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Water Level Factors

Predicting water levels can be difficult, especially in light of the volatile weather events brought on in large part by climate change. Past trends no longer inform projections about future levels for researchers, and rapid transitions between extreme highs and lows in the Great Lakes will continue to represent a new normal. There are, however, several indicators that are important to note when it comes to understanding the environmental influences on water levels. The eight major factors that influence water levels. 


Precipitation takes the form of rain, snow, hail and sleet, and is a factor that adds additional water to Georgian Bay, increasing water levels.

Precipitation occurs when a portion of the atmosphere becomes saturated with water vapor and reaches a point of 100% relative humidity. As water vapour condenses, droplets become bigger. Eventually, these water droplets become heavy enough where they fall, or “precipitate,” to Earth’s surface.

Precipitation may fall directly onto Georgian Bay, but it can also fall on the land that forms the Georgian Bay watershed, percolating through the soil, and running off the land.


The Great Lakes received record precipitation levels over the last 3 years, resulting in more water entering than leaving the system. In fact, from 1951 to 2017, total precipitation increased by 14% in the Great Lakes Basin, representing a huge immediate supply of water to lake surfaces like Lake Michigan and Lake Huron.

Isostatic Adjustment

Evaporation occurs when liquid is turned into vapour and is a factor that subtracts water from Georgian Bay, decreasing water levels.  Driven by water temperature, air temperature, and relative humidity, evaporation occurs primarily in the cooler autumn months, as cold, dry air receives moisture from comparatively warmer bodies of water sitting below. This process occurs until the air reaches a point of 100% humidity. If evaporated water vapour condenses into rain or snow, the process will be reversed. This reversal, however, also releases heat, bringing cool air down to continue the evaporative cycle.

Ice coverage bears an important role in influencing evaporation. Thicker ice takes longer to melt in the spring months, meaning more time is needed for lakes to warm up.  As an outcome, less evaporation will take place the following fall. Conversely, thinner ice means water warms up more quickly, resulting in more evaporation in autumn. As ice coverage has declined by 71% since the 1970s, the impact of evaporation in the coming fall seasons is expected to increase. In fact, the last time Georgian Bay froze over completely was 2015.

The evaporative process is also beginning earlier due to warmer winters, which also result in warmer waters. Lake Superior, for instance, lost an extra 10 inches in 2011/2012 due to an early start evaporative season! 


Since the 1990s, studies show that evaporation has played an increased role in driving water levels.  However, while evaporation has increased in the fall and winter months, it has been more than offset by increased precipitation over the past three years.


Largely due to the intensified human activity which occurred during and after the Industrial Revolution, climate change is in part the result of the release of heat-trapping greenhouse gases like carbon dioxide (CO2), nitrous oxide (N2O), methane (CH4) and chlorofluorocarbons (CFCs) into the atmosphere. This effect is intensified by melting permafrost which reawakens natural methane-emitting processes. Since 2001, 18 of the 19 hottest years on record have occurred! In the Great Lakes Basin, average temperatures have increased by 2.3 degrees Fahrenheit since 1951.

Climate change is believed to alter the long-standing balance between evaporation and precipitation. Both are expected to become more severe, causing higher highs and lower lows in water levels. Heavy precipitation events are up by 35% in the Great Basin since the mid 1950s.

Storm surges are also predicted to strengthen and reoccur more frequently over time, with the wind pushing water across the Bay during a storm, creating temporary increases in water levels on the side wind is blowing into.


Extreme floods and rainfall events are now occurring four times more often than in 1980, due to the increased presence of water vapour in storm systems.


Moreover, warmer temperatures suggest greater ice-melting. Forecasts suggest that we will have completely ice-free years in the Great Lakes in the next couple of decades.


Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations. Flowing and collecting below rock formations at different rates, ground water enters Georgian Bay from a variety of directions within the Georgian Bay watershed.

Record precipitation has increased the Great Lakes Basin ground water reserves by 50 cubic kilometers from 2013 to 2019, signifying that little to no ability remains for the Great Basin land to soak water up immediately.

With increasing precipitation, ground water reservoirs are expected to remain at or near capacity. Uncollected water runs off the land to eventually increase water levels in the Great Lakes. More rain falling on the surface will increase runoff and ground water movement. When surfaces are saturated, the soils in some places become more mobile and there may be increasing sloughs of material into the lake. More surface rainfall that can’t soak into the soils may also result in more surface stream volumes and energies and that may result in increases in erosion. All of these examples are driven by one simple rule: water flows downhill.


