federal Reserve Note) won't likely be worth much. Silver coins, copper coins, Canadian cash, gold coin for large amounts, and items of barter will likely be the new currency, with large transactions in bullion which will have value anywhere in the world.
I can see Canadian cash being worth something in the northern border states but I don’t think it will accepted much south of the Ohio.