Bottom Line: As of June 1, 2009, everyone coming to Canada by land or sea has been required to have a passport or equivalent travel document.
Best Advice: Apply for your passport or passport equivalent today if you haven't already.
If you need a passport right away, get a passport within 24 hrs with Rushmypassport.com
Land & Sea Passport Requirements in Depth: The Western Hemisphere Travel Initiative (WHTI), which was introduced in 2004 by the U.S. government to strengthen U.S. border security and standardize travel documentation requires all U.S. citizens present a valid passport or equivalent travel document to enter or re-enter the United States.
Technically, Canada Border Services does not require U.S. citizens to present a passport to enter Canada, however, Americans do need a passport or equivalent travel document to get back into the U.S. Thus, Canada and U.S. border requirements may be different on paper, but, are in practice, the same. Does it make sense to allow a U.S. citizen into Canada who does not have the proper documentation to return home? U.S. border laws essentially trump Canada's.
For More Information:
Canada Travel Documents & Passport Substitutes
Canada Border Services Agency OR The US Department of State
Further Reading about the Canadian Border:
Tips for Driving across the Border into Canada
Duty Free Shopping at the Canadian Border
Bringing Children across the Canadian Border
How Much Liquor Can I Buy at the Border?