The West begins 140 miles further east than it used to

Population at the turn of the twentieth century: the U.S. Census Bureau's 1914 map, with the break at the 100th meridian.

The American West used to begin at the 100th meridian. It now begins about 140 miles further east. The line was never a border on the ground — it was a line of rainfall — and as the climate warms, the rainfall line is moving.

The line Lewis and Clark crossed

In September 1804 the Lewis and Clark expedition travelled west up the Missouri River from its Big Bend to the mouth of the Bad River, in present-day South Dakota, and in doing so crossed the 100th meridian. On September 24 and 25 they had tense encounters there with the Teton Lakota — a nomadic, horse-based people whose way of life would become the emblem of the high plains and, in the American imagination, of the West itself.

Why the 100th meridian

In 1878 the geologist John Wesley Powell drew the boundary between the humid East and the arid West along it. West of the line, annual precipitation was usually under twenty inches — less than farming needs without irrigation. Population thinned out from east to west across it too, and between it and the Rocky Mountains settlers turned to dryland wheat and free-range cattle, filling the ecological niche once held by the bison that had supported the Lakota and other peoples of the Great Plains.

Moving east

The cause is heat, not less rain alone: as temperatures rise, more water evaporates, and land that was just wet enough to farm without irrigation becomes too dry. The shift is expected to change which crops are grown, how large farms are, how much water they need, and, eventually, how many people the land supports.

The idea of "the West" was always tied to movement — the frontier advancing westward with exploration and settlement. Its boundary is moving again, in the opposite direction, and this time what moves it is the climate.

Sources

Written from the National Park Service's "Bad River Encounter Site — 'The West' Moves East" (Lewis and Clark National Historic Trail), a work of the United States government in the public domain, which cites the expedition journals (William Clark, September 25, 1804), Harvey Leifert in Earth (2018), Kevin Krajick for the Columbia Climate School (2018), and Richard Seager and others, "Whither the 100th Meridian?", Earth Interactions 22, no. 5 (2018).