Anchor Ice – What’s it really look like and how does it form?

This section with all of its text, images and videos was created by
Dr. Edward Kempema at the University of Wyoming.

Frazil video

Frazil from the nearshore bottom of Lake Michigan. It’s about 40 cm in diameter (16”)!
Video shows frazil that formed at -4°C (25°F). Why does the water actually warm up? These crystals are growing in real-time.

Not all ice forms on the water surface. In streams, the turbulent flow can allow the water column to supercool to temperatures below the freezing point, allowing ice to grow in the water column. The first ice to appear is frazil (small disks or spicules of ice suspended in the water column). When turbulence transports frazil to the river bed, the frazil may stick to the bed, forming anchor ice.

Frazil and anchor ice are most likely to form on a cold, clear night when the wind is strong, the humidity of the air is low, and the river is at minimum flow, especially if such a night follows a cold, windy, cloudy day.

Dr. Ed Kempema's Underwater Videos
Cobble Transport


This clip shows mm- to cm-sized frazil crystals suspended in the water column of an artificial stream channel with flowing water.


When frazil sticks to the bed it becomes anchor ice. Individual anchor ice crystals can grow to be several cm in size (more than an inch).


More globs of the stuff.


More globs of the stuff. As it builds up its buoyancy eventually causes it to float to the surface. But it has to overcome the weight of gravel and even cobbles.


Once anchor ice lifts off the bed it drifts away as rafts carrying sediment, gravel and even cobbles downstream.


Watch out Titanic This clip shows a 6 pound rock being floated downstream and then captured by a giant hand.


Dr. Ed paints some rocks to measure just how far material can be transported by this mechanism.

Dr. Ed Kempema

Many thanks to Dr. Ed Kempema from the U. of Wyoming Geology & Geophysics Dept. for allowing us to use his materials. His hard work has helped us to see and understand a phenomenon that occurs in our streams every year and which can be very important to the ecology of the fish and other aquatic organisms.

We encourage you to visit his website to find out about the technical details and scientific importance of his work.