physics question #3982

Lisa Pearce, a 20 year old female from Gympie asks on November 6, 2007,

Q:

In winter in very cold countries like Canada and Finland, most rivers, lakes and ponds freeze over. Cars can be driven over lakes and the ice does not break. But if you have a strong enough saw, you can cut through the ice and there will be water underneath. It is also common to fish through these holes and get good catches. Why don't rivers and lakes completely freeze when barrels of water do freeze solid? How do fish, crustaceans and plants survive these bitterly cold conditions?
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the answer

Barry Shell answered on November 13, 2007, A:
Plants and animals can live under ice because of water's unusual quality of being most dense at 4 degrees C. Ice has a density about 93% of water so it floats on top. Water is most dense at 4°C. Deeper water below the ice stays at 4°C because colder water rises to the top due to its lower density. So at the bottom of a frozen lake, stream or pond, it's always 4°C, warm enough for life-or hibernation at least. For more amazing facts about water check our water facts page on science.ca.
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