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Guest Post by Stephen E. Kesler

Although the Great Lakes formed only a few thousand years ago, they are the result of much older processes. In fact the geologic history of the Great Lakes region covers almost the entire 4,540-million-year span of time since the Earth formed. Some of the oldest rocks in the world are found in the Minnesota River valley and in the Watersmeet and Carney Lake areas in the Upper Peninsula of Michigan. At the other end of the time scale, the Great Lakes shorelines are evolving even today.

Water levels in the lakes and surrounding waterways have been a particular concern for Great Lakes residents in 2019. The U. S. Army Corps of Engineers has reported new record high monthly mean water levels for Lakes Erie, Superior, and Lake St. Clair, and levels are continuing to rise throughout the region. Many factors affect water levels in the Great Lakes, as discussed in the following excerpt from my new book, Great Lakes Rocks: 4 Billion Years of Geologic History in the Great Lakes Region.

From Chapter 2: Landscaping the Continent

The Great Lakes Chain Is More Than 2,000 km Long

The Great Lakes contain 22,250 km³ of water, 20 percent of the world’s surface freshwater. They are a major transport artery into the central part of North America, as well as a major recreation and fishing resource. The Great Lakes system begins at the west end of Lake Superior and flows more than 2,000 km to lower and lower lake levels until it exits the east end of Lake Ontario. From there water travels another 870 km through the St. Lawrence River to the Atlantic Ocean. The famous children’s book Paddle to the Sea by Holling C. Holling follows a toy canoe along this path from the headwaters of Lake Nipigon, which drains into Lake Superior, through the lower lakes, and finally into the ocean. Because the toy canoe had no paddler, it was moved along its journey by long-shore drift currents, which are a major agent of erosion and sediment transport, as we will see later in this chapter.

With the exception of Erie, which is very shallow, the Great Lakes extend to depths considerably below sea level. Their great depths reflect the fact that the glaciers were able to scour deeply into Paleozoic rocks that underlie the lake basins. For Lakes Michigan, Huron, and Ontario, the glaciers cut into the soft shales, carbonate rocks, and evaporites. Lake Superior, which is not surrounded by Paleozoic rocks, owes its depth to glacial scour along the soft Nonesuch Shale, a part of the much older Midcontinent Rift. Depths in the lakes are not continuous, however, and are commonly interrupted by glacial deposits that formed when the lake levels were much shallower during glacial retreats. For instance, in Lake Michigan, the Southern Basin is separated from the Algoma Basin in the north by submerged moraines that formed during the late Wisconsin glacial retreat.

Islands and promontories in the Great Lakes are underlain by rocks that were resistant to erosion. Most obvious are the Silurian-age dolomites that make up the Door and Garden peninsulas on the west side of Lake Michigan and the Bruce Peninsula on the northeast side of Lake Huron. These rocks continue eastward to make the Niagara Escarpment in Ontario and New York, as well as the resistant layer that makes Niagara Falls.

Lake Levels Vary in Response to Natural and Human Factors

Water levels in the Great Lakes vary seasonally by a meter or two, with the highest water levels commonly found during summer when runoff is greatest. Although this level of variation might seem small, it is significant to docks, homes, and other facilities along the shoreline. Great Lakes scientists and administrators are also concerned about whether the lakes will embark on a new path with greater changes in lake levels as the climate warms. The variation of Great Lakes water levels at human time scales depends on wind, precipitation, evaporation, streambed erosion, and glacial rebound. Human features also play a role, particularly dams on the St. Marys River between Superior and Huron and on the St. Lawrence River below Lake Ontario. In the summer of 2017, when Lake Ontario levels were unusually high, the St. Lawrence dam was the focus of debate between upstream interests that wanted water released to alleviate flooding and downstream interests that feared the released water would flood them and by shipping interests that wanted to retain high water in the river for navigation. Three diversions that send water out of the Great Lakes also have the potential to affect water levels but are not as important as the dams and the natural factors.

