Pond Boss Forums INTRO Questions & Observations Why are deeper ponds less prone to fish kills?

I didn't remember that 39 degrees was the magic number, but it sounds about right.

Ice has a crystal structure that takes up more space than when it is a liquid. Therefore ice is less dense than water and floats. H2O starts this "expansion" at about 39 degrees when the water molecules are just beginning to position themselves to form ice crystals. With no other outside factors, the colder water sinking to the bottom of a pond should never be lower than 39 degrees. Anything warmer OR colder will rise. However, wind action could stir up and move water colder than 39 degrees to the bottom. The deeper a pond, the more protected the bottom would be from this stirring. And the temperature of the surrounding ground could act as a heat sink to buffer the bottom temperatures, especially in the South.

Also, I'm guessing that it is partly that a sudden change in temperature of the water is hazardous to fish. A deeper pond would have more water in it than a shallow pond of the same surface acres, and the greater water volume would slow down any temperature change. Plus the fish would have a wider choice of places to be to get at a temperature they could tolerate.