Crystals of sugar form in hot saturated sugar solution as it cools down, due to excess sugar molecules not fitting in the solution and precipitating as crystals.

In a similar situation, if we have a region where temperatures are well below zero, still water should freeze. But in rivers, water doesn't freeze, because it's flowing. Do ice crystals form in such super-cold water?

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  • 1
    Possible duplicate of Is it possible for rivers to freeze completely and stop flowing? – Fred Feb 10 '16 at 7:30
  • It's not a duplicate per-se, as the essence of the accepted answer is what I know already. However, I have added a relevant snippet below that answers my question. – cst1992 Feb 10 '16 at 7:37
  • I think your problem may be in the assumption that the river water is "super-cold". I think that if you stuck a thermometer in a flowing river (or FTM a large lake), it would register 0 C. Two reasons I can think of. 1) The river water is in contact with high thermal mass ground. 2) If any parcel of water does freeze, it's a) probably on the surface, in contact with colder air, and b) releases some heat by freezing, then floats to the surface. – jamesqf Feb 10 '16 at 19:31
  • @cst1992 This link is pretty close to an answer. It's more physics or chemistry than Earth science. phys.org/news/2011-11-supercool-doesnt-.html – userLTK Mar 26 at 2:31

Rivers can freeze, but only on the surface. The water still flows below the ice. When it is colder, the ice layer can become thicker.

The kinetic energy of the moving water in rivers delays this freezing. So when water is more calm it freezes more easily.

A snippet from the linked question:

Fred says:

Also, while water is flowing its potential energy is constantly being converted to heat energy that resists freezing on the molecular level and subsequent crystallization.

For flowing water to freeze, the temperature would have to be exceptionally cold

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