Lake turnover is a seasonal phenomenon that occurs in the spring and fall when the entire water column "mixes" from top to bottom. During the summer, lakes become stratified into layers: a warm, oxygen-rich top layer (epilimnion) and a cold, oxygen-poor bottom layer (hypolimnion). As the air temperature drops in the fall, the surface water cools and becomes denser. When it reaches 39°F (4°C), the temperature at which water is most dense, it sinks to the bottom, forcing the lower water to rise. This "flip" is vital for the lake's health because it redistributes oxygen to the deep water (where fish may be struggling) and brings nutrients from the bottom up to the surface. You can often tell a lake is turning over because the water suddenly looks murky or "dirty" as sediment is stirred up, and there may even be a slight "rotten egg" smell as sulfur gases from the bottom are released into the air. This process ensures that the lake remains a habitable environment for aquatic life throughout the extreme changes of the seasons.