The Fall “Turnover”

The dreaded “Turnover.” No, we’re not talking about football, we are talking about a process which has been going on for weeks in all of our northern Michigan lakes. For many anglers, this not only means cooler and shorter days on the water, but that the fish are also tougher to find.

Courtesy of Univ. of Illinois at Chicago

Although most anglers are well aware of this cycle, few are aware of the complex relationships which exist between falling water temperatures and wind. Lake “turnover” is a process which essentially involves the flipping of the temperature layers (stratification) of a water body.

One little known fact which contributes to this seasonal stratification is that water is most dense (heaviest) at 39º F (4º C). As temperature increases or decreases from 39º F, it becomes less dense (lighter).  During the summer and winter, these temperature layers are more stable and resistant to mixing from the forces of wind.

In autumn, it means that the water which has warmed all summer and formed a top temperature layer (Epilimnion zone) is now cooling and becoming denser.  This cooling continues until the water temperature at all depths reaches approximately 39º F.  Because the water density is very uniform at this stage, they become more readily mixed by wind.

Since our falls and winters never stop their downward temperature trend at 39º, as the water temperature continues to drop most lakes end up with a condition whereby the coldest water is on the top layer, including ice (at 32 ), with the 39º water now found on the bottom.

If you are an angler, understanding this process and its affect on the seasonal movement of fish is frequently this difference between success and “a beautiful fall day for a boat ride.”  The good news is that with the availability of  today’s marine electronics, finding fish during “turnover time” can be far less daunting. For those who may not be interested in the science of what this means, all you need to know is that soon you will be able to “walk on water.”

P.A.R.