Unlike precipitation and evaporation, ground water is believed to be a relatively minor driver in adding water to Georgian Bay.


Connecting Channels

Inflows and outflows through connecting channels have a relatively strong impact on water levels in Georgian Bay. Water flows into Lake Huron and Georgian Bay from Lake Superior via St. Mary's River. The Straights of Mackinac connect Lake Michigan and Lake Huron, maintaining both lakes’ water levels in a state of near equilibrium. Water flows out of Lake Huron through the St. Clair River, and water flows out of Lake Michigan-Huron through the Chicago diversion which diverts water away from Lake Michigan into the Upper Mississippi River basin.

Outflow through the reaches connecting Lake Michigan-Huron to Lake Erie is about double the average evaporative losses from Lake Michigan-Huron itself. Moreover, if sustained, guidelines would result in an impact of 13 centimetres on Lake Michigan-Huron water levels from St. Mary’s River inflows.


The major diversions that affect water levels in Georgian Bay are diversions into Lake Superior at Long Lac and Ogoki and the Chicago diversion out of Lake Michigan.

The Long Lac and Ogoki diversions, located in northern Ontario, divert water from a portion of the Hudson Bay watershed into the Lake Superior basin. The Long Lac diversion began in 1939 and the Ogoki diversion began in 1943. They are operated by Ontario Power Generation.


The Chicago diversion diverts water from the Lake Michigan watershed into the Upper Mississippi River basin. The Chicago diversion began in the early 1800s and increased in 1900 after the Chicago Sanitary and Ship Canal was completed. The first US Supreme Court decree in limiting the Chicago diversion was effective in 1925, and the latest decree of 1967, modified in 1980, limits the annual diversion to 91 cubic meters per second (3,200 cubic feet per second). It is operated by the US Army Corps of Engineers.

Diversions have a much smaller impact on water levels than evaporation or precipitation. For instance, while Ogoki and Long Lac add 6000 cubic feet/second to the system and Chicago diverts about 3200 cubic feet/second out, the average evaporation from the surface of Lake Michigan-Huron alone is about 87,000 cubic feet/second. This is 29 times greater than the outflow from the Chicago diversion!

For more information, visit refer to the IJC's page or Factsheet.


Control Structures

The only control structure impacting water levels in Georgian Bay is the control structure located on St. Mary’s River, which regulate inflows into Lake Michigan-Huron from Lake Superior. There are no control structures in place to regulate the Lake Michigan-Huron outflows. Control structures, like those at St Mary’s, must be regulated for different considerations in determining which gates to open and by how much.

Some of these assessments include upstream and downstream impacts, effects on agriculture, commercial fisheries, commercial navigation, fish, wildlife, power generation, industrial facilities, municipal infrastructure like sewage system, recreation and tourism, and residential shoreline property, and First Nations rights.

In 2016, Georgian Bay Forever selected the renowned international engineering consulting firm AECOM to conduct an examination of over a dozen structural alternatives for lake level control, particularly water leaving lake Michigan-Huron. The study identified three possible control structures that could be used at the St. Clair River: in-stream turbines, inflatable dams, and a park fill/control gates system. Read the full report.

Climate Change

Historically glaciers have had a substantial impact on water levels. Ten-thousand years ago, Lake Superior, Michigan, Huron and Georgian Bay were part of one large lake called the Main Lake Algonquin. This massive body of water drained down through the St Clair Basin into early Lake Erie, to early Lake Ontario and finally through the Niagara River into the Champlain Sea.


Two-thousand years after, the Laurentide Ice sheet moved back. Consequently, changes to the Basin’s hydrology occurred, as Lake Superior went out through the North Bay, into Ottawa River and then into the Atlantic Ocean. No water flowed out from Lake Huron into Lake Erie. Additionally, as the ice sheets melted, the weight of ice removed itself from the North American shield, and the land tilted upwards as weight was released. Humans, however, can do nothing about this kind of occurrence. The modern-day Basin we see today connects from the tip of Lake Superior, through the lakes over Niagara Falls, into Lake Ontario, the St Lawrence River, and out into the Atlantic.

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