As of November 2010, U.S. citizens do not need a passport to enter Canada via the Peace Bridge. According to the official Peace Bridge authority, however, U.S. citizens over the age of 16 must carry with them at least one of the following: driver license, non-driver license or a permanent resident card. U.S. citizens 15 years of age and under must have an original or certified copy of their birth certificate or a certificate of naturalization in the event that they were not born in the United States, a restriction extended to those up to age 19 when traveling with school groups into Canada. If any U.S. citizen lacks these documents, he can bring his passport instead.
As is the case with the many other international borders, Canada can choose to refuse entry to anyone. For example, if you have engaged in what Canada refers to as "serious criminality"--you've committed a crime that could have landed you in jail for 10 or more years--you can be denied entry. Additionally, those who have engaged in terrorism or war crimes stand a chance of being refused. Providing false documentation will result, upon authorities finding out, in immediate expulsion.
Aside from those who wish to permanently emigrate to Canada, the majority of U.S. citizens will return to the United States, some of them across the Peace Bridge. As is the case going over in the first place, you can present alternate official identification if you don't have your passport. When re-entering the U.S., you must also declare the value of any goods or cash you're bringing if their total value exceeds $10,000, as of November 2010. Additionally, you must declare whether you're bringing any agricultural supplies, animals or biological entities into the country.
