And the effects are good.
The paper's abstract reads:
We study the effects of European immigration to the United States during the Age of Mass Migration (1850-1920) on economic prosperity today. We exploit variation in the extent of immigration across counties arising from the interaction of fluctuations in aggregate immigrant flows and the gradual expansion of the railway network across the United States. We find that locations with more historical immigration today have higher incomes, less poverty, less unemployment, higher rates of urbanization, and greater educational attainment. The long-run effects appear to arise from the persistence of sizeable short-run benefits, including greater industrialization, increased agricultural productivity, and more innovation (Emphasis added).