by Helen Bryce, Director of Education


As a child growing up here every summer, I listened to my grandfather talk about the water levels. He described a 5 – 7 year cycle; and, as a child, I noticed the water level gradually rising each summer for a few years until the next summer, the boathouse dock was underwater! Then, just as gradually as it rose, the water level began to recede to its previous lower level. Up and down, like a roller coaster every 5-7 years. It was a predictable cycle that everyone talked about so not too surprising or unexpected for cottagers and those permanent boathouses and docks built on cribs weathered the cycle just fine. Why isn’t it that way now? What has happened to create the more extreme roller coaster of ups and downs in the last several decades with the water levels of Georgian Bay (GB)?

Of course, the answer is complex and involves several processes – each one complicated in its own right. I will attempt, in a series of short articles, to explain in laymen’s terms, what those influences are and to do so in priority order from the most influential to the least. 

Full disclosure! This is not an academic article. The information I will share has been distilled from an amazing resource document called “Water Levels Q&A – Just the facts” that was written by David Sweetnam and Rupert Kindersley, Executive Directors of Georgian Bay Forever (GBF) and Georgian Bay Association (GBA), respectfully. They did so in response to the 259 questions asked during the Water Symposium last Oct. 24, 2020, hosted by GBF and GBA. If you are interested in much more scientific detail, read the entire resource document here. When I get into the finer details of data, diversions, and regulations in this series, I have consulted and freely pulled from many online resources. These will be listed at the end of each article where applicable should you wish to delve deeper. Look for next week’s article which will be a simple explanation of the processes and interactions between precipitation and evaporation which have the greatest impact on our water levels. 

By far the biggest influence over GBay levels is the combined interactions between precipitation and evaporation. Science has demonstrated that there are no other factors in existence today as influential as these natural processes on water levels – not Great Lakes inflows and outflows, not Great Lakes regulatory modifications and diversions as they currently operate.  


What is it?

  • Precipitation is weather that we see when we look out the window each day.
  • It takes the form of rain, snow, sleet or hail that falls to the ground.
  • Groundwater that flows and collects below the Canadian Shield rock formations can be affected by heavy rainfall, which eventually will flow into the lake. 
  • The moss and soil can only absorb so much rainfall and then the rest runs off toward the lake (potentially causing erosion).

Where does it come from?

  • The water in the Great Lakes comes in the form of storm systems from many directions: from the Gulf of Mexico, the Northern Pacific Ocean, or the Atlantic Ocean (like the devastating Hurricane Sandy that came up the eastern coast of North American in 2012). 

What is the impact of precipitation?

  • The more precipitation, the higher water levels on GB will rise.
  • Generally, the air temperature decreases when it rains because the moisture is falling from a cooler layer of air above.
  • As it falls into the warmer air near the ground, heat is exchanged through conduction and convection which gets the air moving.
  • Precipitation levels have had the most influence over water levels prior to the late 1990s and again more recently.  
  • Record rainfall over the past three years has resulted in more water entering the lakes than leaving, increasing the levels to the extreme high we had last summer.  

What is the impact of Climate Change on precipitation?

  • The size of a storm depends on the amount of water vapour it carries. 
  • A 1º Celsius increase in air temperature, means a 7% increase in the water vapour contained in a storm system. 
  • As the temperature of the earth’s atmosphere warms due to the accumulation of greenhouse gases, the size of storms increases. 
  • If Global Warming continues to gradually increase the air temperature, the impact of climate change on precipitation can also be expected to increase.

And the Jet Streams?

  • Jet streams are bands of strong wind or air currents, 8-14 km up in the atmosphere (5-9 miles), that generally blow from west to east all across the globe.  
  • Jet streams form when warm air masses meet cold air masses in the atmosphere.
  • The Earth has four primary jet streams: two polar jet streams, near the north and south poles, and two subtropical jet streams closer to the equator.  
  • They have impacts on weather, air travel and other things and are closely monitored through weather satellites.  
  • Monitoring jet streams can help meteorologists determine where weather systems will move next.  
  • But jet streams are also a bit unpredictable. Their paths can change, taking storms in unexpected directions.
  • A recent example of this is Hurricane Harvey that got stalled over Houston, Texas for 3 days dumping the equivalent of 495 days of Niagara Falls rain onto that city.
  • Over the last few decades, the Jet stream has been gradually weakening due to the accumulation of greenhouse gases in the atmosphere. 
  • The historic sharp line separating cold Arctic air from warm tropic air is decreasing.
  • Warmer polar temperatures slows down and weakens the jet stream.
  • The jet stream can get more wavy and block weather systems in these wave traps, known as Rossby waves. 


What is it?

  • Evaporation occurs when liquid is turned into vapour like when rain puddles gradually dry up and disappear. 
  • Over the surface of the Bay, evaporation is influenced by many complex interactions between water temperatures, air temperatures, humidity, wind speed and ice coverage.   
  • A shift in balance towards more evaporation would lead to lower water levels in the future. 
  • Since the 1990s until recently, evaporation has had a greater impact on water levels than precipitation.

How does the process work?

