Question: How is John Kerry Much Like A Minnesota Lake?

Answer: They both like to flip flop, especially in the fall.

All political humor aside, there’s a phenomenon that is either taking place right now or has already taken place in many Minnesota lakes that few sportsmen really understand. It’s called lake “turnover” and it involves the transformation of most, if not all, lakes in Minnesota during the fall and spring seasons.

Here’s what the Minnesota DNR says about Lake Turnover:

• The key to this question is how water density varies with water temperature. Water is most dense (heaviest) at 39º F (4º C) and as temperature increases or decreases from 39º F, it becomes increasingly less dense (lighter). In summer and winter, lakes are maintained by climate in what is called a stratified condition. Less dense water is at the surface and more dense water is near the bottom.

• During late summer and autumn, air temperatures cool the surface water causing its density to increase. The heavier water sinks, forcing the lighter, less dense water to the surface. This continues until the water temperature at all depths reaches approximately 39º F. Because there is very little difference in density at this stage, the waters are easily mixed by the wind. The sinking action and mixing of the water by the wind results in the exchange of surface and bottom waters which is called “turnover.”

• During spring, the process reverses itself. This time ice melts, and surface waters warm and sink until the water temperature at all depths reaches approximately 39º F. The sinking combined with wind mixing causes spring “turnover.”

• This describes the general principle; however, other factors (including climate and lake depth variations) can cause certain lakes to act differently. A more detailed description of the physical characteristics of lakes, including temporal and density interactions, can be found at the Water on the Web site, sponsored by the University of Minnesota – Duluth and funded by the National Science Foundation.

Understanding lake “turnover” is important for sportsmen because it will serve an indicator as to where you will find fish. In other words, late in the fall the cold temperatures found near the lake’s bottom will cause the water to be murky and denser. If you’re a fish, this is not a good thing because this cold water can have oxygen levels so low that it sometimes cannot sustain fish life. Therefore, immediately before the “turnover” period a savvy fisherman will look for a lake’s thermocline and focus fishing activity at depths near this zone or slightly higher than this level.

The thermocline can be explained simply as that level in the lake where there is a zone of rapid temperature change. In the fall, locating this zone can be often accomplished with a good depth finder capable of finding algae and protoplankton. Otherwise, you will need a thermometer allowing you to monitor temps at various depths.

Eventually when the lake turns over the complete lake is mixed of oxygen and it also tends to clarify itself. The bad thing is now the fish are no longer concentrated as they were just prior to the turnover period. For a period of several weeks fishing can be tough as they become difficult to pattern because the entire lake has taken on this new character. Eventually, however, the fish will fall back into their routine where fishermen can find them more predictable.

Certainly fall fishing can be a challenge no matter how you look at it. Then when you add the concept of lake “turnover” it only complicates matters for many fishermen. For this reason, if you want to be a successful fisherman during the month of October in Minnesota you need to understand what happens to their habitat. If you don’t consider turnover, you might just be wasting a whole lot of time wondering why you are not getting any bites.

Indeed, an informed sportsman can expect and deal with flip-flopping when it involves our lakes during the spring and fall seasons…but when flip-flopping involves our politicians it causes us great concern. Either way, when flip-flopping occurs it generally means we can anticipate very few positive results during the experience.

© 2004 Jim Braaten. All Rights Reserved. No Reproduction without Prior Permission.