Re: How do rivers freeze in the winter if the water is constantly moving?

Kinetic energy is certainly a factor in how rivers freeze. Ambient air 
temperature is another. Rivers begin freezing at the banks, in pools or 
eddies. As the ice sheets there grow out into the water, they break off and 
float downstream. Along with the colder weather both at your location and at 
the head of the river, this has the effect of cooling the water, regardless 
of its speed, to near freezing. This accelerates the growth and thickening 
of ice near the bank, allowing it to reach farther into the moving water, 
and it also allows any unmelted floes to grow. Most rivers have shallow 
regions or islands, and floes can become grounded on these. As the floes 
ground, they provide not only seeds for more stationary ice to form, but 
blockages which catch other loose ice. Eventually the river surface freezes 
completely. 

Even though a river may be covered with ice, you cannot assume the ice is 
very thick. Even in shallow rivers in cold winter climates, there may be 
regions where the ice never reaches a thickness of more than a few 
centimetres. More than one skidooer has gone to his death thinking a frozen 
river was a great place to open the throttle.