Well The great lakes are hugh and don't freeze. I don't see them swimming across those lakes. Erie is shallow, so maybe that helps. The rivers between those lakes have major current, I don't see pigs swimming across those rivers. I know Lake St. clair freezes in the winter, but I believe a channel is open year round due to the currents. So I don't think there will be a big invasion of wild boars unless they are shipped here.
http://www.google.ca/search?q=sangl...urce=og&sa=N&tab=wi&ei=RNnFUfKfHai0iQLDw4DYAQ
have a look at some of those pictures ...
they re able to swim in Mediterranean sea but your lakes maybe are different ....


















































