Fall turnover is always a source of frustration for anglers, but knowing a little bit more about the phenomenon can turn a slow season into one of your best.
As the air temps drop this time of year, so do the surface water temps of our lakes. This water becomes denser as it cools, and starts to sink. The sinking water is replaced by water from below, which cools in turn, sinks, and continues the cycle. After some time the circulating water will reach an equilibrium. Wind acts as an accelerant, pushing the surface water and further mixing it.
Pinpointing exactly when a particular lake will turn is practically impossible, but there are general rules to keep in mind. Shallower lakes will turn before deeper lakes, as there is less of a temperature gradient within the water column to begin with. Wind affects smaller lakes much more powerfully than it does larger lakes as well. These turnovers might finish before bigger bodies in the same area even begin to turn. Deep lakes might have bottom temps of around 39˚F, which is the temperature at which water is most dense. These lakes will take longer to begin turning.
When a lake is turning over, it is considered “mixed”. Mixed lakes are fairly easy to spot; water that was clear in summer will now be murky. As cold water from the bottom comes up to the surface, it will bring decaying material with it. You might even be able to smell sulfur. Water thermometers can also be helpful tools this time of year. When the temperature of the water remains the same throughout the water column, you’ll know the lake is turning over.
Temperature is not the only thing that will be equally distributed during this phase. The dissolved oxygen (DO) and nutrient levels of a lake will also be constant, which opens up new habitat for fish that have been confined to static spots in the heat of late summer. Combine this with their changing metabolism, and you have fish that act completely different from two weeks ago.
Turnover is complete when the temperature gradient reestablishes itself. In Minnesota, this happens when ice covers the lakes and the coldest water is immediately below the surface and the warmest is on the bottom. After the spring thaw, there will be another turnover event. The spring turnover is much shorter and has a fraction of the effect on fish activity, though.
During turnover, fish will be suspended in all depths because of the uniform temperature and DO. Luckily, they will be grouped up in extremely tight schools and will associate with specific structure. At this time of year, anglers will have the best luck targeting the most dramatic incline changes they can find, especially those that are near a severe bottom content change.
If you have a fish finder, don’t waste time cruising around your summer haunts: keep your eyes glued to your screen for tight groups and sharp drops. If you aren’t so fortunate, study lake maps and cover water quickly with cranks and spinnerbaits. Be sure to record where you have luck, as these spots will probably keep producing until they’re covered with ice.
Originally published in Outdoor News Minnesota