ICE!

Is that why it's Colder by the Lake?

One of the most pleasurable things to do on a sunny spring day in Duluth is to go down to the beach at Park Point and wander the Lake Superior shoreline when the winter's ice floes are stacked up. Some years a strong Northeast wind will stack the winter's lake ice into piles 5 or 10 feet or more, forming a wonderful maze of ice, snow, gouged up sand and pools of crystal clear water.

The ice itself appears in a striking variety of clear and frosted slabs, needles and flakes. In Winter 2002-2003, after a relatively snowless (until Spring at least) but fairly cold winter, the big lake froze over about 95% of its area (see image below). Lake Superior freezes at least in part every year and less frequently in its entirety. The last year that it froze completely was in February 1994. It almost froze completely in March 2003 and this photograph was taken by the GOES satellite on March 7, 2003.

It is one of a series taken from 17 February - 18 March 2003 during a cold spell in the northland when ice was forming on Lake Superior. This image was taken from the GOES Satellite Gallery developed by the University of Wisconsin.

Traffic Jam?

April 3, 2003. The Indiana failed to get out of the channel. Image courtesy of Duluth Shipping News.

After the weather warmed in early spring, strong winds from the northeast that persisted for several days blew enormous numbers of ice floes into the far western end of the lake where it piled high from eastern Duluth all the way to the beach at Park Point.

The ice piled so thick that on April 3, 2003 the thousand footer iron ore boat Indiana was unable to break out of the Harbor into Lake Superior. It did apparently manage to destroy our water quality data cable connection to our website. The Coast Guard cutter Mackinaw spent the next couple of weeks trying to create and maintain a shipping channel.

Inactive fingerrafting highlighted by new snowfall. Lakewood, MN February 1970.

Finger Rafting and Pressure Ridges

Many strange shapes can be seen in lake and river ice. Rafting refers to the process commonly seen in Duluth region lakes and streams where pressure caused by expanding and contracting ice causes one piece of ice to override another. It most commonly occurs in younger, thinner ice.

Fingerrafting occurs when interlocking thrusts are formed, each floe thrusting "fingers" alternately over and under each other. Professor John Green of the University of Minnesota, Duluth, noted these formations along the Lake Superior shoreline just north of town in the 1960's and reported his observations in a research journal (Green. J.C. 1970. Finger-rafting in fresh-water ice: Observations in Lake Superior. J. of Glaciology 9(57) 401-404). He noted that the phenomenon occurred when the ice was very thin (less than about 3/4 of an inch) and could be caused by a gentle breeze. The image at right is an example.

Ridging is the process where ice is formed into ridges. A ridge is a line or wall of broken ice forced up by pressure. The submerged volume of broken ice under a ridge, forced downwards by pressure, is termed an ice keel.