Great Lake Water Levels – The Long View

The Sources of Knowledge Forum (SOKF), a charitable non-profit organization based in Tobermory, was founded to inform local people and a wider general public about the unique environment that is the Saugeen Peninsula and its surrounding lake. SOKF hosts an annual, three day public seminar in Tobermory to highlight some aspect of this environment. SOKF also has a responsibility to extend its reach and to develop a model for a larger knowledge network. With that in mind SOKF is starting a monthly blog to highlight current issues and resurrect topics from previous forums which are relevant today.

The next annual forum, on May 3 to 5, 2019, will focus on Fathom Five National Marine Park. Ten years ago, as part of the first forum, Steve Blasco of the Geological Survey of Canada, gave an overview of the history of Lake Huron since the end of the last ice age 12,000 years ago. Over that period the lake levels have changed dramatically. There have been at least three extremely low water level episodes and three high ones. 4500 years ago, the Peninsula, including the cliffs along the Georgian Bay shore, was almost submerged.

Between 10,000 to 7,500 years ago the lake levels were generally low and forests grew on what are now the lake beds of Lake Huron and Georgian Bay. Today the remains of those forest are known as the “drowned forest’ because the trunks are still rooted on the land where they originally grew. Those dramatic changes in lake levels, varying by as much as 125 meters, were caused by the receding glaciers of the last ice age, which allowed the land, freed of the weight of the ice, to spring upwards. Climate changes also played a role as did the way the lakes drained into the sea. Originally, the Mississippi was the preferred route to the sea. Ten thousand years ago the lakes drained through the Pettawawa/Ottawa rivers into the St. Lawrence.

Today’s modest changes in the lake levels (as much as two meters) have two principal causes, precipitation (including run off) and evaporation and these are the result of weather. Water levels rise and fall in seasonal cycles, with annual highs typically occurring in the summer and annual low levels in winter. Water levels also fluctuate in longer cycles in response to persistent wet or dry conditions that may last for a number of years. Declining annual ice cover and warmer water temperatures increase the potential for evaporation when water is open and there is a large difference between air and water temperatures. A mild winter produces less evaporation.

Inevitably, humanity has an impact on lake levels, although they are much less than is often believed. Since the late 19th century, Lake Michigan-Huron has been lowered by about 16 inches due to dredging and other channel changes in the St. Clair and Detroit Rivers. The International Joint Commission (IJC), which manages the Great Lakes on behalf of Canada and the US, published a study in 2012, which found that erosion since 1962 had caused another 3 to 5 inches of lowering. However, the erosion is not ongoing. The effect of the diversion at Chicago has removed the equivalent of lowering the Lake Michigan-Huron by about 2 inches whereas the Long Lac and Ogoki diversions into Lake Superior have raised Lake Michigan-Huron by the equivalent of 4 inches..  The table below illustrates humanity’s impact.

Human Impacts on Lake Levels

                                                        Long Lac 
and Ogoki
Chicago WellandNet Effect
+ 0.06 m

– 0.02 m

– 0.02 m

+ 0.02 m
Michigan / Huron
+ 0.11 m

– 0.06 m

– 0.06 m

– 0.01 m
+ 0.08 m

– 0.04 m

– 0.13 m

– 0.09 m
+ 0.07 m

– 0.03 m


+ 0.04 m

  International Joint Commission

Because of climate change, the IJC admitted that there is considerable uncertainty regarding future trends in Great Lakes water levels. In short, the 2012 IJC study indicates that over the next 30 years, lakes Superior, Michigan, Huron and Erie levels are likely to go down – but they may go up.

You can find the full, 236 page report at

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