About 900,000 French Canadians migrated to the US in the late 1800s to early 1900s, which represented approximately 1/3 of Quebec's total population at the time. They mainly went to work in the textile mills of New England, living in tenement slum enclaves known as "Little Canadas".
These were the New England mill towns with the largest French Canadian populations in 1900
Fall River: 33,000
Lowell: 25,000
Manchester: 23,000
Woonsocket: 17,000
http://faculty.marianopolis.edu/c.be...gs/leaving.htm
For comparison, the French Canadian populations of the largest Quebec cities would've been something like
Montreal: 200,000
Quebec City: 50,000
Levis: 18,000
Others: approx 8,000 (ex Saint-Jean-sur-Richelieu, Sherbrooke, Trois-Rivieres, Saint-Hyacinthe, Salaberry de Valleyfield)
The migration into New England largely ended with the Great Depression. Although the "Little Canadas" of New England disappeared in part due to assimilation, it's estimated that approximately half of the immigrants returned back to Canada. Mind you the immigrants were returning back to Quebec in large numbers even prior to the decline of these enclaves since they often came to work in the mills only for a short time to pay off debts or accumulate savings to purchase property or start a business when they returned to Quebec.