Anchor Ice

If natural laws are inviolate, how is it that ice, which is lighter or less dense than water, can sometimes form on the bottom of streams and rivers, and even the ocean? Isn’t that violating a natural law?

Not really. Ice is always lighter than water. That doesn’t change. However, there are conditions that do allow for ice to build up from the bottom of a water body.

It all starts when air temperatures are at least little below freezing for several days. If temperatures are significantly colder, then the process can happen literally overnight.

Fast moving, shallow waters are the next ingredient. Moving water may become super-cooled, cooling below the normal freezing point of water. The substrate, the rocks and other bottom materials, also chill below 32 degrees Fahrenheit as the water column cools.

This sets the stage for the formation of what is referred to as anchor ice. The roiling water prevents surface ice from forming, but eventually, free-floating crystals, called frazil, begin to form in the super-cooled water column. Frazil is sticky, and whenever it contacts an object—for instance, another frazil crystal or something on the bottom—it adheres immediately. Once an anchor point is established, the ice grows quickly by free growth or by collecting additional frazil crystals.

The cloud-like appearance of the gray to blue-green colored anchor ice is interesting to see, but it does create some problems.

Since anchor ice is still ice, it wants to float. Once the adhesion strength is exceeded by the natural buoyancy of the ice, the anchored ice may break free of the bottom and float on the surface. As it floats downstream, it creates ice jams that send floods onto adjacent lands. Anchor ice may reduce flows for hydropower units or plug them altogether.

Biologically, anchor ice in small areas isn’t much of a concern. However, when anchor ice is extensive, it can wreak havoc on stream and river processes. Macro invertebrates, so essential to the health of the stream, can be frozen, smothered or crushed by the forming ice. Fish eggs can also be caught up in the ice and destroyed. The freezing process dislodges aquatic plants and, as the ice lifts from the bottom, can scour the stream or river bottom, reducing productivity all around.

Anchor ice doesn’t have to form a solid sheet to have an impact. In the super-cooled waters beneath the continental ice-shelf of the Antarctic where tides and wave action keep the water in motion, anchor ice can form on bottom dwelling organisms such as sponges. When the natural buoyancy of the ice exceeds the adhesive strength of the organism, it floats up and becomes frozen to the bottom of the shelf or surface ice. Eventually, as the surface erodes and ice is added to the bottom of the ice pack, the organisms actually end up on top of the ice.

Anchor ice is a reminder that nature is in charge of the rules and can create conditions where rules operate differently.