Every time I look at a map of the United States coloured by something proportional to population density, I see a stark vertical line going from northeast North Dakota, through SD, NE, KS, OK, and finally reaching southern Texas. For instance, here it is in a map of light pollution:
Here you can see it in nighttime satellite imagery:
and even in confirmed COVID-19 cases:
Finally, here it is in terms of raw population density data by counties:
My current hypothesis is it's mostly determined by climate---for instance, here is a map of the Köppen climate types of the United States:
However, the cold semi-arid (BSk) climate only starts in the western parts of the states that the line crosses in the east. I found a slightly higher correlation by looking at a map of rainfall:
So, is this line mainly determined by climate? If so, why is the line so stark, and still so cleanly visible, perhaps a hundred years after the main wave of migration? Could this be better explained by specific historical circumstances and demarcations; for instance, a boundary between land that was ceded by the Native Americans, and land that wasn't?
For instance, the map of land cessions seems to line up pretty well with the line in Texas: