I've crossed the border dozens of times with DL only. This is the most current info I could find.
"U.S. citizens and permanent residents traveling to Canada by air are required by U.S. law to present a passport for entry into Canada and will need to present their passport again when returning to the U.S.
For those traveling by land or water, you will need a passport, passport card, NEXUS card,
Enhanced Drivers License, or another Western Hemisphere Travel Initiative-compliant document to enter Canada, and will need to show these documents again to return to the U.S."3
What You Need to Know About Visiting Canada From the U.S.