FTDK
Just how big a catchment do you class the Mackenzie and how long a river is it, in your opinion? In Australia, a catchment system for a river that eventually runs into the sea (Fitzroy) can have a change of name every time a major tributary joins it. We used to live on the Peak Range, which was a watershed between the headwaters of Capella Creek which ran into Theresa Creek which ran into the Nogoa River which joined with the Comet River and became the Mackenzie River. The water falling the other side of the hill ran into a creek that ran into the Isaac River that joined the Mackenzie at the top of its loop about halfway along its south-to-north-to-south-again run. The Mackenzie lost its identity when it joined the north flowing Dawson to become the Fitzroy that held its name to the sea. So you could say all our water went to the Mackenzie, but it all also went to the Fitzroy.
Even the longest tributary of the Murray, -the Darling-, starts as the Condamine, becomes the Balonne at about Surat, then the Culgoa at 'Cubbie', etc, before it ever becomes the Darling River. (have to find my Bourke map)
No wonder people get confused over catchment areas..