Lake turnovers generally occur in the fall but can sometimes occur in the summer. During the hot weather, the surface water of a lake warms much faster than deeper water. This results in a temporary layering effect, with warm water on top and cool water underneath. Scientists refer to this as stratification.

Because the top layer has constant access to the atmosphere, it tends to have more oxygen than the bottom layer-even though it’s warmer. If a heavy wind or cold rain should occur during these conditions, the stratification may be broken, causing the two layers to mix. Once this happens the mixing action can result in low dissolved oxygen levels throughout the water column, and can potentially cause a fish kill.