Grade by Grade
E. Earth and Space Systems · Soils in the Environment

Erosion: When Soil Moves Away

E2.4 A1.2 A1.4 A1.5About 45 minutes
Learning goals
  • I can explain what erosion is.
  • I can name causes of erosion: water, wind, ice, gravity, and people and animals.
  • I can describe how erosion harms soil and water.
  • I can do a fair test to show how plants protect soil from erosion.

Words to know

erosion
When soil or bits of rock are carried away from one place to another by water, wind, ice or gravity.
weathering
When rock is broken into smaller pieces. (Erosion moves the pieces away.)
runoff
Rain or melted snow that flows over the top of the ground instead of soaking in.
gravity
The pull that makes things fall or slide down toward the ground.
gully
A deep ditch cut into the ground by running water.
sediment
Bits of soil and rock carried by water and dropped somewhere else.

Where did the soil go?

It rained hard all night. In the morning, Zoe sees a muddy stripe of soil on the sidewalk below her family's garden. Some of the garden soil is gone! The water in the street gutter looks brown.

What happened? The soil was moved by water. This is called erosion.

A garden bed after heavy rain, with muddy soil washed out across a wet sidewalk in streams.
Rainwater carried soil from the garden and dropped it on the sidewalk. This moving of soil is erosion. AI-generated image

What is erosion?

Erosion is when soil or bits of rock are carried away from one place and dropped somewhere else.

Remember weathering from the first lesson? Weathering breaks rock into small pieces. Erosion moves the pieces away. A good way to remember: weathering breaks, erosion takes.

WEATHERING BREAKSEROSION TAKESbig rock breaks intosmaller pieceswater or wind carriesthe pieces awaywater
Remember: weathering breaks rock into pieces, and erosion takes the pieces away.

Causes of erosion

  • Water. Raindrops hit bare soil and splash bits loose. Water running downhill (runoff) carries soil with it. In spring, lots of melting snow runs off. Rivers wear away their banks, and waves wear away lake shores. Along parts of the Great Lakes, like the Scarborough Bluffs in Toronto, waves slowly wear away the cliffs.
  • Wind. Wind picks up dry, loose, bare soil and blows it away. In the 1930s, the Canadian Prairies had years with very little rain. Strong winds blew away huge amounts of dry farm soil. People called those years the "Dirty Thirties".
  • Ice. Long ago, giant glaciers scraped soil and rocks and pushed them across Ontario. Today, ice in rivers in spring can scrape banks too.
  • Gravity. On steep hills, soil and rocks can slide down, especially when the soil is soaking wet.
  • People and animals. Many feet walking the same path wear it down. Cutting down plants and trees, and digging for building, leave soil bare, so it erodes faster.
bare soil washes awayplants hold the soil
Rain falls on two hills. On the bare hill, water runs down and carries soil to the bottom. On the hill covered with plants, the roots hold the soil and the plants slow the water.

Why plants are soil protectors

Bare soil erodes much faster than soil covered with plants.

  • Roots hold soil together like a net.
  • Leaves and stems soften raindrops, so they don't splash soil loose.
  • Plants slow down running water and wind.
  • Fallen leaves and mulch cover the soil like a blanket.

Why is erosion a problem?

Erosion is a natural process. It happens slowly all the time. But when it happens too fast, especially because of people, it causes problems:

  • The topsoil, the richest layer, is the first to go. Without it, plants grow poorly. Remember, it takes hundreds of years to make new soil.
  • Nutrients and humus are carried away too.
  • Running water can cut gullies (deep ditches) into fields.
  • The eroded soil ends up in creeks, rivers and lakes as sediment. It makes the water muddy. Mud can cover the places where fish lay their eggs and make it hard for fish and water plants to live.
  • Sediment can fill up ditches and drains, so they flood.
Worked example

Let's do a fair test: Do plants stop erosion?

Owen asks: "Does covering soil with plants protect it from erosion?" How can he test it fairly?

  1. Prediction: "The covered soil will lose less soil, because roots hold it."
  2. He uses two baking pans the same size, filled with the same amount of the same soil. On Pan A he leaves the soil bare. On Pan B he lays a piece of grass sod (or a thick layer of leaves) on top. Only the cover changes.
  3. He props both pans at the same slope on a book, with the low end hanging over a clear container.
  4. He pours 500 mL of water from a watering can, from the same height, onto the top of each pan.
  5. Pan A (bare): the water in the container is very muddy, with a layer of soil on the bottom.
  6. Pan B (covered): the water is almost clear, with very little soil.

