What is water stratification?

Question

What is water stratification?

Answer 1

In a lake (as opposed to a pond) only the top layer is mixed by the wind. This portion (epilimnion) has a fairly even temperature throughout. Below that is a layer called the thermocline, in which there is a rapid drop in temperature with depth (>1°C/m). Below that is the hypolimnion, where the temperature changes fairly slowly with depth.

This is an oversimplification and is based on summer and winter conditions, ignoring the spring and autumn "overturns". Check a limnology book, such as the one by Welch, if you are interested.

Answer 2

Water of high and low salinity as well as cold and warm water form layers that act as barriers to mixing.