Why do small lakes freeze earlier than rivers?

Small lakes generally freeze earlier than rivers because of differences in their physical dynamics and thermal properties.

In lakes, the formation of ice is primarily governed by the cooling of the water column. As surface water cools toward 4 °C (39 °F), it becomes denser and sinks, mixing the water column until it reaches a uniform temperature. Further cooling below 4 °C makes the surface water less dense, allowing it to remain at the top where it can reach the freezing point and form a stable ice cover, provided wind and currents are sufficiently low. Small lakes are often more protected from wind, which facilitates this stratification and the stabilization of the initial ice layer.12

In contrast, rivers are characterized by moving water, which complicates the freezing process. The flow and velocity of a river can prevent the formation of a stable ice cover, as higher velocities may retard thickening or cause ice to submerge and move downstream. While some rivers form ice along the shorelines first, the central regions often remain open longer due to the constant movement of water. Consequently, rivers typically freeze later than smaller, more stagnant bodies of water.345