According to this article:
Passport Requirements at the Canada Border - Do Americans Need a Passport to Visit Canada?, if you are driving across, no, you do not need a passport, but you DO have to prove citizenship. You may do so with your birth certificate or a passport. With either, you MUST have a photo ID.
Here's a link from the US State Department's website:
Canada, you can find more info there. The site for Canada was down when I checked, but there will be further info there.
Enjoy your trip!