Rivers easily stay clear well below 32, moving water helps with that. The below zero temps are hard even on rivers and probably the low current. I was surprised by the photos how much open water there often was. We boat all winter around here in various places, but a 3 week trip brings risk of unknown forecasts that you either deal with, or don't go. Most wouldn't go, true.
I am a river, babe - I've got plenty of time, I don't know where I'm going, I'm just following the lines..... - "We are water" by Shaye