http://www.cbsa.gc.ca/travel-voyage/td-dv-eng.html#_s2a

According to this page:
Identification requirements for U. S. citizens and permanent residents
If you are a U.S. citizen, ensure you carry proof of citizenship such as a passport, birth certificate, a certificate of citizenship or naturalization, or a Certificate of Indian Status along with photo identification. If you are a U.S. permanent resident, ensure you carry proof of your status such as a U.S. Permanent Resident Card.

Whether you’re entering by air, land or water, we recommend you carry a valid passport for all travel abroad, including visits to Canada from the U.S. A passport may be required by your airline or alternative transportation authority, as it is the only universally-accepted identification document.
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