The surface of rivers freeze when the temperatures falls below freezing. However, the water underneath this layer of ice continues to flow. This phenomenon allows fishermen to fish through holes drilled in the ice the entire winter. Which property of water is the reason the river doesn't freeze completely?

Water retains its temperature for long.

Explanation:

The ice floats on the water of river as it is less denser then the liquid water. This causes ice to float on water. It protects the water at the bottom from freezing. The decrease in the temperature level can cause the death of the marine life by catching organisms in the ice. The floating of the ice insulates the water present beneath. So, the fish and other aquatic animals can survive and the fishermen can catch the fishes alive.