Starting in about 2015 and going forward, the net growth in the American workforce will be entirely due to immigration. The total working age population in 2015 was 173 million. Without immigration, that would decline by 2035 by 4%. With immigration, it will increase by 6%.
Between 2005 and 2014, the native-born American working age population increased by 4.8 million workers. The first generation immigrant workforce increased by 6.1 million and the second generation immigrant workforce by 2.4 million.
After 2015 and through 2035, the native-born working age population will decline by 8.1 million, the first generation immigrant workforce will increase by 4.7 million, and the second generation immigrant workforce will increase by 13.6 million.
From Pew Research