Why and how do rivers freeze?

When we mention the word “river”, our minds first visualise a flowing water body. River water too, however, freezes, and it is, in fact, a common occurrence more than half (56% of all large rivers) of Earth’s rivers freeze over every year.

While the process of freezing is straightforward in calm waters such as those in lakes or even an ice cube tray, the turbulence in moving water bodies such as rivers and even streams, makes it different and difficult as the water molecules try to crystallise. The freezing of river waters occurs in a number of ways and even give rise to distinct formations.

Anchor ice

Some sections of the river freeze from the bottom up. This type of ice, known as anchor ice, forms on the rocks underwater, even if the surface hasn’t frozen. While a lot isn’t known about how anchor ice forms, grows or even detaches, it is well established that it grows rapidly once it has started. As a result, the appearance and flow of a river can change even overnight!

Pancakes and circles

Ice pancakes and ice circles are distinct surface formations that appear when river water freezes. What sets them apart is the process in which they are formed.

Pancake ice is formed in three phases. At first, frazil ice, which consists of small, needle-like structures, begins to form in the supercooled water. Frazil ice can get packed together by wave action, and the resulting floating mass is known as slush ice. The third and final phase sees the formation of pancake ice, which consists of frazil and slush ice along with a raised rim.

Ice circles, meanwhile, form in slow-moving rivers and look like a giant circle of ice slowly rotating on water. A change in river speed in a section where there is a large amount of ice could lead the ice to rotate until it becomes circular. Alternatively, a huge chunk of ice that breaks away from an ice sheet can become an ice circle in the presence of necessary forces.

Anchor ice, ice pancakes, and ice circles are only a few ways in which the river water freezes. The freezing of river water occurs in complex ways and also leads to more obscure formations at times.


Picture Credit : Google