How can river water become supercooled below its freezing point?

In rivers, water becomes supercooled due to the effects of turbulence and water velocity. Unlike in lakes, where cooling is often restricted to the surface, turbulent mixing in rivers causes the entire water column to cool uniformly even after the temperature drops below 4° C (39° F).1

As the water temperature reaches the freezing point and continues to lose heat to the air above, the water temperature can fall slightly below freezing—a state known as supercooling. This process typically involves only a few hundredths of a degree Celsius.1

Once supercooling occurs, the introduction of ice particles leads to the formation of frazil ice. This freezing action releases latent heat of fusion, which helps return the water temperature toward the freezing point, balancing ice production with the rate of surface cooling. These supercooled conditions are significant because they cause ice particles to become unstable and adhere to one another or to surfaces, potentially forming anchor ice on riverbeds.1