Understanding the water structure in the lake where you fish can help you be successful on your fishing trips. Most all lakes have 3 levels of water in them. These levels of water are based on temperature that is dictated by the time of year and weather conditions. Naturally as you go down in the depths the temperature of the water goes down. The deeper water has lower temperatures because sunlight cannot reach there to warm the water.
The top level of water is called the epilimnion and contains the most oxygen. Wind actions and currents allow this level to be mixed. The epilimnion is the warmest level of water since it is struck directly by sunlight. Depending on the time of year, the depth to which this level reaches can vary quite a bit. The second level of water is called the metalimnion. The top part of this level is the thermocline, an important target area for fishing. At the top of the thermocline fish like to get out of the heat that is present in the epilimnion, but where there is adequate oxygen. Below this level is the hypolimnion where oxygen levels are lowest.
So if you are fishing for largemouth bass your task is to find out at what level is the thermocline. You want to do your fishing near that level. The depth of the fish in this level can also be affected by the clarity of the water. If the water above the thermocline is not clear then the fish will be near the top of the thermocline as long as the tmeperature is to their liking. If the water is clear down into the thermocline then the fish will tend to be lower in the thermocline. It is all about the comfort level for the fish, they want to be in water that has sufficient oxygen and where the temperature is comfortable to them. This level is generally somewhere in the thermocline.
The depth of the thermocline varies quite a bit from lake to lake. Lakes that are very deep have water that is cooler. Because of the depth these lakes can have a thermocline that if 50 or 60 feet deep or more. Lakes that are not very deep can have a thermocline that is only a few feet deep. The thermocline has an impact on the depth where fish are because the water in it usually has satisfactory oxygen levels and temperature for fish to be comfortable.
You can see the thermocline displayed on the screen of your Humminbird Fish Finder. Make sure your unit is set to Sonar Mode. You may have to adjust the sensitivity of the unit to see the thermocline. If the difference in temperature above and below the thermocline is great enough, the sonar will reflect the thermocline. This occurs because the density of water changes with the difference in temperature. Sonar can reflect that difference in density.