To become covered in ice, often of a body of water. I know the lake has frozen over, but I still don't think it's safe for you kids to go skating on it.
freeze over
[for a body of water] to get cold and form a layer of ice on top. The pond froze over, so we went skating.
freeze over
v.
1. To freeze completely at the surface: Once the pond freezes over we can go skating.
2. To become covered with a layer of ice: The bridge has frozen over, so drive very carefully.