They were almost extinct by the 1920's,all across North America.There were some in the far north but they were highly protected.However there was a black market for the skins and people poached them.
There was extensive re stocking programs for beaver in the 40;s and 50's.Lots of places in the states only started having beaver again starting in the 1960's.
There were indian beaver preserves in the far north, Rupert house beaver reserve was one of the big ones.Certain Indian trappers were known as tallymen and they looked after the beaver by counting the houses every year and telling the natives which areas to trap and how many could be trapped.This was basically the start of fur management.
Many of the beaver used for restocking in north america came from the northern cree beaver reserves,basically the only places left with any number of beaver.

The James Bay beaver preserves were-
Fort George
Old Factory
Rupert
Nottaway
Kesagami(where I am)
Albany
Kapisko
Attawapiskat