Above: An ice shove on Mille Lacs Lake sent huge chunks of ice into the backyard and a house in Isle, Minnesota, on Monday, April 20, 2020. (SCV/Doug Kiesling)
I remember the first time I experienced an ice shove when I lived on the eastern end of Lake Erie near Buffalo. As the season headed into spring and the ice on the lake was beginning to melt, a strong wind out of the southwest began to blow down the long axis, or fetch, of Lake Erie. Ice begin to slowly creep and pile up onshore and nothing was going to stop it. The sounds were eerie, and the potential for damage to anything in its path was significant.
An ice shove is defined as a surge of ice that piles up on the shore of a body of water, typically an inland lake. The movement of the ice is modulated by strong winds and currents as well as temperature changes.
When the break-up and melting of the ice is underway in the spring, the large mass of ice covering the body of water is subject to movement from strong winds. Sustained periods of strong winds down the long axis, or fetch, of the lake can begin to slowly move the ice sheet toward the downwind shoreline.
Since the ice sheet can cover hundreds of square miles on a lake, there is plenty of force behind that wind-driven mass of ice. Jay Doering, a University of Manitoba civil engineer, notes "It's a little bit like thinking of a train hitting a solid barrier. It has momentum and the cars just keep coming in behind and continuing to add to the wreckage." In what seems to be a slow-motion version of a frozen tsunami, the leading edge of the ice sheet beings to pile up on shore. It’s almost like the slow movement of a glacier, except the glacier moves by gravity whereas the ice shove is moved by the wind and water currents.
As long as the winds maintain a relatively strong velocity, say 25 mph or more, for an extended period of time, the sheet of ice will continue to move and the downwind shoreline will feel the effects of ice piling up—as high as 40 feet at times!
Across North America, there are several lakes in the U.S. and Canada that experience the correct combination of conditions to produce ice shoves during the springtime. These include but are not limited to Dauphin Lake and Lake Manitoba in the province of Manitoba in Canada; Mille Lacs Lake in Minnesota, about 100 miles north of Minneapolis; and Lake Winnebago in Wisconsin.
Ice shoves can occur on even bigger lakes, such as the Great Lakes. Favored areas include the south shore of Lake Superior and the east end of Lake Erie around Buffalo. Back in February 2019, a major ice shove on Lake Erie produced spectacular video as the ice encroached on the Canadian shoreline across from Buffalo. Using an ice thickness of 6 inches, it was calculated that there could have been in the neighborhood of 13.5 billion tons of ice covering the 225-mile-long lake, being pushed downwind by wind gusts up to 70 mph. That gives you some idea of the forces involved when Nature flexes its muscles.
The video below shows that this ice shove was far from the slow-motion events we often see with ice shoves.
Below is a comparison of the ice cover on Lake Erie before and after the event, to show just how much the ice sheet can change over a few days.
The recent ice shove on Mille Lacs Lake, pictured at top, was a great example of nature’s forces in slow motion. The lake, which is the second largest inland body of water in Minnesota, covers an area of over 200 square miles. People watched helplessly as the ice slowly encroached on the shore, piling up over 20 ft. in some locations. In the video you can actually see the ice moving across a back deck of a home and smashing through the sliding glass door. Trees are jostled around as the heavy piles of ice spill forward on the momentum created by winds that gusted in excess of 50 mph.
I reviewed the setup and weather associated with the event on Mille Lacs Lake to show how those winds were able to push the massive sheet of ice toward the shore that was so severely impacted. Strong northwest winds developed on the morning of Monday, April 20, and by noon CDT were gusting as high as 51 mph at St. Cloud, MN, about an hour south of the lake. The southeast portion of the lake is highlighted by a narrow bay where the town of Isle is located. That bay likely compounded the force of the ice sheet as it is pushed into the narrow opening.
The movement of the ice sheet can be seen from the low-orbiting polar satellite in two snapshots that were made before and after the ice shove, as shown in the animation below. The grayish white appearance of the lake is the ice cover. Notice the northern shore of the lake on April 19 and two days later as clouds cleared once again. There is a measurable difference in the northern edge of the ice, verifying that the prolonged strong northwest winds pushed that entire mass of ice southwards. Since the ice sheet effectively covered the entire lake before the event, there was nowhere for the ice to go once the winds picked up, thus pushing the leading edge onto the southeast shoreline of the lake.
Each spring season, residents in areas prone to ice shoves maintain a constant vigil. The combination of significant ice cover from the winter season, coupled with drastic temperature changes, and most importantly, a strong, sustained wind, out of a favorable direction to move the ice, can produce these amazing—and sometimes dangerous—events.
