Definition
Migration streams are the discrete corridors — specific rail lines, highways, river routes, and origin-destination pairs — that a large migration uses, rather than spreading evenly across all possible paths.
Streams are how historians and demographers measure who went where. The same migration that looks like a featureless wave in aggregate is, on closer inspection, a small number of well-defined channels carrying most of the volume. Identifying the streams converts a vague "people moved" into a precise sociology: which county of origin, on which carrier, to which destination neighborhood, in which year.
Why it matters
How it works
A migration stream is identified by the persistent over-concentration of an origin-destination pair beyond what chance would predict. If twenty percent of a migration's flow goes between two counties separated by two thousand miles, while most county pairs send essentially none, that pair is a stream. The mechanism behind any stream is usually some combination of three things: a carrier (a rail line, a highway, a recruiting agent) that physically connects the two points; an anchor (an early migrant or a destination-side institution) that gives newcomers a place to land; and an information channel (letters, hometown newspapers, returning migrants) that keeps the route alive in the origin's imagination.
The U.S. Great Migration is the textbook case of stream structure. Three corridors carried the vast bulk of the six million migrants. The Illinois Central corridor moved Mississippi Delta and Alabama migrants north through Memphis to Chicago, Milwaukee, and Detroit. The Eastern Seaboard corridor — the Atlantic Coast Line and its successors — moved Carolinas, Georgia, and Florida migrants up to Washington, Philadelphia, New York, and Boston. The Sunset Limited corridor of the Southern Pacific moved Louisiana and East Texas migrants west to Los Angeles and Oakland. Maps of the migration look less like an expanding cloud than like three parallel arrows pointing in three different directions.
Streams persist because of the same kin-and-information mechanics that drive chain migration. Once an Illinois Central stream from Sunflower County, Mississippi to South Side Chicago is established, it keeps running for fifty years — long after the original push factors have changed — because each new migrant has aunts, cousins, and former neighbors at the destination making their arrival cheaper than any other destination would be.