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:

Source: http://darksitefinder.com/maps/world.html#4/39.00/-98.00

Here you can see it in nighttime satellite imagery:

Satellite imagery of the United States (from NASA), showing the dividing line.

and even in confirmed COVID-19 cases:

source: JHU

Finally, here it is in terms of raw population density data by counties:

Source: census.gov

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:

Source: Adam Peterson on wikimedia commons

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:

rainfall map of the US

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:


Are there any historical reasons other than "geography"for this divide?

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    This doesn't appear to have an historical basis - as your geographic research already bears out. – Pieter Geerkens Mar 29 '20 at 20:12
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    !! New contributor with a well researched, well phrased question. I don't know. – Mark C. Wallace Mar 29 '20 at 20:41
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    This is a Geography/Geology question rather than a History one. See the Rocky Mountains and the Gulf Stream Myth. – Denis de Bernardy Mar 29 '20 at 20:46
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    The line matches well with that between cropland to the east and grazing/pasture land to the west. if you are looking for an historical basis for that divide - the 1930's Dust bowl would fit. But that in turn has to do with rainfall, and susceptibility to drought further west and north. – Pieter Geerkens Mar 29 '20 at 21:04
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    I wish I could say this is on-topic here but I don't think it is. Congrats, though, on a well-researched, clear question. – Lars Bosteen Mar 30 '20 at 6:22

The answer is related to the map you posted about rainfall. The population of the eastern third of the continental US is denser because of settlement patterns that reflect the local availability of water resources. This quirk of human geography occurs because of the need for irrigation. As detailed by Harvey Leifert in "Dividing line: The past, present and future of the 100th Meridian":

In his 1878 “Report on the Lands of the Arid Region of the United States,” [John Wesley] Powell identified the “arid region” as the land west of the 51-centimeter-per-year rainfall line, which closely tracked the 100th meridian. This amount of rainfall per year is about the minimum that permits farming without irrigation, and it also greatly influences the types of crops that can be grown. The line Powell noted as dividing the arid and humid sections of the continent has become known as the “effective” 100th meridian.

That population is much denser on the east side of the line is apparent in historical demography nearly as soon as masses of settlers arrived there in the mid-1800s; check out this neat animated map.

With more limited water, farms had to be bigger to make money west of the line. Possibly related to farming conditions and market size, insurance was more available east of the line.

  • 1
    Thanks so much for the excellent sources! Fascinating. – 416E64726577 Mar 30 '20 at 4:35
  • I can't upvote this, without a reference to the lessons learned from the Great Dust Bowl of the 1930's. Irrigation is insufficient for much of the terrain between that line and the Continental Divide because drought will blow away everything ever turned over by a plough. – Pieter Geerkens Mar 30 '20 at 13:43
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    @PieterGeerkens - That is indeed probably why the Dust Bowl hit western Oklahoma much harder than eastern Oklahoma. – T.E.D. Jun 27 '20 at 23:16

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