Final answer:
The Homestead Act caused immigrants to migrate north after the Civil War, as it offered free land for settlement and economic opportunities.
Step-by-step explanation:
The event that caused immigrants to migrate north after the Civil War was the Homestead Act.
The Homestead Act was passed by Congress in 1862 and offered 160 acres of free land to households willing to move west. This act encouraged westward migration and settlement, as individuals and families saw an opportunity to obtain their own land and improve their economic prospects.