A couple additional thoughts on vessel movement:
Poles and sails will be useless as a means of propulsion in this circumstance. They will be moving so quickly downriver, any effort to connect with the bottom using a pole will either be fruitless or profoundly dangerous. As other have said a sail would be shredded, but wind in this circumstance will also most likely be funneling along the length of the river, not blowing across it in the direction they want to go.
To calculate how far down the river they go, I'd assume the water will be carrying them downstream at a rate of about 10 mph. I would reduce their forward motion across the river to between 20 and 30 feet to reflect hazardous conditions. Based on the width of the river, determine how long it will take them to cross barring disaster. The amount of time they're on the water multiplied by the downstream movement rate will give you some idea where they end on the other side. Assuming they actually make to the other side in one piece.
Depending on how well they know the area, getting back to where they want to be from where they end could be a whole other adventure.
Carl