Plants protect soil from erosion. The bare soil washed away much more. Owen's prediction was supported.

Worked example

Let's explain: Why is the river brown after a storm?

After a big storm, Hana sees that the river near her home is brown instead of clear. Why?

  1. Heavy rain made lots of runoff.
  2. The runoff flowed over fields, construction sites and bare ground and carried soil with it.
  3. The soil washed into the river as sediment, so the river looks brown and muddy.
  4. Later, when the water slows down, the sediment settles to the bottom.

The river is brown because erosion washed soil into it. That soil was lost from the land, and it can harm fish.

Stay safe, scientist!

  • Do the erosion test outside or in a sink or bathtub, so mud doesn't make a mess.
  • If you test wind by blowing through a straw onto dry sand, blow gently, point the straw away from faces, and wear sunglasses or safety goggles. Never blow sand toward someone.
  • Stay away from the edges of riverbanks and cliffs, especially after rain. Eroding ground can collapse.
Just for fun

Why did the soil leave the hill without saying goodbye?

It got carried away!

Hands-on activity

Erosion experiment

You need: 2 baking pans or two plastic food containers with one short end cut off (a grown-up cuts), soil, a piece of grass sod or lots of fallen leaves, a book, 2 clear containers, a measuring cup, a watering can or a cup with holes poked in the bottom (a grown-up pokes them).

  1. Fill both pans with the same amount of soil. Cover one with sod or leaves. Leave the other bare.
  2. Prop each pan on a book so it slopes. Put a clear container under the low end.
  3. Predict which pan will lose more soil.
  4. Pour 500 mL of water slowly over the top of each pan.
  5. Compare the water in the containers. Let it settle for an hour and look at the layer of soil on the bottom.
  6. Draw and label your results. Use the words erosion, runoff and roots.

Extra (wind): Put a spoonful of dry sand and a spoonful of damp sand on a tray. Blow gently through a straw at each one (goggles on!). Which one blows away more easily?

Safety: Wear gloves, or wash your hands with soap when you are done.

Your turn

1

What is erosion?

2

Name three causes of erosion.

3

What is the difference between weathering and erosion?

4

How do plant roots help stop erosion?

5

Why is losing topsoil a big problem?

6

Which soil blows away more easily: dry, bare soil or damp soil covered with plants?

7

How can erosion harm fish?

More practice: warm-up

1

True or false: Wind can cause erosion.

2

What is rain that flows over the ground called?

3

Which erodes faster: bare soil or soil with grass?

4

What is a deep ditch cut by running water called?

5

Name one thing that can cover soil like a blanket to protect it.

More practice: on your own

1

Why does a lot of erosion happen in spring in Ontario?

2

A path across a school field has no grass and is lower than the field. What caused this?

3

How does gravity cause erosion?

4

In Owen's test, what was the one thing he changed? Name two things he kept the same.

5

What happened in the Dirty Thirties on the Prairies?

Challenge

1

Two pans lost soil in a test. The bare pan lost 9 spoonfuls of soil. The pan with leaves lost 2. How many more spoonfuls did the bare pan lose? What does this show?

2

A builder clears all the plants from a hill before building houses. A big rainstorm is coming. What might happen, and what could the builder do?

3

Why is erosion sometimes called a problem "for land and water at the same time"?

Check your understanding

Quiz

1.

Erosion is when soil is...

2.

Which is not a cause of erosion?

3.

Which hill will lose the most soil in a rainstorm?

4.

Weathering ___ rocks. Erosion ___ the pieces.

5.

Why can a river look brown after a storm?

6.

Which layer of soil is lost first by erosion?

For parents and teachers

Big idea (E2.4): Erosion is the movement of soil and rock by water, wind, ice and gravity, often sped up by people and animals that remove plant cover or compact soil. It strips topsoil and nutrients and sends sediment into waterways.

How to help: The two-pan test is the heart of this lesson (A1.2). Ask your child to name what changed and what stayed the same, and to explain the results with the new words (A1.5). After a rainstorm, go for a walk and look for evidence of erosion: soil on sidewalks, small channels in bare ground, muddy puddles.

Common difficulties:

  • Mixing up weathering and erosion. "Weathering breaks, erosion takes."
  • Thinking erosion is always caused by people. It is natural, but people can speed it up a lot.

Safety (A1.4): An adult does any cutting or poking. Goggles for the wind test. Gloves or hand-washing after handling soil and sod. Stay well back from eroding banks, bluffs and fast water, especially in spring.