I’ve noticed on this mapand others that US population density tails off abruptly in a rough line south from the North Dakota/Minnesota Border down to eastern Texas. East of that line most parts of most states are somewhat densely populated with of course the big cities of the the midwest and northeast being decidedly more densely peopled and with plenty of areas like the Appalachians which are relatively sparsely populated. West of the line most of the states seem far more devoid of human populations, except of course in major cities and in the PNW and California. Presumably climate accounts for some of this difference but is there a particular geographic feature of the continental United States that this divide reflects? Or is it a combination of different geographic, climatic, and historical causes that accounts for this divide?
Rainfall, pure and simple. Here is a map of average annual precipitation. Notice the correlation? When average rainfall drops below about 20 inches the optimal land use changes from farming to ranching, which historically supported a much lower population density and thus spawned smaller markets, fewer industries, and smaller cities.
The correlation between population density and average annual precipitation is striking!
Oregon is a good example of this, in and of itself. Look at the population density of Oregon, and then look at the average rainfall. You see the same exact thing happen.
Excellent. Thank you for your swift and clear reply!
that is a cool map: thanks for posting it.
Most of that sparsely populated western area is also mountainous, or, at any rate, at high elevation. (Of course the correlation of this with rainfall is not a coincidence.)
Actually, the correlation breaks down for most of the West Coast.
There actually is a bit of a human geography in play here. Before there was good weather data for the whole country, the 100th meridian was somewhat arbitrarily chosen as the official eastern boundary of the Great Plains. When the vast middle of the country was being settled, it was very difficult to get farm loans or crop insurance on a farm out past the 100th meridian. The Desert Land Act and the Enlarged Homestead Act also used the 100th meridian as the easternmost point where you could get a larger allotment more suitable for ranching as opposed to the smaller farm allotments under the original homestead act.
As actual climate data started rolling in and mechanical irrigation began the 20th century, the 100th meridian became somewhat less important for the actual distribution of farms (the “background” density on that map), but it’s still pretty clear when you look at where the small farm-support cities on the eastern plains grew in.
But why, you may ask, is the rainfall divide around there (yes, I know it’s a fuzzy line along a continuum)?
A big part of the answer is the Gulf of Mexico. Look at a map of North America. Follow the Gulf coast of Mexico northward toward the southern tip of Texas. When the shoreline starts to curve eastward, keep going north. This is approximately the line you’re describing (I’m on it right now).
Storms generated in the Gulf transport moisture to the US east of this line during much of the year.
Interesting. That definitely would reinforce the relative sharpness of the more gradual climate boundary, part of whose underlying cause I just described. (Another cause is the rain shadow effect of the Rockies as weather systems move from west to east, but you still need a new moisture source to then replenish those air masses… Something similar occurs in China, but only once you’re east of the western shore of the South China Sea.)
Wallace Stegner wrote a book called “Beyond the 100th Meridian” about John Wesely Powell. The book also addresses the failure of the homestead act in the more arid West. The standard parcel of 40 acres was enough to sustain a family in the midwest, but was too small to provide for a family in the drier areas of the Great Plains west of the meridian.
I’d add that the Rockies are a barrier that keep the eastern side much drier than the western. Precipitation from the Gulf of Mexico (look at the remnant of Isaac dumping rain onto the eastern part of the drought areas in the midwest) explains where the rainfall starts picking up again, probably farthest west than might otherwise occur.
Actually, it doesn’t. Once you enter the rain shadow in eastern Washington, Oregon and California the population density drops as well. The anomaly of Southern California is explained by the artificial transportation of water to sustain crops and the population.