On the other hand, because of a friendly border crossing agreement between Canada and the United States, Passport Requirements Flying into Canada
United States Canadian Passport Rules

Travel between the United States and Canada is a fairly simple affair. Entry into Canada from the U.S. requires one of three documents: a U.S. passport,
Nexus. Expedited Entry into the US Canada

NEXUS: The NEXUS program allows pre-screened travelers expedited processing by United States and Canadian officials at dedicated processing lanes at designated
Canada Border Services Agency Pacific Crest Trail Association Home

For entry back into the U.S.A. (only allowed at an official border crossing) It is to be presented to US Customs or Canada Border Services offices
Crossing the Canada/U.S. Border Suite101

This article deals the problems of legal entry into the United States for Canadians who have criminal records including the differences between pardons and waivers.
Canadians with mental illnesses denied U.S. entry Canada CBC News

Canadians with mental illnesses denied U.S. entry Data entered into national was blocked entry to the United States because Canada would share that with us