Right from the Canadian Border Services website.
International visitors to Canada
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All international travellers must carry acceptable identification and a valid visa (if necessary) when entering Canada. A passport is recommended because it is the only reliable and universally-accepted travel and identification document for the purpose of international travel.
International transport companies, such as airlines, must make sure that travellers have proper, valid travel documents. If you do not have the proper documents, you may be delayed or refused entry into Canada. What you need will depend on where you are from, how you are travelling, and what documents you are travelling with. Consult entry requirements by country.
When you enter Canada, a border services officer will ask to see your passport and a valid visa, if you are arriving from a country for which one is required. Visit the Immigration, Refugees and Citizenship Canada website to learn more about what travel documents are required.
Citizens of the U.S. who are members of the NEXUS program may present their membership card as proof of identification and as a document that denotes citizenship, when arriving by air (when coming from the U.S.), land, or marine modes.
Citizens of the U.S. who are members of the FAST program may use their cards as proof of identity when arriving by land and marine modes only.
Permanent residents of the U.S. who are members of the NEXUS or FAST programs must travel with a passport and proof of permanent residence, and may be asked to present these documents to the officer upon arrival at the border.
All visitors arriving from or transiting through the U.S. are encouraged to visit the U.S. Customs and Border Protection website for information concerning the U.S. Western Hemisphere Travel Initiative, and the requirements to enter or return to the U.S.