Will the lake water freeze if the temperature is -15C?

Even at an air temperature of -15 degrees Celsius (-15C), the water in the lake does not freeze to the bottom. The water will only cool down to +4 degrees Celsius (+ 4C). Water in a reservoir at an air temperature above 4 degrees C circulates from the bottom to the surface, since warm water is less dense and, accordingly, it rises more easily to the surface. When the temperature drops below 4C, this circulation stops, as ice on the surface of the lake interferes with it. Thus, water in all reservoirs remains in a liquid state and does not freeze, complete freezing is possible only with severe frosts of the order of -30-40C, which last for a fairly long time, about several weeks.