The Science of Lake Turnover
Every fall, anglers experience a period of tough fishing often attributed to "lake turnover." But what exactly is this phenomenon? To understand turnover, we must first understand stratification.
Summer Stratification
During the summer, the sun heats the surface layer of the lake (the epilimnion). Because warm water is less dense than cold water, it floats on top. The bottom layer (the hypolimnion) remains cold and dense. Between them lies a narrow band of rapidly changing temperature called the thermocline. In deep lakes, the bottom layer can become devoid of oxygen by late summer, forcing all fish to live above the thermocline.
The Fall Turnover Process
As autumn air cools, the surface water also cools. Eventually, the surface water becomes colder (and denser) than the water immediately below it. This heavy surface water sinks, forcing the oxygen-depleted bottom water to the surface. This mixing process is called turnover.
During turnover, the entire lake briefly becomes a uniform temperature. You might notice the water becoming murky, smelling like sulfur, or seeing decaying plant matter float to the surface.
How Turnover Affects Fishing
Turnover is notoriously tough for fishing. The sudden mixing of oxygen-depleted water can stress fish. Furthermore, because the temperature and oxygen levels are now uniform from top to bottom, fish are no longer constrained to specific depths. They can scatter anywhere, making them incredibly difficult to locate.
Pro Tip: Turnover usually happens when surface temperatures drop into the low 60s (°F). If your local lake is turning over, your best bet is to fish highly oxygenated areas, such as incoming creeks, or wait a week or two for the fish to settle into their winter patterns.