Below the thermocline — in an area called the hypolimnion -the water has low oxygen and this discourages fish from spending much time there. Decomposing weeds and dead fish that lay on the lake bottom also contributes to the low oxygen level. Turnover is aptly named. Water is at its heaviest at 39 degrees F, so as the surface temperature of a lake drops to around 50 degrees that top layer of water becomes heavier.
Because of this that surface water wants to sink. Lakes can take a couple of days to turnover or it can happen in a day. A cold front, cold rain, or a heavy north wind can increase how fast the turnover occurs.
When a lake turns over, the surface water descends and the warm water from the bottom rises to the top. At times, the bottom water can carry mud and rotting weeds, and it can create murky looking water. This will clear in time. At the same time, the highly-oxygenated surface water falls to the bottom, and spreads oxygen throughout the lake.
Finding fish right after turnover can be a trick. The whole lake basically becomes oxygen rich and an even temperature. The spots you normally find walleye in the spring and summer, like at river mouths, points or along shoreline reefs, may not have fish.
Walleye tend to drop into deeper water after turnover and are often found at specific depths. However, find the fish at a certain depth, and you will find them at that depth pretty much everywhere else you look.
The reality of fall walleye location is it can be like finding a needle in a haystack. So you have to put the odds in your favour.
The easiest way to do that is to mark fish with a depth finder or look for underwater spots that are promising like humps, rockpiles or drop offs. McComas' twin cities based firm specializes in all things water. In a message to McComas, I wrote; "In the lakes that do stratify and turn over, is there a precise temperature when one would expect a lake to undergo the fall turnover or is the relationship relative to underlying water temperatures? McComas ; "The quick answer is when the top water temperature is lowered to the underlying water temp, the lake can turn over.
To add a little back story, bottom water the hypolimnion is cold. The densest water is 4C or about 40F and many lakes have bottom water with that temp. That means the surface water has to cool down to 40F to get to the same density as the bottom water and then the whole water column will mix with a little wind action.
However, in some shallower lakes the bottom water may be a tad warmer than 40F, but still denser than the warmer top water. The same principle still applies, when the surface water temp is lowered to the bottom water temp, a little wind will mix the lake. Some lakes have turned over already; others will have to wait until the surface water temps get down to 40F and that might be a while".
Knowing this makes me think that most fishermen probably never actually fish during "The Turnover" and that means that the absolute scientific event is probably not the only thing that influences fish locations and feeding patterns during the cold water period.
Most of us rarely, if ever see surface temperatures colder than 40 degrees and for me, 50 degree water is enough to produce dramatic changes in both location and feeding patterns. In other words, it's the fish that force me into finding alternative locations and feeding patterns well before the scientific version of turnover actually occurs.
Although there are times when a lake make may actually have completely turned over during our fishing season, it seems more likely to me that it's fish metabolism, the loss of shallow water weed cover and the influence of moving forage that determines where the fish will be and how easily they can be caught.
After reading between the lines of the Lake Detective's answer, it seems more likely that what we anglers really experience are "Mini-Turnovers". Eventually the top layer fades and mixes with the middle layer. The thermocline sinks in the water column as the heavier upper water layer forces it deeper.
Soon, though, the temperature in this mixed upper layer equals that of the bottom layer; wind easily mixes the entire water column because similar temperature and density exist throughout. This mixing allows oxygen and nutrients to be distributed across the whole water column again, and is called fall turnover.
The timing and duration of fall turnover depends on the size and depth of the lake. Sometimes it can occur in a couple days and sometime it takes a week or more. The deeper the lake, the longer the water column takes to become uniform in temperature since the bottom stays cooler.
Most lakes in Minnesota that are deeper than about 20 feet turnover in early to mid October. You can track fall turnover in an individual lake a couple different ways. The easiest way is with a Secchi disk and a water thermometer. A Secchi disk measures water clarity. During fall turnover, the clarity of a lake usually decreases because mixing brings up nutrient rich water from the bottom of the lake and causes the lake to look cloudy.
Sometimes there is even a distinct smell as decomposing plants, algae and other matter surfaces.
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