Why doesn't Niagara Falls freeze in the weather of MINUS tens of degrees Celsius?

Water is a typical crystal material. The mechanism of water molecule solidification is not only related to temperature. In micro-thermodynamics, the transition point of solid and liquid molecules is often combined with the expression of Gibbs free energy between molecules. However, when the water flow is too large, the Gibbs free energy between water molecules is more liquid because of its greater fluidity.

The macro explanation is that water will form a relatively large crystal nucleus at low temperature, but the large crystal nucleus will be broken due to the stirring action of water flow, so it is not easy to form a large crystal nucleus and it is not easy to freeze. But as long as the time is long enough, more and more tiny crystal nuclei will appear in the water, which will make the viscosity of the water increase and the fluidity become worse, and eventually it will freeze, but the process will be longer. If impurities such as fine sand are added to water, the supercooling degree of water can be greatly reduced when it turns into ice. This is because crystal nuclei are formed during crystallization, and the addition of fine sand improves the nucleation rate and reduces the temperature required for freezing water.

The growth rate of crystal nucleus and the flow rate of water are the key measures to determine whether supercooled water freezes. If the temperature is low enough and there are enough nodules, if the growth rate of crystal nucleus can be faster than the flow rate of water, the water will definitely freeze. This is why some waterfalls with small falls freeze in winter. When the falls are large enough and the water flow is fast enough, it is difficult to freeze.