It’s not just water but all kinds of resources including biological ones. That part of the world is just generally inhospitable to life. All kinds of life. The biodiversity of the United States falls off a cliff the further west of the Mississippi you go until you get to the west coast. Not just animals, but plants too. There can be a ton of trees, but not many different kinds of them. And they don’t support that many different animals.
This is an issue noticed in the very earliest explorations of the continental US and was a problem early on. Life is harder out west and so there is generally less life around because of that.
OPs map contains yellowstone which has the highest concentration of wildlife in the lower 48. The great plains are only part of it and they have been inhabited by humans for thousands of years
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u/NathanArizona_Jr 26d ago
The Missouri River runs through OPs map. You can see it, it's massive. I think historically it was difficult to navigate though