The Western Hemisphere Travel Initiative (WHTI) is a U.S. law that requires all travellers, including U.S. and Canadian citizens, to present a valid passport or other approved secure document when entering the United States from within the western hemisphere.

When entering the United States by land or water, Canadian citizens are required to present one of the following valid documents: a passport, a NEXUS card, a Free and Secure Trade (FAST) card or an enhanced driver's licence/enhanced identification card or a Secure Certificate of Indian Status (when this certificate is available and approved by the United States).

Canadian citizens flying to the United States must present a valid passport to confirm their citizenship and identity. A NEXUS card is also acceptable when using a self-serve kiosk located in the pre-clearance area at designated Canadian airports.

Information for travelers can be found here  at http://www.cbsa-asfc.gc.ca/whti-ivho/menu-eng.html