Original post made by Justin W. Cantrell(Justin) on April 13, 2002 I hear alot of people talk about Fall Turnover. I've heard about the thermocline. I bet most fisherman have, but I didn't know exactly what it meant for along time. So I decided to do some research and thought I would share it with my brothers at the BOC! Hopefully understading more about the water your fishing in will help to learn more about where the fish are, and why! The info below will expose some of the facts about Thermal Stratification and will go through a seasonal cycle from start to end. Hope it helps.. The layering of warm water over cooler water, is refered to as thermal stratification. This happens because cool water is heavier than warm water, so it sinks to the bottom. Cooler water being more dense than warmer water then establishes a density gradient so that normal wind action will only circulate the upper-warmer portion of the lake. This creates three seperate layers in the lake. the warmer water on top is refered to as the Epilimnion and the cooler water on the bottom is refered to as the Hypolimnion. During stratification these two layers are seperated by a layer called the metalimnion, or more commonly know as the thermocline. The thermocline offers the best mixture of water temperature and oxygen.Figure 1: These layers change or disappear from season to season, and continue this cycle year after year. We'll go through the seasonal pattern of a lakes water, starting with Spring. After the ice melts on a lake, the lake water is basically the same temperature all the way through. Wind allows circulation of the water, surface water can be push to the bottom and bottom water can rise to the top. This allows large amounts of oxygen to reach the bottom. This mixing of the lake water in the spring, is called spring overturn. Figure 2: In late Spring the air temperature rises and heat from the sun begins to warm the lake. Since the amount of solar radiation absorbed decreases, the upper water warms first. The warm water that is now on top is less dense than the lower water, which makes a layer of warm water that floats over the cold water. Stratification begins. The layers are well seperated by Summer. With the lower water not mixing with the upper more oxygenated water, the bottom tends to have a very low amount of oxygen. Respiration by animals and bacteria deplete the dissolved oxygen and theres not enough light for photosynthesis. This stage is know as, Summer Stagnation. Figure 3: As Autumn moves in the air temperature decreases and the upper/warm water begins to cool, so the upper epilimnion level begins to get smaller. Eventually the thermocline will rise far enough that the upper layer can no longer be maintained,losing stratification. As in the Spring, the water now has a fairly even temperature and wind can again mix the entire lake water again. Surface water is now in contact with cold air, it cools quickly and falls to the bottom. This helps even further,to mix the oxygen and nutrients evenly throughout the lake. This process is called Autumn Turnover. figure 4: Winter brings much colder weather with it. The lake will remain the same as in Fall, unless the water freezes. When the water freezes, it prevents the wind from mixing the lake water, causing stratification again. A layer of low density water thats colder than 4 degrees celsius and to warm to freeze, forms just underthe ice. The water below remains around 4 degrees celsius throughout.This is refered to as Winter Stagnation. Then the new year comes around and as Spring approaches, the seasonal cycle begins again. Hope that helped fill in some of the grey areas for some of us. It may help picking a depth when your on the water alittle bit easier as well.