The event that signaled the start of the Great Migration in the United States was the Great Flood of 1927. This catastrophic flood displaced thousands of African Americans from the rural South, prompting many to seek better opportunities in northern cities. As they moved, they sought to escape systemic racism and the economic hardships faced in the South. This migration had significant demographic impacts, reshaping urban areas like Chicago and New York, and contributing to cultural movements such as the Harlem Renaissance.