I believe it's the beginning of the Great Plains and generally lacks water. It's not dry like a desert, but compared to the temperate forests and rivers of the East, South, and Mid-West, it is. It is intensively farmed now, using pumped aquifer water (https://en.wikipedia.org/wiki/Ogallala_Aquifer), but it never developed the population density of the East. Geology maps also show this divide, so it is based on the underlying rocks, as well as weather patterns from the Rocky Mountains I suppose.

Curiously this lines up with the locations decimated by the Dust Bowl [1]. There was a mass migration of millions of people westward towards California during the 1930s once the agricultural industry there collapsed.

The proximate cause for that line might be farming techniques poorly adapted to the prairie ecosystem which caused a massive ecological collapse.

[1] https://en.wikipedia.org/wiki/Dust_Bowl

My guess would be that the land is not as conducive to farming west of that line, both in terms of soil and weather. So early settlements didn't occur there, and thus cities did not emerge from those areas.

The cities that did evolve in the West were based on different resources: ports around the ocean, mining, oil, railroad stops etc.

That’s correct, the US population density correlates fairly closely with the average annual rainfall: https://en.wikipedia.org/wiki/United_States_rainfall_climato...

Yeah, there is a pretty distinct line from Winnipeg to San Antonio, separating light from dark.

If you look at a population density map, it looks quite similar: http://dafi1637.blogspot.com/2017/10/united-states-populatio...

I think that explains the immediate effect. You can of course then wonder why the population is distributed that way, but I feel I've done enough here :)

>Yeah, there is a pretty distinct line from Winnipeg to San Antonio, separating light from dark.

That's the I-35 corridor. An interstate highway running north/south all the way from Mexico to Canada. It was probably the most hyped/utilized part of NAFTA. I live in Dallas where I-35 runs through (actually splits in Waco to cover both Dallas and Ft. Worth, reconnecting north around Denton). I grew up in a small city south of Dallas, and remember all of the highway construction that got spurred by NAFTA. That area (Ellis county) also had a construction boon due to the Super Conduction Super Collider.