I can tell you for certain that the Western Cottonmouth is as far west a Rough River Lake. I have one in my freezer if anyone wants to check it out. I lived in the swamps of Mississippi when I was younger and have caught many cottonmouths on a cane pole. It is 3 feet long. I am in the woods almost everyday with my job so I have encountered many different posion snakes. I also have photos of it if anyone is interested.