Grade by Grade
E. Earth and Space Systems · Water Systems

Watersheds: The Land That Drains to Your Water

E2.2 A1.2 A3.1 A3.2About 50 minutes
Learning goals
  • I can explain what a watershed is and identify its divide, tributaries and outlet.
  • I can explain why anything that happens on land in a watershed affects the water downstream.
  • I can explain why the watershed is used as the unit for water management and planning in Ontario.
  • I can build and test a model watershed.

Words to know

watershed
An area of land where all the water from rain and melting snow drains to the same place, such as a creek, river or lake. Another name for a large one is a drainage basin.
divide (height of land)
The high ground that separates one watershed from the next. Rain on one side flows one way; rain on the other side flows another way.
tributary
A smaller stream or river that flows into a larger one.
headwaters
The streams, springs and wetlands where a river begins, at the high part of a watershed.
outlet (mouth)
Where the water leaves a watershed, for example where a river flows into a lake.
floodplain
Flat land beside a river that is covered with water when the river floods.
conservation authority
A local agency in Ontario that manages and protects the water and natural resources of one or more watersheds.
source water
Untreated water in lakes, rivers and aquifers that is used for drinking water.

Remember from Grade 7

In Grade 7 geography you learned that all the land draining to one ocean is a drainage basin, and that Canada has five of them. A watershed is the same idea at any size. Your local creek has a watershed. That creek's watershed is part of a river's watershed, which is part of a Great Lake's watershed, which is part of the huge Atlantic drainage basin. Watersheds fit inside each other like nesting dolls.

Atlantic Ocean drainage basina Great Lake's watershed (e.g. Lake Ontario)a river's watershedyour local creek's watershedcreek
Watersheds nest like dolls (not to scale): your local creek's watershed is inside a river's watershed, which is inside a Great Lake's watershed, which is part of the Atlantic Ocean drainage basin.

A puzzle

On a rainy day, two raindrops land 10 metres apart on top of a hill. One ends up in Lake Huron. The other ends up in Lake Ontario. How can that be? The top of the hill is a divide, and the two raindrops fell into different watersheds.

Parts of a watershed

  • The divide is the boundary, along the highest ground.
  • Headwaters (springs, wetlands, small streams) are high up in the watershed.
  • Small tributaries join to make bigger streams and a main river.
  • Water leaves at the outlet, such as a lake or a larger river.
  • Water also moves underground: groundwater flows slowly toward streams and lakes and keeps them running in dry weather.

A watershed is not only water. It includes all the land: forests, farms, towns, roads, wetlands and the people and animals who live there.

View from a hilltop over green farmland and woods, with a small winding creek flowing toward a larger river.
The creek is a tributary. All the land that drains into it is its watershed, which is part of the larger river's watershed. AI-generated image
dividetributarymain riverlakeABnot to scale
A schematic watershed map (not to scale). The dashed line is the divide. Rain at point A, inside the divide, runs into a tributary, then the main river, then the lake (the outlet). Rain at point B, outside the divide, drains into a different watershed.

Why everything upstream matters

Water flows downhill and carries things with it. Whatever happens on the land in a watershed (fertilizer on a field, salt on a road, oil from a parking lot, soil from a construction site) can wash into the streams and end up downstream. A town at the outlet drinks water that has crossed the whole watershed.

That is why scientists say: we all live downstream.

The watershed: the basic unit for water management

Water does not stop at town or county borders, but it does stay inside a watershed. So in Ontario, water is managed by watershed.

  • Conservation authorities. Ontario has had 36 conservation authorities, each responsible for one or more watersheds. They map floodplains, issue flood warnings, run dams and reservoirs, monitor water quality and quantity, plant trees and protect wetlands. In 2026, Ontario passed a law to combine them into nine larger regional conservation authorities, with the change planned to start in 2027. People disagree about whether this will help or hurt local watershed management, but the new authorities are still meant to manage water across watersheds.
  • Flood planning. In October 1954, Hurricane Hazel caused deadly floods in the Toronto area, and 81 people died. Many homes had been built on floodplains. Afterwards, Ontario strengthened watershed-based flood control: in many places, new homes are no longer allowed on floodplains, and land along rivers became parks.
  • Protecting drinking water at the source. After the Walkerton tragedy in 2000, Ontario passed the Clean Water Act (2006). Local source protection plans, organized by watershed, protect the lakes, rivers and aquifers that supply drinking water.
  • Planning. Towns use watershed studies to decide where to build, how to handle stormwater, how much water can safely be taken, and which wetlands and forests to protect.

Example: the Grand River watershed (managed by the Grand River Conservation Authority) covers about 6 800 km², a bit larger than Prince Edward Island. It is the largest watershed in southern Ontario. About a million people live in it, mostly in Kitchener, Waterloo, Guelph, Cambridge and Brantford, and the river empties into Lake Erie at Port Maitland.

