The boundary between the United States and Canada west of the Great Lakes mainly falls on latitude 49N until Vancouver Island in British Columbia. The boundary then swings south and west in the middle of the main channel between Vancouver Island and the United States mainland.
There are two interesting features of this boundary. First, the latitude 49N cuts through the southern tip of a small peninsula in Boundary Bay south of Vancouver, B.C. This small section of the United States, called Point Roberts, is only accessible by land through Canada!
The second feature has to do with San Juan Island. If you look at a map with the international boundary, you will see a "kink" in the north/south line to the east of Vancouver Island. The island that this "kink" goes around is San Juan Island. There was a dispute over who owned San Juan Island, since is appears to be in the middle of the channel. There were even two small military camps on the island during part of the 19th century, the British camp on the northwest corner of the island, and the American camp on the southeast corner.
Finally, this international dispute had to be decided by a third party, who had to interpret the treaty which originally set the boundary. The leader of Germany at the time (who was called the Kaiser) decided that the main channel was to the west of San Juan Island. As a result, all of San Juan Island is in the United States, and the boundary has a "kink" in it.