A lake has turned over when water temperatures are the same from the surface to the bottom. The process can take days or even months to complete, depending on lake shape and depth, and air and water temperatures.

Lake "turnover"—the process where the surface and bottom water mix—typically happens twice a year, in the spring and fall, and usually takes a few days to two weeks to complete. The process is driven by water reaching its maximum density at 4°C (39.2°F). In the fall, as the surface water cools to this temperature, it becomes denser and "sinks" to the bottom, pushing the nutrient-rich (but oxygen-poor) bottom water to the top. This mixing is essential for a lake's health as it redistributes oxygen and nutrients. In 2026, scientists are noting that "climate shifts" are causing turnover to occur later in the fall and earlier in the spring, which can sometimes lead to longer periods of "stratification" and a higher risk of late-season algae blooms. The actual duration depends on the lake's depth, surface area, and the intensity of the winds that help "churn" the water during these transition periods.