Under the Western Hemisphere Travel Initiative (WHTI), Canadian citizens entering the United States by land or sea from within the Western Hemisphere will be required to present a passport or other acceptable document showing their citizenship and identity. This U.S. law will take effect June 1, 2009.
WHTI acceptable documents for Canadians include: trusted traveller program cards, such as NEXUS, FAST cards, SENTRI, and enhanced driver’s licenses (i.e., licences that require, among other things, proof of citizenship). Currently, only British Columbia is authorized to issued enhanced driver’s licences, however, other provinces are expected to become authorized to issue them in the coming months.
Canadian citizens 15 years of age and younger who wish to enter to United States by land or sea will only be required to present proof of citizenship, such as an original copy of a birth certificate. Canadian citizens between the ages of 16 and 18 will have to present WHTI acceptable documents unless they are travelling with a school or other organized group, under adult supervision with parental or guardian consent. In these circumstances, Canadian citizens between the ages of 16 and 18 will be entitled to enter the United States by presenting proof of citizenship alone (i.e., a birth certificate or a Canadian citizenship card).
(Link to Canada Border Services Agency)