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Drivers of Change

This page is part of the project: Great Lakes Climate Change and Lake-Levels

Drivers of Lake Level Change

Lake level changes are the result of changes to that lake's net basin supply. Below shows the general net basin supply (NBS) equation for the lakes. These changes in the NBS terms can be the result of natural variability along with impacts from humans. 


Net Basin Supply Equation:

NBS = I + P + R - O - E - CU +/- D


NBS = Net Basin Supply
I = inflow from an upper lake
P = over-lake precipitation
R = runoff into the lake (includes over-land precipitation and evapotranspiration)
O = outflow from the lake
E = evaporation from the lake surface
CU = consumptive use
D = diversion


Past & Present Variability

Annually, there is an increase in lake levels from spring runoff from melting snow and a decrease in lake levels from high fall evaporation rates. The spring melt increases the runoff term in the NBS equation leading to increased water levels in the lakes; conversely the higher evaporation rates in fall increase the evaporation term in the NBS equation leading to a decrease in water levels. As the climate changes, the timing of the spring runoff can shift, occurring earlier than normal leading to an earlier rise in the lake levels. If the timing and amount of lake ice changes, with less ice forming later in the year, this allows the lake water to be in contact with the atmosphere for longer allowing for higher evaporation amounts compared to normal which in turn lowers lake levels more (when compared to the norm).

A visual representation of the lakes annual water levels (Lake Superior used for this example) and where/how climate change will affect and change the annual water level curve.


The natural processes of precipitation and evaporation are the largest factors in controlling the long-term levels of the Great Lakes. Changes in the typical patterns of precipitation and evaporation change the net basin supply and lead to changes the levels of the lakes, on decadal timescales. Changes in the timing and amount of lake ice also influence the lake levels as it changes precipitation and evaporation rates during the winter months. The past 20 years have shown examples of how changes in these natural processes led to low and high lake levels. Low lake levels seen throughout the early 2000's were the result of warmer temperatures, decreased ice cover, increased evaporation, and decreased runoff. This changed in the mid 2010's when a combination of low temperatures, high precipitation, and increased ice cover led to record increases in water levels on the upper Great Lakes. High springtime precipitation and basin runoff triggered the record high water levels on Lake Ontario and subsequent flooding in 2017.


Climate change will continue to cause changes in the patterns of precipitation, evaporation, and lake ice. The Third National Climate Assessment found there has been an 11% increase in region-total precipitation since 1950 as well as a 37% increase in the heaviest 1% of precipitation days in the Midwest between 1958 and 2012. Annual average air temperatures have increased by 2.0°F in the US Great Lakes region since 1900 with lake temperatures increasing even faster. For example, Lake Superior summer surface waters have increased by 4.5°F and are projected to continue to rise as much as 7°F by 2050 and 12.1°F by 2100. Annual maximum average ice cover declined to 43% during 2003-2010 timeframe, compared to the 1962-2013 average of 52%. This decrease in ice coverage impacts evaporation from the lakes for the following seasons. Annual days with snow cover decreased by 15 days from 1974 to 2004, but average snow depth has increased by an average of 2 inches, affecting the timing and magnitude of springtime runoff from snowpack melt. Warming temperatures enhance evaporation over the lakes and in the drainage basin. Increases in evapotranspiration together with reduced ice cover duration can lead to lower water levels. Climate models project increases in precipitation frequency and intensity which would lead to rising water levels. Warmer temperatures can also reduce snowpack and soil moisture (in the fall and winter seasons in particular) leading to decreased runoff and lower water levels. 


Water Management

Man-made diversions (into and out of the lakes) along with water taken out of the lakes for drinking water, agriculture, industry, etc. are small in comparison to the natural processes of precipitation and evaporation. Shown below is a map of the Great Lakes with diversions labeled. The inflow and outflow rates from these diversions is regulated by the International Joint Commission Boards of Control. Lake Ontario and Lake Superior each have their own board of control to regulate outflow from the Moses-Saunders Dam and St Marys River control structure, respectively. There is a control board for the Niagara area as well which focuses on the Chippawa-Grass Island Pool control structure above Niagara Falls, and supervises the annual installation and removal of an ice boom at the outlet of Lake Erie. These boards try to regulate flow rates at the control structures to keep the lake water levels in an approved range that helps protect the ecosystem while also maintaining adequate flow for hydroelectric power generation, minimum depth for municipal water intakes and safe navigation, and protect against flooding. However these structures are not able to fully control lake levels as natural factors such as precipitation, evaporation, and runoff can't be controlled. These factors are complex and difficult to predict. This lack of full control of water levels is shown with the record high water levels in the mid-2010's which resulted in flooding around Lake Ontario in 2017. However, the man-made control structures do help to make water levels less extreme overall. 


Image result for Map of the Laurentian Great Lakes system including state and national boundaries, interconnecting channels, and water management structures (U.S. Army Corps of Engineers)
Source: US Army Corp of Engineers


The International Joint Commission amended the agreement between the governments of the United States and Canada on Great Lakes Water Quality of 1978; this was called the Great Lakes Water Quality Agreement of 2012. The 1978 agreement was made to provide a framework for binational consultation and cooperative action to restore, protect and enhance the water quality of the Great Lakes to promote the ecological health of the Great Lakes basin. The 2012 agreement reaffirmed those objectives along with updating/strengthening the agreement to address current impacts on the quality of the Waters of the Great Lakes, and anticipate and prevent emerging threats to the quality of the Waters of the Great Lakes. As part of this, the 2012 protocol created Annexes 1 through 10 with each one focusing on a specific topic relating to the Great Lakes (for example, Annex 2 focuses on lake management and Annex 6 Invasive Aquatic Species). Annex 9 focus is to identify, quantify, understand, and predict the effects of climate change on the Great Lakes which can be shared with Great Lakes resource managers who can actively address these impacts. The research is to look at the effects of climate change on the biological, chemical, and physical integrity of the waters of the Great Lakes. This is to be done by:

  1. develop and improve regional scale climate models to predict climate change in the Great Lakes Basin Ecosystem at appropriate temporal and spatial scales;

  2. link the projected climate change outputs from the regional models to chemical, physical, biological models that are specific to the Great Lakes to better understand and predict the climate change impacts on the quality of the Waters of the Great Lakes;

  3. enhance monitoring of relevant climate and Great Lakes variables to validate model predictions and to understand current climate change impacts;

  4. develop and improve analytical tools to understand and predict the impacts, and risks to, and the vulnerabilities of, the quality of the Waters of the Great Lakes from anticipated climate change impacts; and

  5. coordinate binational climate change science activities (including monitoring, modeling and analysis) to quantify, understand, and share information that Great Lakes resource managers need to address climate change impacts on the quality of the Waters of the Great Lakes and to achieve the objectives of this Agreement.

Source: International Joint Commission