Answer
Since the water at the surface is in contact with the air, the water at the surface freezes first.
Work Step by Step
The water in lakes and rivers freezes because the temperature of the air is cold. The water that is in contact with the air will freeze first as it takes time for all parts of an object to reach thermal equilibrium, when the whole object has the same temperature. Since the water at the surface is in contact with the air, the water at the surface freezes first.