Angler’s Guide to Thermocline and Lake Turnover
by Paul E. Moore

So let’s start with a short question and answer session. Are we in the midst of the heat of summer? The answer is obviously yes. Is the water temperature increasing as well? Again, the answer is most definitely yes. Are the fish seeking out the deepest water they can find? Well, the answer to that one is maybe. It really depends on the fish and the individual water body.

There is a lot of talk at this time of year about the thermocline and that anglers need to keep it in mind when fishing. Later, in just a few weeks, there will be talk about lake turnover. Magazine articles, TV shows, and technical writings are littered with big words and complicated sounding phrases like “epilimnion” or hypolimnion” or perhaps “a thermally stratified body of water.” For instance, if you looked up the word thermocline in the American Heritage Dictionary of the English Language, you would find the definition as: “A layer in a large body of water, such as a lake, that sharply separates regions differing in temperature, so that the temperature gradient across the layer is abrupt.”

That all sounds big and important huh? Sometimes these definitions and articles, with all their lofty scientific technical jargon, can be downright complicated to understand. What are they really saying and how does it affect fishing? Well, to be quite honest, it is really not that hard to understand. It just needs to be said in a different more simple way.

Let’s look at “a thermally stratified body of water” as an example. The word stratified simply means something that is layered. Thermally obviously relates to temperature. So a thermally stratified body of water is merely a lake with different layers of water temperature. Have you ever gone swimming or wading and hit a pocket of water that was a lot colder than the water you were just in?

During summer, a lot of lakes experience stratification, or the development of different layers of water temperature. This usually happens in lakes that don’t have enough current to keep the water mixed. The upper layer (identified with the big scientific word Epilimnion) is warmer and well oxygenated. The bottom layer, labeled the Hypolimnion, is colder and oxygen deprived. The difference in temperature between these two layers is quite dramatic (remember the cold spot from swimming). The slim layer of water that separates this warm water from the underlying cold water is called the thermocline.

Most everyone knows that cold air falls and hot air rises. That’s why you’re supposed to crawl out of a burning building. The colder, denser air is near the floor, while the hotter air is higher up. It’s the same with water. Colder water is denser and sinks to the bottom. But, more on that later.

As the sun gets more intense through summer and the air temperature continues to rise, the water temperature warms. This begins at the surface and gradually works downward in the water column. Eventually, this results in a “layer” of water from the surface downward that is warm and well oxygenated. However, the sun and warmth can only penetrate so far, so below that point, the water is colder and has less oxygen. Again, the thin line separating these layers is the thermocline.

On lakes with plenty of current, such as a riverine type of lake where the river is constantly flowing, the water stays mixed really well and stratification does not usually occur. Also, shallow water lakes don’t usually form a thermocline because the warmth penetrates all the way to the bottom. Wind also plays a factor (not just on shallow lakes, but large waters as well) in mixing the water and keeping stratification from happening.

Anglers may wonder why the fish don’t just drop below the thermocline to enjoy the cooler water. The reason is simply because there isn’t enough oxygen present to sustain them. Without current or wind turbulence, the water is not agitated and kept oxygenated. Furthermore, organic and decaying matter usually sinks to the bottom and the process of decomposition further deprives the bottom section of the lake of oxygen.

In a stratified lake, fish will typically not venture very far below the thermocline, except for brief forays in search of food or other meanderings. They will, however, often hold right at or just above the thermocline. This is why knowing whether or not a lake develops a thermocline and if so, at what depth, is so vitally important to fishing. If an angler is fishing on a stratified lake and putting all their effort at depths below the thermocline, the results will generally not be very pleasing.

The thermocline will actually show up on most depth finders used these days. Learning to read what the electronics are saying can be a bit tricky at first and varies in display on different types of units. With a little research on your particular unit and some experimentation with the display and intensity, finding the thermocline usually isn’t that tough.

Another way to locate the thermocline with electronics is simply by cruising around and paying close attention to the depth at which the fish are marked on the display. If you motor around and see lots of fish congregated at and above 22 feet and see virtually nothing below that depth, it’s reasonable to assume there might be a thermocline at that depth.

How do you fish the thermocline? Well, that depends a lot on the fish species and could be the subject of numerous other articles. Anglers have to consider the habits of the fish and then use methods appropriate to intercept them.

One particular trick that works on a lot of different species is to first locate the depth at which the thermocline is set up. Then seek out structure that intersects that depth. For instance, if you are seeking a fish like a channel catfish that oftentimes hugs near the bottom, simply follow the thermocline depth over until you find bottom contour at that depth. Bass and other species will often be right where the thermocline intersects structure too and not necessarily just the bottom. Rock piles, bridge pilings, downed trees, standing timber, and other structure often intersects at the point of thermocline. It just takes a little effort to seek these areas out.

The summer stratification period can yield good fishing and it can also be somewhat frustrating at times. However, it is no where near as frustrating as when lake turnover occurs later in the year. This phenomenon is often marked by dark water, a foul smell, and a lot of leaves and other debris floating on the surface. It looks like the bottom of the lake has flipped up to the surface and this is pretty much what actually happens. During lake turnover, fishing is generally not very good for most species of fish.

Lake turnover is difficult for some folks to understand. Again though, I believe it to be a fairly simple process that is often not explained very well. Let’s take a stab at it and see if we can make sense of it.

Remember that cold air settles and hot air rises and water reacts the same? Through the spring and summer months, the upper layers of the lake continue to warm while the bottom is cool. But, once fall arrives and the mercury starts plummeting, the water temperature begins cooling down at the surface.

As the surface layers of water get colder, they become more dense and heavier. Eventually, the temperature of top section of water becomes colder than the section that has been sitting on the bottom all summer. When this top section gets heavier than the bottom, it simply begins to sink. This causes the bottom portion of the lake to be displaced and forced upward which results in all of the decaying matter on the bottom to be boiled up to the surface in the process.

Sometimes on some lakes, this process happens slowly over a period of time. Other times, lake turnover can occur rather suddenly. It is not uncommon to go fishing one day and enjoy good water color and good fishing, and then come back a couple of days later to find the lake in turmoil.

When lake turnover occurs, the fishing often goes in the toilet for a while and many anglers simply find other things to do and wait it out. The amount of time a lake takes to stabilize depends on the lake, the depth, and other prevailing weather conditions. Generally, anglers can look at the stain of the water and determine how close to normal the water looks.

Hopefully, if you’ve ever had questions about the thermocline or lake turnover, these few paragraphs have helped answer them. Although there is much more to discuss regarding these annual lake phenomenon, we have at least covered the basics. Now, we just need to get out there on the water and find those doggone fish!

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