Why do rivers and lakes not freeze easily?

Answer 1

While water is moving, its temperatue can drop below zero and it doesn't freeze. Obviously, faster moving rivers are less likely to freeze.

For lakes, water's unique density behaviour protects them from freezing. Unlike almost all substances, the solid form of water (ice) is less dense than the liquid form. Ice cubes float. (For almost all other substances, the "cubes" would sink.)

So when lakes freeze, the ice stays at the top. This insulates the remaining water from the colder air above. If ice cubes sank, then lakes could freeze all the way to the bottom: the ice that formed would fall to the bottom, continually exposing the top water to the cold air.