@OP: Can't you figure out how to use Google to find this information? What others have said on this thread is erroneous or incomplete. Here is the latest information from the correct Canadian and U.S. government sites (and, of course, you can get additional detailed information from these sites, too):
Identification documents required [to enter Canada]
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[from
http://www.cbsa.gc.ca/publications/pub/bsf5082-eng.html#s1x1]
When you enter Canada, a border services officer may ask to see your passport and a valid visa (if you are arriving from a country from which one is required). If you are a United States citizen (U.S.), you do not need a passport to enter Canada; however, you should carry proof of your citizenship such as a birth certificate, a certificate of citizenship or naturalization or a Certificate of Indian Status, as well as a photo ID. If you are a permanent resident of the U.S., you must bring your permanent resident card with you.
All travellers, including U.S. citizens, are encouraged to visit the U.S. Customs and Border Protection's Web site at www.cbp.gov for information on the U.S. Western Hemisphere Travel Initiative and its traveller requirements to enter or return to the United States.
http://www.cbp.gov/travel/us-citizens/whti-program-background
WHTI Program Background [ Documentation requirements for entering the U.S.]
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[From
http://www.cbp.gov/travel/us-citizens/whti-program-background]
The Western Hemisphere Travel Initiative (WHTI) is the joint Department of State (DOS) and Department of Homeland Security (DHS) plan to implement a key 9/11 Commission recommendation and the statutory mandates of the Intelligence Reform and Terrorism Prevention Act of 2004 (IRTPA). IRTPA, in part, required the DHS and DOS to develop and implement a plan to require all travelers, U.S. citizens and foreign nationals alike, to present a passport or other acceptable document that denotes identity and citizenship when entering the United States.
WHTI document requirements for air travel went into effect in January 2007. All U.S. citizens and nonimmigrant aliens from Canada, Bermuda, and Mexico departing from or entering the United States from within the Western Hemisphere at air ports of entry are required to present a valid passport (or NEXUS card, if utilizing a NEXUS kiosk when departing from a designated Canadian airport).
WHTI was successfully implemented on June 1, 2009 for entry into the U.S. by land and sea. U.S. citizens entering the United States at sea or land ports of entry are required to present a WHTI-compliant document such as a valid passport, U.S. passport card, Trusted Traveler Program card (NEXUS, SENTRI, Global Entry or FAST), or an Enhanced Driver’s License.
For purposes of establishing identity and citizenship, Canadian citizens entering the United States at sea and land ports of entry are required to present one of the following documents valid for the entire period of intended stay: a valid passport issued by the Government of Canada; a Trusted Traveler Program card (NEXUS, FAST, or SENTRI); or an Enhanced Driver’s License.
U.S. and Canadian citizens under age 16 may present only proof of citizenship, such as a birth certificate or Naturalization Certificate.