Oxygen from the surface mixes with the bottom, while nutrients trapped near the bottom are free to mix throughout the lake. This is why, sometimes, in the spring and fall the lake can smell unpleasant. Decomposing organic materials are churned up from the bottom of the lake, bringing a signature sign of lake turnover.

You can tell a lake is "turning over" in the spring or autumn by observing several distinct physical and sensory changes. The most obvious sign is water clarity; the water will suddenly become murky or "stained" brown as sediment from the bottom is stirred up. You will often see organic debris, like green or black "blobs" of decaying vegetation and algae, floating on the surface. Another high-value indicator is a strong sulfuric or "rotten egg" odor, caused by the release of hydrogen sulfide gas that was trapped in the oxygen-depleted bottom layer (the hypolimnion). For anglers, a major clue is that the fishing suddenly "turns off" for about a week as the fish become disoriented by the rapid changes in temperature and dissolved oxygen levels. This natural process is essential for the lake's health as it redistributes nutrients, but for the casual visitor, it is marked by a "messy" appearance and a temporary assault on the senses as the lake's layers literally swap places.