Well, just in case my wording is straightforward, please tell me so my words are going to be appealing.
I in its essence agree with your assessment. As the fork is about 50km east of P.A., maybe a dam allowing river traffic could be built for hydroelectricity which supplies P.A.
In the future though, the map above shows that Edmonton, Calgary, Saskatoon, Winnipeg are connected by rivers. Then a
For example a ship from Edmonton goes to P.A., then into Lake Winnipeg, upstream to Winnipeg for rest, then upstream thru the Winnipeg River to the Lake of the Woods, then upstream thru Rainy River to Rainy Lake. Then how to connect from Rainy Lake to Lake Superior? Maybe a canal. Also all the rivers east of the City of Winnipeg may need to be widened....
I confess IDK.
No, I'm saying bypass the City of Winnipeg, because that's a dead end. The goal is the Atlantic,
via Great Lakes, so you leave Lake Winnipeg for LotWoods, from there to what's now Thunder Bay, & from there by Lakes freighter to wherever. It looks like there's a water connection from LotW to LWinnipeg, so no canal needed. Size of barge on Winnipeg R & Rainy R (from LotW to Superior) may be a limit.
This covers all the Prairies, more/less, & even south into Minnesota (by Red River to Winnipeg, then north to the lake).
Winnipeg might be a staging point for barges or tugs, I'll agree, but not an ultimate end point; that's much more likely to be OTL Thunder Bay, maybe Banff & Jasper at the other end (which would hurt the National Parks...if they still happen).
The rationale that comes to mind for me is defense. Perhaps if you have a more militaristic and anti-British United States there might be a push to try to settle the prairies and develop them in order to resist American attempts to seize them and to secure supply lines between eastern and western Canada. It would be similar to many other attempts to settle frontiers with hostile powers to provide some organic defense capability.
That crossed my mind, too. Which seems to want a Black Hills Gold Rush sooner than OTL; otherwise, there's not big U.S. population pressure. The Cherry Creek Rush is a bit far south, but it might do the trick. If it does, you've created Canada sooner, IMO, because desire for population expansion probably means a rail line demand sooner, & that was BC's condition to join--if fear of the U.S. doesn't drive building the railway & creating *Canada no matter what BC wants.
More U.S. "intrusion" still has to overcome the ice for river transport to be a Thing. I hate to be a party pooper about that, but I'm not seeing any way around it.
If you're prepared to go back farther, you could make the Prairies & BC a part of Canada with a purchase from HBC any time after about 1817, provided HMG would sign off. With a bit more attention on the HBC freight canoes, & the idea of steam propulsion... And, unfortunately, you hit ice as sure as
Titanic.
Sorry, folks.