Yes you will need one to get back into states. They also have a visitation card you can get that's not as expensive as a passport, check with your local post office.
Make sure you don't take any guns either, Canada frowns on them also. Just went there on bikes over the 4th...they asked my buddy if he owned any guns...not if he had any but if he owned any. Needless to say when he said yes about 300 of them he had the pleasure of getting searched. They weren't very friendly either!