Why do some rivers freeze only on the surface?

As a lake or slow river cools, water near the surface loses heat first. Because fresh water is densest at about 4°C, that water sinks until the whole body is close to that temperature. Once the surface water cools below 4°C, it becomes less dense and stays on top, where it can freeze. The ice layer then insulates the water below, slowing further freezing. This is why fish and other aquatic life can survive under winter ice.