Visitors from any country other than the U.S. have always needed a passport to enter Canada. On the other hand, because of a friendly border crossing agreement between Canada and the United States, Canada Border Services did not require U.S. citizens to present a passport to enter Canada. This friendly border crossing agreement used to be mutual; however, now the WHTI requires that U.S. citizens have a passport to return home. In this way, passport requirements for Canada and U.S. borders are different on paper, but, are in practice, the same. Canada will not allow a U.S. citizen into the country who does not have the proper documentation to return home.
(so now you have to have a passport)