Why do lakes turn over in spring and autumn?

Freshwater is densest at about 4°C, so small temperature changes can strongly affect how lake layers arrange themselves. In summer, warmer light water stays on top of cooler deep water, creating stratification. In autumn and spring, surface water cools or warms enough that the density difference weakens and wind can mix the lake from top to bottom. This process is called turnover. It is important because it brings oxygen to deep water and nutrients back to the surface, affecting fish, algae, and overall lake health.