They are. Also you can get to several of the Rockies, even past them,
e.g. The Fraser River will take you to Yellowhead Lake by Jasper Alberta.
Mackenzie River in the Northwest Territories will get you to Athebasca Lake, which also drains into Hudsons Bay.

For that matter, most of the Canadian Shield is linked by rivers and creeks to Hudson’s Bay. Which is how that company used to be one of the largest “countries” on the planet. Their deed was to all land all water which flowed into the bay. The English King who did that was an idiot.