Of course there are a number of portages on this route around rapids and falls. In addition, there are swifts and minor class 1's leading up to portages from both direction. This being Wabakimi, there are downed trees and other obstacles to paddling in the upstream direction. All I can say is that I was able to paddle these, but I do a fair amount of upstream paddling for excercise. When my saw broke I was nervous about my ability to do the route upstream, partly because I wouldn't have a way to clear a path if i needed one. The water didn't seem high when I was there, but high water would have made it much more difficult to travel in the upstream direction. If you do a youtube search on "Flindt River Wabakimi" there is a video series from a fellow who went through there just a few weeks after me, after the portage crew had cleared the trails. This will give you an idea of the extent of the burn.
Mark