there have been 4 urban migrations in US:
1) mid 19th century dominated by Irish & German immigration...Irish settled primarily in urban places in northeast, Germans in both urban & rural places in northeast & midwest
2) late 19th/early 20th centuries immigration, primarily from southern & eastern Europe...settled in both northeast & midwest cities
3) mid 20th century in-migration of African-Americans leaving rural south for north & west...black sharecroppers driven off southern farms & planatations by policies of agricultural subsidies & by mechanization of farm labor...a shorter & smaller in-migration occurred during WW1
4) late 20th century immigration of Latins and Asians...settling in several western cities, in Florida cities, New York, and in some midwest cities such as Chicago
third wave coincided with both suburbanization and a general drift towards the so-called Sunbelt...Michael Hoover