system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What's the deal with Idaho's northern border / Montana's western border?

So in my head, borders are usually divided because of Geographical terrain, otherwise they'd all be square, or at least straight.

States to the east have rivers or mountains that act as natural boundaries between geographical areas. I understand why a state like New Jeresey is drawn the way it is.

But why is Idaho/Montana so strange? It seems like a totally arbitrary scribble on the map. Is there some historical reason? Were Montanaians really into those forests? Maybe Idaho gets all it's Potatoes from the north? It doesn't make any sense to me. Any locals around to explain?
gpt
According to the awesome book "How the States Got Their Shapes"
> The separation between Idaho and Montana begins where the Continental Divide intersects the 111th meridian. It then follows the Continental Divide to the point where it intersects the Bitterroot Mountains. Here the crests of the Bitterroot Mountains becomes the boundary up to the Clark Fork River, where a straight line due north completes the border.