NOTE: This weekend I'll be at the agate show in Muskallonge State Park, located 20 miles east of Grand Marais. My friend, Bob, will be volunteering at the museum so that the normal hours can be maintained. Go to www.agatelady.com to find out information about the show. The directions and hours are posted on the home page.
The local TV news station recently reported that the level of Lake Superior is down significantly from last year, so I decided to try to verify the report. According to government sources, Lake Superior's level is in fact down 11 inches from the average level (calculated since 1918). Lakes Michigan and Huron are down 24 inches and Lakes Erie and Ontario are down 7 inches. The decrease from the all time high level for Lake Superior, which occurred in 1985, is 24 inches. The lowest level ever recorded for Lake Superior was in 2007. The lake's current level is 9 inches above the all-time low. Lakes Michigan and Huron are 58 inches below the all-time high levels recorded in 1986.
Over geologic time, the levels of the Great Lakes have been influenced by the rebound of the crust. Crustal movement, the rebounding of the earth's crust from the removed weight of the glaciers, does not affect the amount of water in a lake, but rather affects water levels at different points around the lake. Crustal rebound varies across the Great Lakes basin. The crust is rising the most, more than 21 inches per century, in the northern portion of the basin, where the glacial ice sheet was the thickest, heaviest and the last to retreat. There is little or no movement in the southern parts of the basin. As a result, the Great Lakes basin is gradually tipping, a phenomenon most pronounced around Lake Superior.
To see what this means for water levels, an analogy can be made using a cup of water. As the cup is tipped, the surface of the water comes closer to the edge of the cup on one side and is farther from the edge on the other side. This is why water levels are measurably higher today at Duluth, Minnesota, and lower at Michipicoten, Ontario, on the opposite side of Lake Superior, than they were several decades ago. This tipping phenomenon is particularly significant for Lake Superior, and somewhat lesser for lakes Michigan, Erie and Ontario as their outlet channels are rising faster than the western shores of these lakes. As such, there is a gradual decrease in outflow capacities for each of the lakes over time.
A conceptual diagram of how high the ice was over the area that is now Toronto is shown below. The height of the ice over the Upper Peninsula was even higher.