Overturns can lead to oxygen depletion fish kills. During summer, water in a pond will become stratified (form layers), with warmer water in the upper level and the colder water near the pond bottom. This stratification is particularly severe in ponds with dense growths of blue-green or “scum” algae on the surface. The colder water may become oxygen-deficient due to interaction with certain elements in the pond bottom and lack of photosynthesis by phytoplankton at greater depths.

An overturning or mixing of this colder, oxygen-deficient water may occur after a heavy rainfall. If the rainfall is of sufficient quantity, it produces a mixing of the pond water as the cold rainwater sinks to the pond bottom. It displaces the oxygen-poor water, which mixes with the water in the upper levels of the pond. The result is a fish kill due to insufficient oxygen in the water.