Answer:
In the 1930s, a large number of people immigrated to California because of the Great Depression and more specifically, because of the Dust Bowl, a phenomenon of severe dust storms in the plains that caused drought and massive agricultural losses.
Most of the immigrants came from Oklahoma and Arkansas, two of the most Dust Bowl affected states (the other two were Texas and Colorado). They settled primarily in the San Joaquin Valley to dedicate themselves to agriculture again. Ever since, the San Joaquin Valley has been an agricultural center in the U.S.
In 1930, the population of California was 5,677,000, and in 1940 was 6,907,000, and increase of 1,230,000 people.
