This is what i copied and pasted from Canadian Tourism Commision
Visitor Requirements: U.S. Residents
Although a passport is the ideal identification, you do not require a passport or visa to enter Canada. Just make sure you carry identification to establish your citizenship such as a Birth Certificate and least one ID card with photo. If you are a naturalized U.S. citizen, you should carry this certificate. Permanent residents of the U.S.A. must bring their "Green Card".
Although it is rare, immigration officials may prevent the entry of visitors:
who appear to pose a health risk.
those they doubt will be able to support themselves and their dependents in Canada.
whose willingness and means to return to the U.S. is in doubt.
As well, those admissible to Canada must not have a criminal record. This includes any convictions for driving while intoxicated.
If you have questions about your admissibility to enter Canada, contact your nearest Canadian Embassy or Consulate for consultation.
here is the link for more info..
http://www.travelcanada.ca/tc_redesign/app/en/us/travelArticle.do?catId=51