How do waterfalls freeze while in motion?
In: Chemistry
There seems to be a common idea here that moving water can’t freeze, but that is not correct.
Liquid water can exist well below freezing, in a state called “supercooled”. You can see an example of supercooled water in [this video](https://www.youtube.com/watch?v=Fot3m7kyLn4). Also in that video, you can see an example of exactly how a waterfall might freeze over.
The most important part missing from a smoothly flowing river of supercooled water is what is called a nucleation site. This is just a spot where conditions are ideal for the water molecules to start building an ice crystal. A nucleation site can be a wide variety of things: a snowflake dropping into the water, a frozen leaf, a turbulent rapid beside a rock, or even more ice. Once a piece of ice forms in moving water, it moves about the same speed as the surrounding water. Since flowing water can (and does) freeze while moving, some water is already ice before it goes over the falls.
In free fall, water increases its surface area meaning that more water is in direct contact with air. In really cold air, this would mean that the water would get even colder. As water splashes, little drops also get exposed to colder air. The supercooled water will more readily crystallize against existing nucleation sites. In practice, this means that ice chunks will become larger. Assuming the rocks behind the waterfall are also below freezing, they would also act as nucleation sites for ice to grow. At the bottom of the falls, solid ice tends to accumulate. This also acts as a spot where the water can crystallize while in freefall (a lot like a stalagmite).
As supercooled water keeps pouring over the waterfall, the ice gets thicker. As more and more ice accumulates near the top of the falls, the river above the waterfall gets backed up. This encourages the surface water above the river to freeze over.
In essence, waterfalls are just big nucleation sites for supercooled water.
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