On March 6th ice coverage on the Great Lakes peaked at 92.2 percent, the second greatest concentration on record (records have been kept since 1973). The greatest amount recorded was 94.7 percent in the winter of 1978-1979.
As of yesterday (March 16) ice coverage was down to a little over 75 percent and should continue to decline as the weather warms.
Ice cover is one indicator of the severity of winter in the region. It takes prolonged periods of cold weather to produce the coverage and thickness of ice seen this winter. Ice thickness exceeded 30 inches in some areas, although exact thickness is undetermined as the ice augers bottom out at 30 inches.
Although the negative impacts of ice seem readily apparent (limiting navigation and shipping), there are benefits to the ice cover on the lakes.
Once the lakes become ice covered, the potential for lake-effect snow and lake enhancement of snow diminishes. Lake-effect snows result from the development of precipitation as cold air moves over the relatively warm waters of the lakes. Once the source of "heat" and moisture is cut off so is the potential for enhanced snowfall. In the same vein, the ice cover prevents evaporation from the lakes which can help contribute to higher water levels on the lakes, important to navigation and to those communities who rely on the lakes for water supply. The ice cover can benefit the fishing industry in the lake. Whitefish, for example, spawn in shallow water, and the ice cover protects their eggs from damaging wind and wave action. When there is little or snow cover on the ice, light penetration promotes the growth of algae, an important nutrient in the food chain. Of course, thick, stable ice on the near-shore areas is beneficial to recreational activities such as ice fishing.
How long is ice likely to remain on the lakes? On the shallower lakes (Erie and Huron, for example) ice cover develops more rapidly and with greater concentration, but also will decline more rapidly than on the deeper lakes. On average the date of last ice is generally sometime in March in the southern Great lakes, but late April in some of the bays in the northern Great Lakes.
However, this is not an average year. In 1979, the year with the greatest ice cover, the date of last ice was mid to late May across the western half of Lake Superior.
There will be some impacts on spring and early summer weather as a result of the extensive ice cover. On a regional basis, air masses that cross the Great Lakes will be modified (cooled) more than normal and that could result in a cooler than normal spring. The delayed warming of the waters due to the ice will mean that near-shore areas subject to lake breezes are likely to find those to be very chilly breezes through early summer.
If you would like to learn more about Great lakes ice, visit the Great Lakes Ice Cover page of NOAA's Great Lakes Environmental Research Laboratory located in Ann Arbor, MI.