Erin-
Just to clerify what document will be required for your return trip to the U.S.
For now in order to be WHTI compliant at crossing the border you will need proof of citizenship (such as a birth certificate) and a form of gov't photo I.D.
Passports,passport card or enhanced drivers liscence will be required starting JUNE 1st to be WHTI compliant. You can find more inf. at CBP.GOV website. Hope this helps.