A glance into the history of the Southwest reminds us that the climate and rainfall patterns have varied tremendously over time, with stretches of drought many decades longer than the one we are experiencing now.
Long dry stretches during the Medieval centuries (especially between 900 and 1350 CE) had dramatic effects on the native peoples of the Southwest (the ancestral Pueblo, Hohokam, and Sinagua), including civilizational collapse, violence, malnutrition, and forced social dislocation.
These earlier Americans are a warning to us.
The past 150 years, which we have used as our baseline for assumptions about rainfall patterns, water availability for agriculture, water laws, and infrastructure planning, may in fact be an unusually wet period.
Let’s look at the past few hundred years first and then explore the region’s climate in geological time.
Recent Droughts and the Arid Regions of the United States
John Wesley Powell stands as one of the most extraordinary scientists and explorers in America in the second half of the 19th century.
In 1869 he became the first white man to lead an expedition down the Colorado River and through the Grand Canyon, a feat all the more remarkable considering Powell had lost most of his right arm during the Civil War.
Ten years later, Powell published Report on the Lands of the Arid Regions of the United States, a careful assessment of the region’s capacity to be developed.
In it, Powell argued that very little of the West could sustain agriculture. In fact, his calculations suggested that even if all the water in western streams were harnessed, only a tiny fraction of the land could be irrigated.
Further, Powell believed that growth and development ought to be carefully planned and managed, and that boundaries drawn for new western states ought to follow watersheds to avoid inter-state fighting over precious water resources.
When Powell presented his findings to Congress, politicians howled. Powell found himself denounced by pro-development forces, including railroads and agricultural interests.
Prescient as Powell’s study has proved to be, it was almost entirely ignored at the time.
Instead, those development boosters responded to Powell’s data about the aridity of the west with a novel climatological theory: “Rain follows the plow.” They insisted that agriculture could cause the rains to fall, so like magic the more acres brought under cultivation the more rain farmers would enjoy.
The years surrounding the turn of the 20th century turned out to be unusually wet across much of the region. Hopeful pioneers continued to flock to the West, despite the visible signs of aridity.