Why Do Lakes "Turn Over"? An Expert Hydrological Analysis
Every autumn, anglers across the northern and midwestern United States experience a sudden, dramatic drop in fishing success. The water turns murky, debris floats to the surface, and an unmistakable sulfur smell fills the air. Local bait shops buzz with the phrase: "The lake is turning over."
But what exactly is lake turnover? To understand this massive hydrological event, we must first understand the physics of water density and the concept of thermal stratification.
1. The Physics of Water Density
Water is a unique substance. Unlike most liquids that continually become denser as they freeze, water reaches its maximum density at exactly 39.2°F (4°C). Any water warmer than 39.2°F is lighter (less dense), and any water colder than 39.2°F (down to freezing) is also lighter. This simple scientific fact is the reason why lakes don't freeze solid from the bottom up, which would kill all aquatic life.
2. Summer Stratification (The Layer Cake)
As the intense summer sun beats down on a reservoir, the surface water heats up rapidly. Because this warm water is less dense, it literally floats on top of the colder, denser water below. By mid-summer, deep reservoirs separate into three distinct layers:
- The Epilimnion (Top Layer): This is the warm, sun-heated surface layer. It is highly oxygenated due to wind action and photosynthesis from algae.
- The Metalimnion / Thermocline (Middle Layer): This is a thin layer characterized by a rapid drop in temperature. You can often see the thermocline on a high-quality fish finder as a distinct line of clutter.
- The Hypolimnion (Bottom Layer): This layer consists of cold, dense water (near 39.2°F). Because it is cut off from the surface and from sunlight, biological decay eventually consumes all the oxygen in this layer. By late summer, the hypolimnion is a dead zone where no fish can survive.
3. The Fall Turnover Event
As autumn arrives, the cool night air rapidly lowers the temperature of the surface water (the epilimnion). As the surface water cools, it becomes denser and heavier. Eventually, the surface water reaches a temperature where it is colder and heavier than the water in the thermocline below it.
Because heavy water cannot sit on top of light water, the surface water physically sinks to the bottom. This massive sinking action forces the oxygen-depleted, sulfur-rich bottom water (the hypolimnion) to the surface. The layers violently mix, effectively "turning the lake over."
"During turnover, the entire water column becomes a uniform temperature from top to bottom. The restrictive barriers of the thermocline are destroyed."
4. How to Fish During and After Turnover
Turnover is incredibly stressful for fish. The sudden mixing of anoxic (zero oxygen) bottom water into the main lake drops the overall dissolved oxygen levels drastically, often causing fish to become lethargic or even triggering minor fish kills.
Furthermore, because the temperature is now uniform, fish are no longer restricted to specific depths. A bass that was previously trapped above the 15-foot thermocline can now swim at 40 feet without suffocating. They scatter wildly, making them very difficult to locate.
Strategies for Success:
- Find the Oxygen: During turnover, fish moving water. Incoming creeks, spillways, and areas with heavy wind exposure will oxygenate faster.
- Wait it Out: The turnover process usually takes 7 to 14 days. Once the water clears and temperatures stabilize, fish will lock into predictable winter patterns.
- Downsize: Fish are stressed and not aggressively feeding. Downsize your lures—use finesse jigs, drop shots, and slow-moving baits.