Daily Read / Geography and community
Geography and community · Use cause and effect and sequence to explain how glaciers shaped the Great Lakes, and compare how two sources report the same number
How Did the Great Lakes Form? Glaciers, Basins and Shared Water
Ice carved the Great Lakes. Learn which four touch Ontario, how Canada and the US share the water, and why 60 per cent of Ontarians drink from them.
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A lake you cannot see across
Stand on the beach at Toronto, Pickering or Oshawa and look south. You see water, then more water, and then the sky. You cannot see the other side, even though it is there. You are looking at Lake Ontario, one of the five Great Lakes.
The Great Lakes sit on the border between Canada and the United States. Environment and Climate Change Canada, a department of the Government of Canada, says the Great Lakes hold 18 per cent of the world's fresh lake water. That is almost one fifth of all the fresh water sitting in lakes on the whole planet.[2]
Five lakes in a chain
The five Great Lakes are Superior, Michigan, Huron, Erie and Ontario. Lake Michigan is entirely in the United States. The other four are partly in Canada, and all four touch Ontario. A much smaller lake, Lake St. Clair, sits between Lake Huron and Lake Erie. The Canadian Encyclopedia explains that it is not really a 'great lake', but it is part of the same chain.
The lakes are like a staircase of water. Lake Superior sits about 183 m above sea level. Lake Ontario, at the bottom, sits about 74 m above sea level. Water flows down from one lake to the next. The biggest drop is at Niagara Falls, between Lake Erie and Lake Ontario. From Lake Ontario, the water flows into the St. Lawrence River and on toward the ocean.
The lakes are also a highway for ships. When the St. Lawrence Seaway opened in 1959, ships could travel between the ocean and the lakes, and the Great Lakes became an international waterway.[1]
Carved by ice
Long ago, there were no Great Lakes. There were low places, called basins, in the rock. Then the glaciers came. During the last glacial period, which lasted more than a million years, huge sheets of ice spread over the land four separate times. Each time, the moving ice scraped and wore away the rock and made the basins bigger and deeper.
As the climate warmed, the ice began to melt and pull back. Water from the melting ice pooled in front of it and filled the basins. More than 14,000 years ago, early lakes in the Erie and Michigan basins drained south toward the Mississippi River. The Lake Ontario basin was free of ice by about 12,000 years ago.
The lakes did not stay the same size. When the ice melted away from the St. Lawrence Valley, water could escape that way, and the level of the lake in the Ontario basin dropped by about 150 m. Even today, the land is still slowly rising after losing the weight of all that ice. The Encyclopedia says the northern shorelines rise about 0.4 m every 100 years.[1]
Why the lakes matter to Ontario
If you live in Toronto or Durham Region, there is a good chance your tap water started in a Great Lake. The Canada-Ontario Great Lakes Agreement says that municipal water sources around the Great Lakes directly provide drinking water to 60 per cent of Ontarians. It also says the region is home to about 33 per cent of Canada's population, or about one in every three people.
The same agreement says the Great Lakes hold approximately 20 per cent of the world's surface fresh water. That is close to the federal number, 18 per cent, but not exactly the same. Different sources can measure or round a number in different ways, so a careful reader notices which source said what.
That water is not quickly replaced. The agreement says less than 1 per cent of the water is renewed each year by rain and snow. The lakes are big, but they are not a supply that refills overnight. Ontario also has the longest coastline of any province or state on the Great Lakes.[3]
Two countries, one set of lakes
Water does not stop at a border. A problem in one part of a lake can travel to another part. So in 1909, Canada and the United States signed the Boundary Waters Treaty. The treaty created the International Joint Commission, or IJC, to investigate, resolve and prevent disputes about the waters the two countries share. The IJC first met in 1912, and it has dealt with more than 100 matters raised by the two governments.
Scientists working with the IJC studied pollution in the lakes. Their study helped officials in both countries agree on clean-up actions. In 1972, Canada and the United States signed the Great Lakes Water Quality Agreement. The IJC says building sewage treatment plants led to big improvements in the water.
The agreement has been updated over the years, most recently in 2012. The Canada Water Agency says the updated agreement includes plans about invasive species, habitat and climate change. It also says the two countries will work with provinces, states, First Nations, Métis, Tribes, cities and towns. Canada and Ontario have their own agreements too, and have worked together on the Great Lakes since 1971.[4]
Fast lesson
Use these checks before you answer the practice questions.
Look at a map together. Trace the water from Lake Superior to Lake Huron, then Lake Erie, over Niagara Falls into Lake Ontario and out the St. Lawrence River. Ask which lake is entirely in the United States.
