River Ice Floats
Life on earth would be very different if ice sank - for example, the oceans, lakes, and even rivers would be frozen solid except for a thin surface layer of warmer water that melted during the summer. It is difficult to imagine how life could even have evolved under those circumstances.
When I think of the first ice forming in early winter it is of a thin transparent sheet skinning over the surface a pond or lake. Just after the freeze, you can pick up pieces of this ice and it can be almost as clear as glass. As the winter progresses the ice gets thicker until we can stand, or even drive, on it.
But on the river, the freeze up is very different because the water is always in motion and the large sheets can't form as they do on lakes
The first stage in river ice formation is when crystals start to form and grow. But the water movement interrupts crystal growth and the crystals don't join together to form a sheet. Instead, you get a mixture of ice crystals and water that looks like a wet slushie. The crystals are called frazil and the mixture of crystals and water is called frazil slush.
Large Pancake Ice
Solid ice usually begins to form along the shore of the river. There is less movement of water here so ice forms more easily and the temperature in the shallow water along the shore drops faster on a cold night. Both factors lead to a margin of ice along each shore, a condition referred to as border ice or shore ice. Border ice may continue to grow gradually toward the center until the river freezes over. Or ...
Whether by the border ice meeting in the center or by a more rapid freeze up of the pancake/frazil ice, when the river is completely covered it is referred to as sheet ice. As the cold weather continues the ice becomes thicker and thicker. But unlike lake ice, where the thickness is uniform and safe to walk on, river ice may have thin spots.
Under the ice the river continues to flow and where the flow is turbulent slightly warmer water may be thrust up against the underside of the surface ice. Where this happens the ice either doesn't freeze as thickly or may thin out leaving places where it is possible to fall through. Walking on river ice demands paying attention to areas where this thin ice is likely to occur.
If you watch an ice cube in a glass of water melt it just gets smaller and smaller. In a lake or river the melting at the end of winter is a little more complex, mostly because the melting involves a series of partial thaws and refreezing as weather patterns change or as days follow nights. These freeze-thaw cycles produce a honeycomb of water and long hexagonal crystals of ice called candles. This ice is referred to as "rotten ice" and even though it may still be many inches thick is not solid enough to walk on safely.
As the water begins to rise during the spring melt the rotten candle ice gives way, cracking into blocks that float downriver. If there are constrictions in the river, such as where bridge abutments have been placed out into the river, the breakup ice may jam the river and cause flooding. Unfortunately, it is often our human manipulation of the river that causes these constrictions and since we usually do this in our towns and villages, it is usually the towns and villages that are damaged by ice jams.
During the flooding breakup, ice may be pushed back up onto the riverbank and onto the floodplain. Both the flooding and the ice blocks can damage buildings and other structures on the floodplain. After the ice in the center of the river has been carried away it may leave the bank of the river piled with blocks of ice. These piles may form a solid wall called a shear wall.
Ice attached to the streambed is called anchor ice. It forms under a variety of circumstances in areas where the water is shallow enough to freeze all the way from the surface down to the bottom. It is more likely to form in the headwaters, in the shallows along the edges of the river, or in riffle areas. It is more likely to form in colder winters and when there is less snow insulating the surface. It is devastating to the animals caught under it.
As the anchor ice forms and thaws, it lifts up the streambed and moves it along like a bulldozer. In this fashion, it can wipe out the small fish and aquatic insects living in the streambed. Each winter sections of the streambed affected by anchor ice are depopulated. It is only by resettlement from adjacent streambeds that these areas are inhabited during the summer.