Grade by Grade
A. Physical Patterns in a Changing World · Physical Patterns in a Changing World

Water Bodies and Systems: Patterns and Changes

A3.4 A3.5About 50 minutes
Learning goals
  • I can describe the patterns and physical characteristics of oceans, lakes, rivers and drainage basins around the world.
  • I can explain what a drainage basin is and name Canada's ocean drainage basins.
  • I can describe natural processes and human activities that change water bodies and systems.

Words to know

drainage basin
All the land that drains into one river, lake or ocean. Also called a watershed.
divide
High land that separates one drainage basin from another.
tributary
A smaller river or stream that flows into a bigger river.
mouth
The place where a river ends by flowing into a lake, sea or ocean.
source
The place where a river begins.
aquifer
A layer of rock, sand or gravel underground that holds groundwater.
reservoir
A lake made by building a dam across a river.
irrigation
Bringing water to crops through canals, pipes or sprinklers.

Where does your water go?

When it rains on your street, where does the water end up? It flows into a storm drain, then a creek or river, then maybe a lake, and finally an ocean. If you live in southern Ontario, that rain could travel through the Great Lakes and down the St. Lawrence River all the way to the Atlantic Ocean!

All the land that drains to one place is a drainage basin. Every home in Canada is in one.

A small creek flows out of a forest and across a sandy beach into a wide blue Great Lake.
Water always flows downhill toward a larger water body. This creek drains into a Great Lake, which drains into the St. Lawrence River and on to the Atlantic. AI-generated image

Earth's water: patterns

  • About 97% of Earth's water is salt water in the oceans. Only about 3% is fresh water, and most of that is frozen in ice sheets and glaciers or stored underground as groundwater.
  • The five oceans are the Pacific (the largest), Atlantic, Indian, Southern and Arctic (the smallest). Canada touches three: the Pacific, the Atlantic and the Arctic.
  • Lakes are often found where glaciers dug basins (like Canada's Shield lakes and the Great Lakes) or where plates pulled apart (like the deep lakes of the Great Rift Valley in Africa). Lake Superior has the largest surface area of any freshwater lake on Earth.
  • Rivers flow downhill from their source (often mountains or hills) to their mouth. Two of the world's longest rivers are the Nile, which flows north through Africa to the Mediterranean Sea, and the Amazon, which carries more water than any other river. The Mackenzie is Canada's longest river and flows north to the Arctic Ocean.
97 squares: salt water(oceans)3 squares: fresh waterMost of it is frozen in iceand glaciers or storedunderground.If all of Earth's water were 100 squares...
If Earth's water were 100 squares, about 97 would be salty ocean and only about 3 would be fresh water, and most of those 3 are frozen or underground.
dividetributarymain rivermouthoceandrainage basinnot to scale
A drainage basin: rain that falls inside the divide (dashed line) flows into tributaries, then the main river, which empties into the ocean at its mouth. Not to scale.

Drainage basins

A divide is high land that sends water one way or the other. Everything inside the divide drains to the same river. Small tributaries join to make bigger rivers, like branches joining a tree trunk.

Canada has five ocean drainage basins, named for where the water ends up: the Atlantic, Hudson Bay, Arctic, Pacific and Gulf of Mexico basins. The Hudson Bay basin covers a huge part of the middle of Canada, including northern Ontario. Most of southern Ontario drains to the Atlantic through the Great Lakes and the St. Lawrence.

Why does this matter? Anything that goes into the water upstream, like fertilizer, road salt or garbage, flows downstream to other communities in the same basin.

How water systems change

Natural processes:

  • Changes in rainfall: droughts shrink lakes and rivers; heavy rain causes floods.
  • Melting glaciers: many mountain glaciers, including the Athabasca Glacier in the Rockies, are getting smaller. Rivers fed by glaciers may have less water in late summer in the future.
  • Rising sea levels: as the climate warms, ice on land melts and ocean water expands, so sea levels rise and low coasts can flood.
  • Deposition: rivers build deltas and fill in parts of lakes.