Among natural factors, wind varies in effectiveness over the shortest time frame. Storms generate large waves that last only a few days but can be very large, especially in the largest lakes. During an investigation of the sinking of the ore carrier Edmund Fitzgerald in Lake Superior in 1975, the captain of one “salty” (oceangoing ship) in the same area at the time reported that the waves were the highest he had seen anywhere in the world. Sustained winds in one direction can push water downwind, generating a standing wave called a seiche in which water sloshes back and forth from one end or side of a lake to the other. Lake Erie, which is shallow and oriented along the path of prevailing winds, develops especially strong seiches. During one storm in November 2015, the water level at Buffalo rose 2 m within a few hours and at the same time the water level at Toledo dropped almost 2 m.

Meteotsunamis are also related to wind but in a different way. They form when a storm leads to rapid changes in atmospheric pressure that cause a corresponding abrupt change in local water levels. Waves formed in this way sweep across the water just like a tsunami in the ocean; according to a study by Adam Bechle and his coworkers, this happens most commonly in April through June. Most meteotsunamis generate waves only about 1 m above normal, although much larger ones have been observed. On July 4, 1929, a 6 m wave hit the pier at Grand Haven, Michigan, drowning ten people, and in 1954 a 3 m wave swept the Chicago shoreline, drowning seven.

Precipitation causes lake level changes over longer periods of at least a year or more. In the simplest sense, levels should go up when there is a lot of precipitation and down when there is less. As climates have warmed over the region, however, evaporation has increased in importance. Evaporation is relatively continuous during the warm months, of course, but it also happens during winter months in years with low ice coverage, and this is becoming more common. The overall trend of maximum ice coverage for the Great Lakes has decreased from about 60 percent in 1974 to about 45 percent in 2017. Jay Austin and others, working on Lake Superior, have shown that the lakes are actually warming faster than local average temperatures, possibly because the darker water (darker than ice) absorbs more heat and the warming causes more evaporation, loss of water, and lower lake levels. Increased variability in ice cover from year to year will probably lead to larger fluctuations in future water levels and increased problems with shoreline erosion.

There appear to be additional lake level changes under way beyond the annual time frame. For instance, many people are convinced that since about 1930 there has been a gradual decline in the average water level of Lakes Michigan and Huron (which are connected and act as a single body of water) and that there has been a corresponding increase in the level of Lake Erie over the same time frame. One possible cause of this change, if it is indeed true, is erosion of the St. Clair riverbed at the outlet of Lake Huron. The erosion is also thought, at least by some, to have been caused by the mining of sand and gravel from the riverbed as reviewed by Frank Quinn. A more recent study by Xiaofeng Liu and others that attempted to evaluate the erosion issue was inconclusive. They found sedimentary bed forms in the river that could have resulted from erosion but concluded that present flow rates in the river were not sufficient to account for them.

Isostatic rebound refers to the rise of land from which a heavy weight has been removed. In our case, the heavy weight was that of the glaciers, and Earth’s crust is still recovering from this load. This process is slow and operates barely within human time scales. It has accounted for much of the long-term history of the Great Lakes, especially in northern regions where the ice was thickest and longest lived. Since the ice left the Great Lakes about 10,000 years ago, Thunder Bay, Ontario, has risen about 200 m whereas Toledo, Ohio, has risen about 14 m. Although the rate of uplift has slowed, it continues today and is highest in the north. In Lake Superior, Michipicoten Island on the north side is rising about 35 cm per hundred years faster than Marquette, Michigan, on the south side. A similar difference in rates is seen between the north and south ends of Lakes Michigan and Huron, which, operate as a single unit. This tilting of the lakes to the south causes water to leave northern regions such as Georgian Bay and pile up along the southern shores. The result is receding shorelines for property owners in northern areas and increasing storm damage for those in the south. It can even move water from one lake basin to another. Studies of modern tilting indicate that the Michigan-Huron system is decanting into Lakes Erie and Ontario, which are gaining water.