Lake ice has it easy. The water just sits there waiting to freeze. Ah, but trying to form ice on river water is quite a different kettle of fish. Because the water is ever moving, large sheets of ice can't form. So Khione (the Greek Goddess of snow, daughter of Boreas and Oreithyia -- there was no Greek god of ice) has her work cut out for her as she attempts to freeze river water. She attacks the problem a different way.
As the temperature of the water dips to freezing, ice crystals begin to form, but the constant movement of the water results in crystalis interruptus. (That's an invention: actually, crystal formation is broken up by the river's current.) Instead of a nice smooth sheet of ice, all Khione can accomplish is frazil. The ice crystals begin to form, but the water movement prevents them from forming sheets, so these crystals just hang around loosely together forming frazil, a natural version of a slurpee, or mister misty, if you will. (When this takes place on the Mississippi River, it would be referred to as mister missi misty).
If the temperature continues to drop, the frazil chunks freeze together, forming plates or pancakes. These cool looking plates of ice are usually round with raised edges caused by the pancakes smacking into each other. So, on a very cold day in December the river may be covered with pancake ice separated by unconsolidated frazil.
If temperatures dive quickly enough, this process may happen rapidly and the frazil and pancakes will freeze together. The river's surface will then indeed freeze solid. Because of the river's current, this ice is not of uniform depth. In one spot there may be 6-8 inches of ice, while the ice may only be one inch thick a couple feet away. River ice should never be considered safe! Please enjoy river ice formation from the safe and solid shore.
Dennis Kalma has a great website that contains more information about river ice and some photos to illustrate the process. Read more…