The only full-time resident on 130 miles of river is Ralph Conaster, whose spacious cabin at Eagle Island was the checkpoint until it burned down a few years ago. There is always the chance of some overflow with glacier areas from water seeping from the high banks or flowing out of side streams. Once in awhile you may also see overflow from the river itself, but hopefully the trail will avoid it. It is critically important to stay on the marked trail on the big river. The Yukon is notorious for stretches of open water and thin ice.
People run snowmachines into the river every winter, and some die.
The other major factor is the wind, which always seems to be blowing downstream, which means in your face. While it's perfectly possible to get fine sunny days with light breezes, wind chills on the river can be very low, pushing 100 below in the worst cases. For long stretches there is no shelter at all and you must keep plugging. Make sure you have coats for your dogs, especially your leaders, since they will be bearing the brunt of the wind chill and breaking the wind for the dogs behind them. You'll head from the checkpoint right down the bank onto the river.