The Northern Frontier: By River

Much of the United States-Canada border is a straight line, cutting across mountains, prairies, and boreal forests.  The border itself is little more than a path of cleared land with the odd dirt road cut on either side.  Further to the east, however, the Great Lakes and associated rivers form the frontier between the two nations, often in relatively level terrain.  The rivers are almost always a lovely aquamarine, and usually have ships plying the waters.

As you can see, even in the more "defensible" parts of the border, there is little that would otherwise remind one that this is an international boundary.  The far side is Ontario, the near side is Marine City, Michigan.