Press your fist into a tray of damp sand and drag it slowly to show ice scraping a basin. Then pour a little water into the hollow to show meltwater filling it. Say clearly that this is a quick model; real ice sheets moved over thousands of years.
Write 18 per cent and about 20 per cent side by side with the name of each source. Talk about why careful readers keep the source next to a number.
When discussing the treaty and agreements, focus on what they set up and how the two countries work together, not on judging any government.
Common mistakes
Why this fails: Lake Michigan is entirely in the United States; only Superior, Huron, Erie and Ontario are partly in Canada.
Why this fails: The moving ice wore away rock to deepen the basins, and meltwater from the retreating ice filled them.
Why this fails: Sources may word or round differently; note which source said what before judging.
Word Lab
Use the meaning, hear the word, then try it in your own sentence.
Great Lakes
proper nounthe five large connected lakes on the Canada–United States border: Superior, Michigan, Huron, Erie and Ontario
Four of the Great Lakes touch Ontario.
basin
nouna low, bowl-shaped area of land or rock that can hold water
Glacial ice made the lake basins deeper.
glacier
nouna very large, slow-moving mass of ice
A glacier scraped the rock as it moved.
retreat
verbto move back; for a glacier, to melt back from the land it covered
Lakes formed in front of the retreating ice.
fresh water
nounwater that is not salty, like the water in most lakes and rivers
The Great Lakes hold a large share of the world's fresh water.
border
nounthe line where one country, province or state ends and another begins
Lake Ontario is on the border between Canada and the United States.
treaty
nouna formal written agreement between countries or nations
The Boundary Waters Treaty was signed in 1909.
agreement
nouna plan or decision that two or more groups accept and promise to follow
The 1972 agreement focused on water quality.
pollution
nounharmful material that makes air, water or land dirty or unsafe
Scientists studied pollution in the lakes.
municipal
adjectivebelonging to or run by a city or town
Municipal water plants take water from the lake.
waterway
nouna river, lake or canal that ships and boats can travel on
The Seaway made the lakes an international waterway.
per cent
nounout of every 100; 60 per cent means 60 out of every 100
About 60 per cent of Ontarians get drinking water from sources around the Great Lakes.
Check your understanding
Choose an answer, explain your choice, and then check. Hints and retries are welcome.
Try the questions here. Public practice does not save a learning record.
Support · Find it in the text
Support · Find it in the text practice
Each answer is stated in the article. Point to the sentence.
1. Name the five Great Lakes.
2. Where is the biggest drop between the lakes?
3. In what year did the St. Lawrence Seaway open?
4. About when was the Lake Ontario basin free of ice?
5. What share of Ontarians get drinking water directly from municipal sources around the Great Lakes?
6. In what year did Canada and the United States first sign the Great Lakes Water Quality Agreement?
Apply · Ice, water and people
Apply · Ice, water and people practice
Explain how the lakes formed and why they matter.
7. Why did the lake level in the Ontario basin drop by about 150 m?
8. Why is the land on the northern shores still rising today?
9. Why does water flow from Lake Superior toward Lake Ontario, and not the other way?
10. Why do Canada and the United States need to work together on the Great Lakes?
11. Lake Superior is about 183 m above sea level and Lake Ontario is about 74 m. How much lower is Lake Ontario?
12. The Boundary Waters Treaty was signed in 1909 and the IJC first met in 1912. How many years apart were these events?
Reason · Chains and sources
Reason · Chains and sources practice
Build cause chains and think about evidence.
13. Build a cause chain with four links that starts with 'ice sheets spread over the land' and ends with 'the Great Lakes'.
14. Sort into Land and ice, Water, or People: glacial ice, Niagara Falls, Boundary Waters Treaty, basins, drinking water for Ontarians, St. Lawrence Seaway.
15. A classmate says, 'The Great Lakes are so big, we can never run out of clean water.' Use the article to respond.
16. How are the Boundary Waters Treaty (1909) and the Great Lakes Water Quality Agreement (1972) alike and different?
17. Why does the article say which source gave the 18 per cent number and which gave about 20 per cent?
Practice answer key
- 1. Superior, Michigan, Huron, Erie and Ontario.
See 'Five lakes in a chain'. - 2. At Niagara Falls, between Lake Erie and Lake Ontario.
See 'Five lakes in a chain'. - 3. 1959.
See 'Five lakes in a chain'. - 4. About 12,000 years ago.
See 'Carved by ice'. - 5. 60 per cent.
See 'Why the lakes matter to Ontario'. - 6. 1972.
See 'Two countries, one set of lakes'. - 7. When the ice melted away from the St. Lawrence Valley, the water had a new way to escape, so the level dropped.
Opening a new outlet lets water drain out. - 8. The land is slowly rising after losing the heavy weight of the ice that once pressed on it.
