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How does ice form on a pond?

How does ice form on a pond?

How does ice form on a pond?
Jonathan – Denver, CO

You look forward to and enjoy the ice formations that appear on your pond each winter but have you ever wondered how it begins? Why is it that the ice in your pond expands when it freezes instead of shrinking and why does ice float?

Unlike those horror flashbacks of falling asleep in science class we’ll keep this as simple and interesting as possible. As the air cools off outside your water looses heat and becomes heavier. This cool and heavy water sinks to the bottom of the pond as the lighter warmer water rises to the top to cool. This process continues until the pond water reaches about 4 degrees Celsius or 39 degrees Fahrenheit and the water cools enough to freeze. Water is unique in the fact that as is it freezes its molecules form crystals that are spaced farther apart causing ice to expand and take up more space. Once the water in your pond begins to form these crystals and expand it actually becomes lighter than warmer unfrozen water and once again begins to rise to the surface of your pond where it begins to form a sheet of ice. If this all seems a bit too wordy or confusing to you visualize a glass of water with ice cubes in it. The spaced out, crystallized, water molecules of the ice cubes make them lighter than the water in the glass and cause them to float at the surface of the water. The water in the glass that is cooled by the ice cube (but not cold enough to freeze) becomes more dense and sinks to the bottom of the glass and the warmest water rises to the top where it is cooled by the floating ice cubes.

The layer of ice formed by this crystallized frozen water can become fairly strong as it becomes thicker. An inch of ice can be strong enough to hold a small animal without cracking. 3 inches of ice typically is enough to bear the weight of the average person and once a pond freezes to 6 to 8 inches thick it is ready to play host to a hockey game or hold up a snowmobile. Some have even taken their cars out on the ice! While we don’t recommend you give it a try, 8 to 12 inches of ice can support a slowly moving vehicle.

Pond Talk: What do you use your pond for in the winter? Do you skate? Ice fish?

4 Responses

  1. A frozen pond can be fun for humans, but what about the fish? Last year we had a vast fish kill in our 1/2 acre pond. A smaller and a much larger pond were not affected.

    • Carol,

      The risk of a fish kill increases, if a pond does not have aeration, completely freezes over and remains that way for an extended period of time. Even though the water temperatures drop, the decomposition of leaves, grass clippings, fish waste, uneaten fish food continue to break down and release toxic gases into the pond. Without a way for these gases to escape the pond, they can build up to a level that is toxic to the fish. It is very possible for one pond to have a fish kill and the others not. Each body of water has its own environment, nutrient level and fish population. Aeration in a pond, increases the dissolved oxygen concentration in the pond, keeps a hole open on the surface of the pond to vent the toxic gases and helps the beneficial bacteria in the pond to work effectively at breaking down the nutrients in the pond.

      If you use your pond for recreation, such as hockey or skating, then having an aeration system operating not be recommended due to thin ice or open water on the surface of the pond.

      For additional information on this subject, please refer to the Question & Answer link below on “Why Did My Fish Die Over the Winter.”


      • I turn my aeration system off in the winter to that we can skate on our pond. Will chopping a couple of holes in spots around the pond and re-opening them regularly make any significant difference in the toxic gas build-up?

        If not, my alternative would be to put the aerator at one end of my long, thin pond (180′ x 60′). This is a shallow part of the pond, away from the skating area. Is there any point in considering this? It would mean moving the aerator each spring and fall (to get it back into the central, deepest part – kind of a pain in the neck to get it re-situated, if it’s not going to make much difference.

        Another possibility would be to put in two aerators (attached to one pump) and turn off the central one each winter.

      • Hi John,

        If you plan to skate on the pond the best bet is to not create any holes or run the aeration system, even in the shallow end of the pond the ripple effect can compromise the safety of the ice in the rest of the pond.

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