Why do lakes and rivers only freeze on the surface and not all the way through?
In: 10
Because ice floats.
That’s the short answer. Let’s unpack this a little.
Water is one of a very few substances that expands when it freezes. This means that frozen water, i.e. ice, is less dense (i.e. lighter, for the same volume) than liquid water. And therefore, when you freeze water, ice floats to the top.
Why doesn’t all the water freeze all at once when the temperature falls below 0°C? Well, because it takes a lot of energy to freeze water. That is, to turn a drop of liquid water at 0°C into ice, a lot of heat energy needs to escape. For comparison: to lower the temperature of 1 g of water by 1°C, requires 4.18 J of energy to be lost (to the environment). On the other hand, to freeze 1 g of water that is at 0°C (i.e. just to turn it into ice that is still at 0°C), takes a whopping **334** J of energy loss.
So, when water is placed in an environment that is below 0°C, its temperature will drop to 0°C fairly quickly, as it only takes about 4 J per °C, per gram of water. But then when it hits 0°C, now it has to overcome this barrier of a 334-joule energy loss in order to turn into ice.
Now let’s think about where the freezing is happening. This is mainly at the surface of the water, where the water is exposed to the cold environment. This is the main place where energy loss happens. And here’s the kicker: once ice is formed, it will float to the top, and form a layer of ice exactly on the surface where energy loss happens. This layer of ice insulates the liquid water below from the cold air above. Not perfectly, of course, but it does slow down the transfer of heat energy out of the liquid water. And as more ice forms, the insulating layer on top of the water only grows and slows the freezing process even more. If any snow happens to fall on the ice, then this provides even more insulation.
If the environment stays below 0°C for long enough, then eventually an entire lake will in fact freeze from top to bottom. And sometimes, in shallow waters, this does indeed happen. But your typical lake is too deep, and your typical winter is not long or cold enough, for it to freeze solid before spring comes and the ice starts to melt.
If ice didn’t float, then it would not form this insulating layer, and freezing would continue at a constant rate instead of slowing down as the ice grew, causing bodies of water to freeze faster.
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