query: What caused the Dust Bowl?
passage: Dust Bowl
The Dust Bowl was a period of severe dust storms that greatly damaged the ecology and agriculture of the American and Canadian prairies during the 1930s; severe drought and a failure to apply dryland farming methods to prevent the aeolian processes (wind erosion) caused the phenomenon.[1][2] The drought came in three waves, 1934, 1936, and 1939–1940, but some regions of the high plains experienced drought conditions for as many as eight years.[3] With insufficient understanding of the ecology of the plains, farmers had conducted extensive deep plowing of the virgin topsoil of the Great Plains during the previous decade; this had displaced the native, deep-rooted grasses that normally trapped soil and moisture even during periods of drought and high winds. The rapid mechanization of farm equipment, especially small gasoline tractors, and widespread use of the combine harvester contributed to farmers' decisions to convert arid grassland (much of which received no more than 10inches (~250mm) of precipitation per year) to cultivated cropland.[4]
passage: History of San Bernardino, California
The Depression and the Dust Bowl caused a wave of migrants from Oklahoma and Arkansas to arrive in San Bernardino to work the fields in and around the City.