Basically a really great research fit from Dalhousie has reached out to me. I wasn't planning on applying to grad school until 2019 (i'm a senior in undergrad); however, this is just too great of an opportunity to pass up. What roadblocks will I face after graduating if I get my PhD in Canada and want to practice in the U.S?
This is not so much a roadblock after graduating but before graduating. Were you able to discuss funding? Funding becomes a bit complicated when studying abroad, especially if the professor does not already have a grant for you.
As an American citizen, you will not qualify for most of the funding from the Tri-Council Agencies (Canada Institutes of Health Research, Natural Sciences and Engineering Research Council of Canada, and Social Sciences and Humanities Research Council); the one exception is the Vanier Canada Graduate Scholarship, which is one of the most competitive and lucrative fellowship programs in Canada. The application process involves being nominated by the institution.
Furthermore, as an American citizen abroad, you will not be able to take many of the major predoctoral fellowship awards (Ford Foundation, NDSEG, NSF GRFP) to a Canadian institution. I have heard that one can use NRSA funding abroad if one can justify why one is seeking training outside of the United States, but I haven't yet run into someone who has done so successfully.
This is not to say that attending a Canadian school is impossible without funding, as the American graduate students who have been generous enough to let me ask have made it work. In particular, because most Canadian graduate students bring Tri-Council funding with them, the department may be able to fund you after a selection process and/or through assistantships (at least in the cases of the programs I investigated). However, it's something that I'd recommend checking with international students attending Dalhousie before applying.
Another thing worth noting is that your study permit will allow you to work only on campus. If you want to work off-campus, then you will need a study + work permit.
Good luck!