I think that in order to answer this question in a scientifically responsible
manner, we would have to define "without
" and "Canada".
In the case that Canada as a land mass would be gone, as in *poof*, America would be left with some rather complicated reasoning, and a really bizarre border on the north of their land mass. Also, with the instant removal of such a large amount of matter, the winds would sweep rapidly to fill the void, sending the world into a heated debate of whether the artist formerly known as Canada sucks, or whether it is (A)merica that blows.
If Canada as a nation was dissolved, or was never colonised in the first place, then America would hold the land, and the title to the largest (un?)defensible border in the universe.
To get to what I believe to be the subtext of the Question, I think that America would be little different, as there are other countries that represent similar roles in the life of America. On a global level, little would differ as the environmental standards of each country are comparable. Of course, the world would react to such a leviathan (imagine one state holding the landmass of 2/3 of a continent! -- Like Australia, damn!) Anyway.
America would have to take on some Canadian traits if Canada did not exist per se, as the the immigration would continue, and communities would form on linguistic/cultural levels, perhaps critically enough in mass to declare nationhood, and demand distinction... perhaps even within the "charter" of the United States of America and Canada.