Just for fun

Why was the raindrop that landed exactly on top of the divide so stressed?

No matter which way it went, it was all downhill from there.

Worked example

Let's trace a spill through a watershed

In the map above, a truck spills fertilizer near the top of the tributary on the right. Where will the fertilizer go, and who should be told?

  1. Water flows downhill from the spill into the tributary.
  2. The tributary joins the main river downstream.
  3. The main river flows into the lake, the outlet of the watershed.
  4. Anyone who takes water from the tributary, the main river below the spill, or the lake could be affected. Places upstream of the spill or outside the divide are not affected by surface flow.
  5. The spill should be reported right away (in Ontario, to the provincial Spills Action Centre). Downstream water treatment plants and the conservation authority can then act.

The fertilizer travels tributary → main river → lake. Everyone downstream in the same watershed should be warned.

Hands-on activity

Build a crumpled-paper watershed (A1.2)

You need: a sheet of paper, washable markers (one blue, one other colour), a spray bottle of water (or a sprinkle of water from your fingers), a baking tray or a sheet of plastic underneath.

  1. Crumple the paper into a ball, then open it partly so it has ridges and valleys like hills. Set it on the tray.
  2. With the blue marker, trace along the ridges where you think the divides are (the high points).
  3. Predict: where will water collect? Mark your predictions with an X.
  4. With the other marker, put dots on the ridges and slopes to stand for pollution (fertilizer, oil, road salt).
  5. Gently spray "rain" over the paper. Watch where the water and colour flow.
  6. Record: were your predictions right? Which "lakes" got the most pollution? Did any water cross a divide?
  7. Extend: find your own watershed. Search your conservation authority's website, or ask a librarian for a local watershed map. Which creek or river does the water from your street drain to? Which Great Lake (or which other water body) does it reach?

What to look for: water runs down from the ridges into the valleys; colour from every slope ends up in the low spots. The child should connect this to real pollution moving downstream.

Safety: use washable markers only; wipe up spills.

Your turn

1

What is a watershed?

2

What is a divide?

3

Put these in order from smallest to largest watershed: Lake Ontario's watershed, your local creek's watershed, the Atlantic Ocean's drainage basin.

4

Why can fertilizer spread on a field end up in a lake many kilometres away?

5

Name three jobs that Ontario conservation authorities do.

6

Why is water managed by watershed instead of by town?

7

What happened in Toronto in 1954, and what changed afterwards?

More practice: warm-up

1

What is a tributary?

2

Where are headwaters found?

3

What is the outlet of a watershed?

4

Into which Great Lake does the Grand River flow?

5

True or false: a watershed includes only the rivers and lakes.

More practice: on your own

1

Two neighbours live on opposite sides of a hilltop divide. One pours paint thinner on the ground. Whose watershed is affected? Explain.

2

Why are wetlands in the headwaters important to the whole watershed?

3

What is source water protection, and why did Ontario make it law?

4

A city wants to build houses on a river's floodplain. Give one reason a conservation authority might say no.

5

The Grand River watershed is about 6 800 km². If about 1 000 000 people live in it, about how many people live per km²? (Round.)

Challenge

1

Explain how paving a large area in the headwaters could cause more flooding downstream.

2

Use the idea "we all live downstream" to explain why towns in the same watershed should plan together.

3

In 2026, Ontario decided to combine 36 conservation authorities into nine regional ones. Give one possible benefit and one possible concern, and say what evidence you would want before deciding.

Check your understanding

Quiz

1.

A watershed is...

2.

The high ground that separates two watersheds is called a...

3.

Why is the watershed used as the unit for water management in Ontario?

4.

After which event did Ontario pass the Clean Water Act (2006) to protect sources of drinking water?

5.

Which of these is NOT usually a job of an Ontario conservation authority?

6.

A smaller stream that joins a larger river is a...

For parents and teachers

Big idea (E2.2): a watershed is all the land draining to one outlet; divides separate watersheds; smaller watersheds nest inside larger ones. Because water crosses political borders but stays within a watershed, Ontario manages flooding, source water and planning by watershed through conservation authorities (A3.1 connects to jobs such as water resources technicians and planners; A3.2 connects to geography and math).

How to help: look up your local conservation authority together and find your watershed map. Walk to the nearest storm drain and talk about where that water goes. Do the crumpled-paper model.

Common difficulties:

  • Thinking a watershed is only water. It's all the land too.
  • Not seeing nesting. Use the nesting-doll comparison.
  • Thinking storm drains go to a treatment plant. In most places they flow straight to creeks.

Current events note: Ontario's plan (Bill 97, passed in spring 2026) to consolidate its 36 conservation authorities into nine regional authorities was planned to begin in 2027. Check your local authority's website for the latest.

Sources: Grand River Conservation Authority (6 800 km², about one million people, outlet at Port Maitland); Toronto and Region Conservation Authority on Hurricane Hazel; Ontario Clean Water Act, 2006.