Human activities:

  • Dams hold back rivers to make reservoirs for hydroelectricity, drinking water and flood control. They change the river downstream and flood land upstream.
  • Irrigation takes water from rivers and aquifers for crops. The Aral Sea in Central Asia was once one of the world's largest lakes. Starting in the 1960s, the rivers that fed it were diverted to irrigate cotton fields, and most of the lake dried up.
  • Pumping groundwater from aquifers for cities, farms and bottled water can lower water levels faster than rain refills them.
  • Canals and pollution: canals connect water bodies; pollution from farms, factories and cities harms water quality.
Worked example

Let's trace a drainage path

A raindrop falls on a farm near London, Ontario, and runs into the Thames River. Trace its path to the ocean.

  1. The Thames River flows southwest into Lake St. Clair.
  2. Lake St. Clair drains through the Detroit River into Lake Erie.
  3. Lake Erie drains through the Niagara River (over Niagara Falls) into Lake Ontario.
  4. Lake Ontario drains into the St. Lawrence River, which flows northeast past Montréal and Québec City.
  5. The St. Lawrence empties into the Gulf of St. Lawrence and the Atlantic Ocean.

The raindrop is in the Atlantic drainage basin: Thames River → Lake St. Clair → Detroit River → Lake Erie → Niagara River → Lake Ontario → St. Lawrence River → Atlantic Ocean.

Just for fun

Funny but true: Lake Superior holds so much water that it could cover all of North and South America about 30 cm deep. That's one very big puddle.

Hands-on activity

Crumpled-paper watershed

You need: a sheet of paper, washable markers, and a spray bottle of water. Do this on a tray or baking sheet.

  1. Crumple the paper into a ball, then open it partly so it has ridges and valleys. Put it on the tray.
  2. With a blue marker, trace where you think water will flow. With a dark marker, trace the ridges (divides).
  3. Draw a few dots of another colour on the high parts. These are "pollution" (like fertilizer or road salt).
  4. Gently spray water over the whole map, like rain. Watch where the water and colours go.
  5. Label one drainage basin, a divide, a tributary and the mouth (where water collects at the lowest point).

What to look for: your child sees that water flows from ridges into valleys, joins together, and carries "pollution" downstream into the low areas.

Your turn

1

About what percent of Earth's water is salt water?

2

Name the five oceans. Which is the largest?

3

What is a drainage basin?

4

What is the job of a divide?

5

Name Canada's five ocean drainage basins.

6

What happened to the Aral Sea, and why?

7

Give one natural and one human change to water systems.

More practice: warm-up

1

Which river is Canada's longest?

2

Which freshwater lake has the largest surface area on Earth?

3

What is a tributary?

4

What is a lake made by a dam called?

5

What is underground water stored in rock, sand or gravel called?

More practice: on your own

1

Why is only a small part of Earth's water easy for people to use?

2

A factory upstream dumps waste into a river. Explain how this could affect a town downstream in the same drainage basin.

3

Explain two reasons sea levels are rising.

4

Give one benefit and one drawback of building a dam.

5

Which Canadian ocean drainage basin does most of southern Ontario belong to? Explain the route.

Challenge

1

Mountain glaciers are shrinking. Explain how this could affect people who live downstream on a glacier-fed river, now and in the future.

2

The Aral Sea is used as a warning. What lesson does it teach about using water systems?

3

Using spatial thinking, explain why water is shared across borders, using an example.

Check your understanding

Quiz

1.

All the land that drains into one river or lake is called a...

2.

About how much of Earth's water is fresh water?

3.

Which is the largest ocean?

4.

Most of southern Ontario drains to the...

5.

The Aral Sea shrank mainly because...

6.

High land that separates two drainage basins is a...

For parents and teachers

Big idea: Expectations A3.4 and A3.5 ask students to describe the patterns and characteristics of major water bodies and systems (oceans, lakes, rivers, drainage basins) and the natural processes and human activities that change them (glacier melt, sea-level rise, dams, irrigation, groundwater pumping).

How to help: Find out which drainage basin (watershed) you live in; most Ontario conservation authorities post watershed maps online. Trace your local creek's path to the ocean on a map. Talk about what goes down storm drains.

Common difficulties:

  • Thinking rivers flow "down" to the south: rivers flow downhill, which can be any direction. The Mackenzie and the Nile flow north.
  • Mixing up source and mouth: a river starts at its source and ends at its mouth.
  • Thinking fresh water is plentiful everywhere: only a small share of Earth's water is easy-to-use fresh water.