The Liard is the largest 
The Liard is the largest tributary emptying directly into this river. At Lake Athabasca it joins the Athabasca River and the Finley-Peace River System to form the second longest continuous stream on the continent at 2,600 miles long. Originating in the Great Slave Lake, it flows generally northwest into the Arctic Ocean through a great delta. FTP, what is this great rivet of the Northwest Territories of Canada?
The Liard is the largest tributary emptying directly into this river.
Mackenzie river (accept Finley-Peace-Mackenzie)