Removing a heavy load lets the land rise back slowly. - 9. Lake Superior is higher (about 183 m above sea level) than Lake Ontario (about 74 m), and water flows downhill.
Compare the two heights. - 10. The lakes sit on the border and water does not stop at a border, so a problem in one part of a lake can affect the other country.
Shared water means shared problems. - 11. About 109 m (183 − 74 = 109).
Subtract the smaller height from the larger. - 12. 3 years (1912 − 1909 = 3).
Subtract the earlier year from the later year. - 13. Ice sheets spread over the land → the moving ice scrapes rock and deepens the basins → the ice melts and retreats → meltwater fills the basins, forming the Great Lakes.
Each link should cause the next. - 14. Land and ice: glacial ice, basins. Water: Niagara Falls. People: Boundary Waters Treaty, drinking water for Ontarians, St. Lawrence Seaway.
Some facts could fit two groups; explain your choice. - 15. The lakes are big, but less than 1 per cent of the water is renewed each year, and the article describes pollution that needed a clean-up, so the water still needs care.
Use two pieces of evidence. - 16. Alike: both are agreements between Canada and the United States about shared water. Different: the treaty set up the IJC to handle disputes; the 1972 agreement focused on cleaning up and protecting water quality.
Use 'both' for the similarity and 'but' for the difference. - 17. So readers can check each number and see that the difference comes from different sources and wording, not from a mistake in the article.
Naming sources lets readers check the evidence.
Related skills
Put it in your own words
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Check your own explanation
This is self-review, not automatic marking.
Compare with a model response
The Great Lakes are five connected lakes on the border between Canada and the United States. Lake Superior, Lake Huron, Lake Erie and Lake Ontario all touch Ontario, but Lake Michigan is entirely in the United States. First, huge sheets of ice spread over the land and scraped the rock, so the basins became bigger and deeper. Then, as the ice melted and retreated, meltwater filled the basins and made the lakes. The lakes matter to Ontario because, according to the Canada-Ontario Great Lakes Agreement, they directly provide drinking water to 60 per cent of Ontarians. Ships also use them as a waterway. Because the water is shared, Canada and the United States signed the Boundary Waters Treaty in 1909, which created the International Joint Commission to help resolve and prevent disputes about the water.
Sources and revision notes
Source check: 2026-11-03. Grade guidance is an editorial suggestion, not a standardised reading score.
- The Canadian Encyclopedia — Great Lakes
- Environment and Climate Change Canada — Water sources: lakes
- Government of Ontario — Canada-Ontario Great Lakes Agreement
- International Joint Commission — History of the IJC
- Canada Water Agency — Canada-US Great Lakes Water Quality Agreement
What changed in this edition?
The Canadian Encyclopedia supports: Lake Michigan entirely in the United States; Lake St. Clair not properly a 'great lake' but part of the chain; elevations from 183 m (Superior) to 74 m (Ontario) with the most dramatic drop at Niagara Falls; the St. Lawrence connection; the Seaway opening in 1959 making the lakes an international waterway; basins as bedrock depressions eroded by glacial ice over a glacial period of more than a million years with four glacial stages that each enlarged the basins; lakes forming in front of retreating ice in the Erie and Michigan basins and draining south to the Mississippi more than 14,000 years ago; the Ontario basin ice-free about 12,000 years ago; a drop of about 150 m in the Ontario basin when the St. Lawrence Valley opened; postglacial uplift of about 0.4 m per century on northern shorelines. Environment and Climate Change Canada supports 18 per cent of the world's fresh lake water, the lakes straddling the border, and Superior, Huron, Erie and Ontario being partly or entirely in Canada (Michigan not). The Canada-Ontario Great Lakes Agreement supports 60 per cent of Ontarians' drinking water, about 33 per cent of Canada's population, approximately 20 per cent of the world's surface fresh water, less than 1 per cent renewed annually by precipitation, Ontario's longest coastline and Canada-Ontario cooperation since 1971. The IJC history page supports the 1909 treaty creating the IJC to investigate, resolve and prevent boundary water disputes, its first meeting in 1912, more than 100 matters resolved, its scientific study helping officials in both countries agree on clean-up actions, and substantial improvements from sewage treatment plants. The Canada Water Agency page supports the 1972 signing, the 2012 amendment with annexes on aquatic invasive species, habitat and species, and climate change impacts, and work with state, provincial, tribal, First Nations, Métis and municipal partners. The beach opening, 'staircase of water', 'highway for ships' and 'one in every three' are descriptive framing; 'all four touch Ontario' reflects that Ontario is the only province on these lakes' Canadian shores. Questions e05 and f01–f04 ask for reasoned inferences beyond the sources.