Very soon (if not already) a passport will be required to re-enter the US. My uncle didn't take the new requirements seriously when he took a fishing trip into Canada a few years ago. When trying to return, since he didn't have his daughter's birth certificate, the border guards wouldn't let them back into the country. They said he could, but she couldn't. You can imagine how traumatic it was for a five year old. It was at night and they were shining their flashlights at her. She was paniced. screaming and crying. They finally let them go when she was able to recite the address on her dad's drivers license.