The Canadian government updated its US travel advisory last week stressing that people need to ensure they can properly show legal status if requested at any time while in the US.
“Authorities may request proof of legal status in the U.S. at any time."
“Be prepared to show evidence of your legal presence in the U.S.”
There is an agreement between the two countries that while within either country a drivers license is accepted as legal ID. But the Canadian government is warning that that may not be good enough at the moment. This means carrying your passport and having access to a copy of your I-94 form at all times.