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Lake Turnover - What Is It?

by: David Coulson 11/9/2013

Every year I see several conditions report stating the lake is turning over.  Often these are made during the summer or early fall.  I’m fairly sure most of these “turnover” reports are actually algae blooms. There’s little doubt in my mind that many use the term “turnover” to describe any change in water conditions, especially if those conditions are visual, smelly and/or associated with a fish kill, all symptoms of events other than turnover. 

In order to understand turnover, one needs to understand that water is densest at approximately 39 degrees Fahrenheit.   Which means ice is less dense allowing it to float on its liquid counterpart.  Without this property, large bodies of water would be frozen solid anywhere it gets cold enough to freeze, as frozen water would sink to the bottom until only a skim of liquid water existed on the surface.  Consequently, in the spring and the fall around ice-off and ice-up there is a short period when the water temperature of a lake is 39 degrees throughout, allowing for water to mix or turnover if you will.

Turnover is best understood by walking through the four seasons and looking at the water processes that are occurring. Let’s start with a Colorado lake that is moderately deep, say 50 feet, just prior to spring thaw.  At this point the lake will have a coat of ice that will be colder than 32 degrees Fahrenheit at the lake surface.  Working down through the ice, we will ultimately find liquid water, which at the ice’s edge will be approximately 32 degrees.  The deeper we go the water temperatures will warm to as much as 39 degrees at the reservoir’s bottom. This is why the discharge for dams in the winter is warmer than the lake’s surface.  It’s obvious that with an ice coating and barring a major flow of water through the lake from streams, rivers, or springs, there is very little mixing of waters under the ice. 

If our lake had abundant vegetation in the summer it could be in trouble.  The vegetation may die back with the ice/snow covering blocking light.  The decomposition of the vegetation sucks the oxygen out of the water, starting at the bottom and working up, potentially stratify the water with dead zones, where the fish cannot survive.  This is an important aside for ice fishers, in that as winter progresses on high mountain lakes with minimal water flows, the fish may be shallow as that is where the highest oxygen levels are to be in late winter.

At ice off, the surface of the lake is still stratified, but as the air warms the water it is denser than the ice (but colder than 39) and sinks, mixing with the waters below.  This continues until the water temperatures are uniformly 39 degrees allowing the waters to be stirred up by wind created currents, bringing life giving oxygenated waters to the depths of our lake.  Sometimes during this mixing process, debris from the bottom is brought to the surface, possibly clouding the water and even releasing a foul smell as rotted organics are brought to the surface.  While this turnover is generally healthy and beneficial, in extreme cases, should the oxygen depleted waters (or toxic material from the bottom) be brought to the surface quickly enough a fish kill is possible.  More often than not though any dead fish seen at this time were the result of “winter kill”, which is primarily due to stress from low oxygen levels.

During the summer as the waters continue to warm, it’s not uncommon for deep lakes to develop thermoclines where the cold water below doesn’t mix with the warmer layer above.  The lower levels can become oxygen depleted forcing the fish to move to the warmer upper layer.  For a better explanation of thermoclines, you might wish to read R. Karl’s article on lake turnover.

Come fall, the waters cool allowing the sinking water to eventually break down the thermocline and once again oxygen rich waters reach the bottom of the lakes. Just like in the spring, sometimes this mixing can be fast enough to bring the oxygen depleted waters and materials with them to the surface,.  However, this is rare, and typically turnover is a healthy rejuvenating of the lake’s depths. Once turnover occurs in the fall, the lake then forms an ice cap and the cycle repeats itself

While turnover, in most cases, is a temperature related phenomenon, typically occurring in the spring and fall, there are cases where lake/pond can turnover that isn’t temperature related.  These situations are generally limited to shallow ponds and associated with extreme weather events, where a heavy storms and/or high winds can mix the lakes waters.

Simply, turnover generally occurs within a week or two before ice-up and after ice-off.  Events outside that time period are not likely turnover.  Furthermore, turnover is a healthy mixing of the lake’s waters, and often goes unnoticed by anglers.

For those wishing to know more about turnover here’s a few online references for your reading enjoyment.

Lake Turnover – how it works, by R. Karl

Fall/Spring Lake Turnover, by Keith C. Heidorn, PhD

Density Stratification

Natural Fish Kills 

Blog content © David Coulson
Member comments
Ajax5240, CO   11/9/2013 8:45:42 AM
Great write up Dave!!! Thanks for sharing it with us!
opencage, CO   11/10/2013 5:53:31 PM
Well-written Dave. Helps clear up a misunderstanding I had. And those links are great resources too.
brookieflyfisher, CO   11/11/2013 12:41:16 AM
Dave, this is a great first reference for all things related to turnover. One thing I will add is that in the summer, the bottom layer of water is usually around 10-15 C (about 55 F). There are many exceptions to these generalized principles, though. Many lakes in the Midwest mix multiple times during the year. Some lakes in the south don't mix for years at a time. Some mountain lakes never get warm enough to set up a thermocline. Luckily, in Colorado our lakes are generally too cool and sterile for thermal stratification to have much affect on fish behavior.
David Coulson (Flyrodn), CO   11/11/2013 6:13:57 AM
Thanks, you are correct mixing of the waters isn't as simple as temperature alone, winds, currents, rains, etc., all play a role. For me understanding what's going on with the water, helps understand what going on with the fish.
brookieflyfisher, CO   11/11/2013 3:18:26 PM
^agreed. Thanks for the great blog!
Dave Mauldin, TX   11/11/2013 6:53:14 PM
Once I asked a top BASS pro how to catch fish when a lake turns over and he said "Go fish another lake". He was right.
Coyute, CO   11/12/2013 10:22:10 AM
Lake turnover, just one of a myriad of excuses we use when we can get the fish to cooperate. Thanks for the info.
Bassnfly, CA   11/14/2013 4:35:56 PM
A news report just this morning reported that a local lake and water source, due to "turnover," may affect the taste of tap water, even after the filtration process.
Lloyd Tackitt, TX   11/14/2013 5:20:47 PM
I was living in Texarkana when the water supply lake turned over. Wright Patman is a big lake and it made a big stink. You could smell it miles away and it turned the city water so bad you couldn't drink it without holding your nose. It was so bad you could taste in ice tea, heavy brewed ice teat at that. It was spring and real windy at the time. Heavy wind from one direction for several days - probably a combination of the wind and a thermo cline inversion due to the warming spring after a hard winter. Man it stank. It stank so bad it blotted out the smell from the pickle factory and the pulp mill both. It stank so bad I could barely stand taking a shower. And it lasted for weeks, the stink just wouldn't go away and the wind kept blowing it right at us. A large lake making a sudden turnover, for the first time probably ever, is a terrible thing to be around. And no amount of filtering can get rid of the taste. It's a great time to be selling bottled water though.
David Coulson (Flyrodn), CO   11/15/2013 8:25:25 AM
There's no doubt that when a turnover event stirs up the bottom of a lake that is anaerobic the results are nasty. Fortunately, such lakes are rare, unfortunately those events are what most folks associate with turnover, thus, an algae bloom which can be equally nasty is often mistaken for turnover. Point is turnover in most cases goes unnoticed and is in fact a natural and healthy event.
David Coulson
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