  • Most evaporation happens in the cooler months because the air is dry and cold and the water is warmer than the air.  
  • When cool dry air sits over warm lake water, the cool air will take moisture from the warmer water (evaporation) until the air reaches 100% humidity.  When the air reaches 100% humidity, it cannot hold any more moisture which sets the stage for the possibility of precipitation.
  • If the evaporated water vapour then condenses into rain or snow, the process will be reversed. 
  • The reversal releases heat and gets the air circulating – warm air rises – which brings the cool dry air down closer to the surface of the lake.
  • This allows the evaporation cycle to continue until the water temperature drops to the point where ice forms in the winter or rises to the point where the warmer spring air melts the ice and warms up the water.

What about ice coverage?

  • Evaporation removes heat (energy) from the surface of the lake.  As the lake gets closer to freezing, evaporation stops (at 100% humidity) and ice forms.  
  • The thickness of the ice has a direct impact on the amount of evaporation that occurs over the lake. 
  • Thicker ice takes longer to melt in the spring and that means it will take the lake water longer to warm up. 
  • When the lake water is cooler, it will cause less evaporation in the following fall.
  • Thinner ice means the lake water warms up more quickly.  Warmer water means more evaporation in the following fall.  
  • The interaction of all these processes, including rainfall, makes it very difficult to predict lake levels. 
  • Ice cover has declined 71% over the past 40 years.  Even though year-to-year ice coverage will vary, the forecast is a downward trend towards zero ice coverage 40 years from now.  


Before Climate Change

  • When we refer to climate, we mean the long-term weather trends (and not the weather we see out the window each day).  Over the last century, before climate change, we had more of a water balance between precipitation and evaporation.
  • Those trends made our climate more predictable.

Climate Change begins

  • Beginning in the middle of the 20th C, human activity intensified during the post-war Industrial Revolution increasing the dumping of greenhouse gases into our atmosphere.
  • Burning fossil fuels like coal and oil has pulled sequestered carbon deposits from deep beneath the Earth’s surface and put more carbon dioxide (CO2) into our atmosphere. 
  • The permafrost in the polar regions is also melting which activates microbes releasing methane into the earth’s atmosphere.  Methane is one of the gases adding to the greenhouse effect and has 80 times the warming power of CO2
  • These accumulating gases have trapped more and more heat in the atmosphere causing the Earth to warm up.  

The impact of Climate Change on Precipitation and Evaporation

  • The warming of the earth is causing changes in precipitation and evaporation. Both of these natural processes are being modified by human activity and are expected to become more intense and less predictable. 
  • This is already happening.  Wind speeds have been measured since the 1980s and show about an 18.8% increase in the last 40 years. 
  • The results of stronger winds include more run-up on the shoreline causing low-lying areas to be overwhelmed where docks, marinas, boathouses, etc. can be found. 
  • Scientists look at models to try and predict how this activity will change water levels over the coming decades, but they do so now with less confidence.  
  • We are already seeing the impacts of changes to the delicate balance between precipitation and evaporation:

1) Changes in the balance between precipitation and evaporation will affect water levels. 

2) The speed at which water levels move between extreme highs and extreme lows will increase.

• A ray of sunshine? … Methods for estimating the hydrological cycle are always improving; both in the Great Lakes and around the globe. Scientists need to present their data within a context that is consistent and transparent. New methods should always be compared with old methods when time and resources allow. This is an ideal area for research and one that is being taken on right now by the University of Waterloo through the Great Lakes Intercomparison Project. (GLIP).

Melting polar ice caps

  • People wear light colours in the summer because light colours reflect heat away from their bodies.  In the winter, darker colours absorb heat into their bodies. 
  • The same applies to the white ice in the polar regions.  It reflects the heat from the sun away.  But when the temperature raises enough so that ice melts, exposing the surrounding darker water. 
  • That dark water absorbs more heat. The extra heat in the water then contributes even more to the melting of the ice caps.  
  • This creates an unwelcome cycle or “feedback loop”. 
  • Global warming will, over time, have a significant impact on precipitation, storms, evaporation and the other processes that affect our water levels. 

What are storm surges?

  • The wind causes disturbances when it blows the waves back and forth across a body of water.  The sloshing waves can cause the water level to go up or down 6 inches in one day.  We’ve all seen this when the West wind blows.
  • This action can even mix the upper level of warmer water with the lower level of cooler water in the lake, creating more complicated, and potentially stronger, interactions. 
  • During a storm, dropping air pressure will cause a bulge in the surface of the lake. 
  • As the storm moves across the lake, the bulge moves with it, pushing the water levels higher as it goes.  In severe storms, like a hurricane, this is called a storm surge.

What about living things?

  • Water holds oxygen, critical to all living things, but colder water holds more oxygen than warmer water.
  • Climate Change effects water temperature and that can have an impact on things living in the water. 
  • Organisms who grow and thrive in cold water, may not do well as water temperatures rise.  
  • Ecosystems have been known to bounce back from major disruptions, like an oil spill or wildfire but often experience permanent impacts.  
  • However, if the conditions that organisms require to survive are no longer present due to the warming of the planet, this could lead to habitat loss and death for those organisms used to consistently cooler water temperatures. We are even seeing many places that are becoming increasingly uninhabitable for humans. 

Next time will focus on inflow, out flows and diversions!  Riveting stuff – stay tuned!!

Online resources