Why does river water cool differently from lake water during winter?

River water cools differently from lake water during winter primarily due to the effects of water velocity and turbulence.1

In lakes, cooling typically leads to vertical stratification based on density. As surface water cools toward the temperature of maximum density (4° C), it sinks and mixes with deeper water. Once the water temperature drops below 4° C, the surface water becomes less dense and remains at the top, eventually cooling to the freezing point to form an ice cover that insulates the warmer water below.2

In contrast, the turbulent mixing in rivers prevents this density-based stratification. Instead, turbulence causes the entire water depth to cool uniformly, even after the temperature falls below 4° C. Because of this constant mixing, river water temperatures tend to track average daily air temperatures more closely than lake water. Furthermore, rivers often undergo supercooling, where the water temperature drops slightly below the freezing point, leading to the formation of frazil ice throughout the flow rather than just at the surface.1