Lake Dynamics. by Mark Thomson

Understanding how environmental conditions can effect a lake is fundamentally important to all freshwater anglers. For many, thermoclines are one of the best places to find bass in stable weather and water conditions. But if conditions change, we don't have the luxury to easily locate good water provided by the thermoclines. I believe that seasonal weather patterns and water levels play a significant role in how anglers can locate fish in dams, the angling methods they use are dictated by the depth that the fish are located. It helps to be able to combine a little science, and different fishing skills to be successful.
The following is my interpretation on how thermoclines in particular change though seasons and under changing water levels. I interpret this from literature and on-the-water experiences over the last few years.

Thermocline dynamics through the seasons

Sring: In spring thermoclines are small in size and are distributed throughout the stratified depths of a lake. Bass tend to concentrate in large schools at this time of year and take advantage of the deep water refuge and food conditions available in these small areas.
Summer: During summer an improvement of weather conditions enhances and expands the size and occurence of thermoclines in a lake. Aquatic life and bass will utilize the expanding presence of thermoclines and tend to distribute over greater areas of the lake.
Winter: Winter will see the reduction of thermoclines to only a handfull of suitable areas of the dam. In fact, thermoclines will only remain in areas of deep water and in the more sheltered or protected parts of a lake that do not turn over.
Lake Turn-over: This is an event where lakes epilimnion tilts and mixes the hypolimnion below, destratification occurs disturbing and mixing water conditions. This event is a common occurrence in temperate lakes that experience long periods of very cold weather. It may affect all or part of a lakes area thus reducing or eliminating the occurrence of thermoclines.
Rain and Flooding: A major rain or flood event will also cause major mixing and destratifaction of water in a lake. Unstable water conditions now prevail throughout the lake reducing or eliminating thermoclines. With the lake now full of water and destratified, water conditions are relatively homogenous offering similar oxygen, temperature and P.H. conditions throughout.