I've been an outdoorsman mostly in SWMO all my life. I've seen exactly one rattlesnake (a pygmy) and one cottonmouth in my 69 years. The rattlesnake was in the gravel near my office in Stonebridge in Stone County. I saved that little snake's life. There was a lynching party forming but they got overruled.
The cottonmouth was at Sam A. Baker State Park. A long time ago. I was collecting a water sample standing on a rock at the edge of Big Creek. Noticed the snake on the rock next to me. It was about the size and shape of my arm, head up with a mouth that looked about four inches wide that was fully open with fangs that had to be close to an inch long plainly showing. I gave him the creek.
Back then, SE MO was supposed to be the northern extent of Cottonmouth range. I hear a lot of reports now much closer to home from people that don't seem all that freaky about snakes. So I buy they're here now. Climate change likely is related to expanding territory for them.