Why do rivers remain unfrozen longer than lakes? Water has an unusual property of being at its most dense at around 38 degrees. As the surface of a lake or river reaches that temperature, dense water sinks to be replaced by colder, less dense water from below. Once the entire system is colder than 38 degrees, the process of freezing can begin at the surface. The deeper the lake, the longer it takes to freeze.
The turbulence of a river, especially fast water, keeps things well-mixed, so the entire body of water must cool to near-freezing before the surface can freeze. Moving water does freeze, although its motion requires a temperature a fraction of a degree colder than 32, at which point tiny ice particles called frazil ice begin to form, which gradually grow together as the water temperature drops.