I assume you mean the upper Churchill in Saskatchewan, but at the end of a trip, I paddled from Fort Prince of Wales to Churchill across the Churchill River where it empties into the Bay.  Although much of the water has been diverted into the Nelson River for hydro purposes by this point, it was a high water year and the volume and current in the Churchill was unbelievable.  The tide was going out as well making matters more difficult. 

I haven't paddled any of the others, but would like to paddle the Hayes some day.  Sounds like you have paddled them all?  Was the Winnipeg worth paddling?  I heard it is full of dams.  I know the kayakers target a few of the major rapids on the Winnipeg for playboating.  You might have more luck at Canadian Canoe Routes (You need to Login or Register to view media files and links) finding canoeists who would like to swap stories about these rivers.  Most paddlers at this site frequent BW/Q, Woodland Caribou and Wabakimi.