The ferry is pulled out of the river at the beginning of winter. There are a few weeks in the Autumn and again in Spring when the crossing is closed and freeze up and breakup.
The Mackenzie crossing is much more impressive than the Peel river as the river is much wider. Here you really do get the feel of driving onto the ice - with the truck leading the way. We had weighed ourselves on the way up (there is a free truck scale just south of Calgary) and we came in at 1730kg with all 4 of us in the car, plus luggage and 3/4 of a tank of diesel. If the truck didn't break through we should be OK :-)