I think their populations are way below carrying capacity, because of over harvest. I've done a great deal of fishing on small streams, especially in the "Big Woods" of NC PA, but also a fair bit in NE and NW PA. And as you explore these streams, you also drive along many miles of stream.
The amount of beaver ponds and other signs of beaver activity that I see seems very low in comparison to the vast mileage of small forested streams that beavers could inhabit.
For those who fish small, forested streams, just ask yourself as you explore these streams: Why are there no beaver ponds here?
Of course I do see SOME beaver ponds on the small forested streams. It just seems like the ratio of beaver ponds/miles of small forested streams seems very low.
There are places where you can walk 9 miles along a small, forested stream on public forest land and there are ZERO beaver ponds. Go back in time to 1700. Would there have been beaver ponds there? I think the answer is yes.