During World War II, the US converted two sidewheel steamships into aircraft carriers and operated them entirely on the Great Lakes. But their mission wasn't to guard America's freshwater border with Canada. The USS Sable and the USS Wolverine (pictured above) were training vessels for pilots.
I learned about these carriers through a post written by Glenn Reynolds. He comments:
You know, Nick McCall and I were going to write a novel about a U-boat in the Great Lakes, being chased by, among other things, these very carriers, but we couldn’t come up with a good way to get it there.
That would be a fine alternate history challenge!