From:

http://www.cbsa-asfc.gc.ca/publicati...c4161-eng.html

If you are a citizen of the United States, you do not need a passport to enter Canada; however, you should carry proof of your citizenship such as a birth certificate, certificate of citizenship or naturalization or a Certificate of Indian Status, as well as a photo ID.

So you don't need a passport to enter Canada but you *do* need proof of citizenship, in addition to your photo ID.

So, for example, you need your birth certificate + your driver's license.

You should also review what you need to get back into the USA