This time of year, frosts on the ground become increasingly common. It may even snow a bit here and there, and when it does, the snow can stick a while before it melts. It is relatively easy for the temperature of the surface of the ground to drop below freezing this time of year, although it rarely stays frozen for more than a few hours at a time until later in the fall. Lakes and rivers, however, are not likely to show any ice until later in the fall when the weather has been consistently cold for a while.
Water has a higher heat capacity than soil, so it takes longer for water to cool down when exposed to cold air. Another factor is that water is at its densest at 38 degrees. When a lake surface reaches 38 degrees, it sinks due to its heaviness, and the surface layer is replaced by warmer water from below. This complicated, dynamic process helps to further delay the freezing of the water.