Erosion by Streams
Dana Desonie, Ph.D.
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Printed: January 6, 2015
AUTHOR
Dana Desonie, Ph.D.
www.ck12.org
C HAPTER
Chapter 1. Erosion by Streams
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Erosion by Streams
• Describe different types of stream erosion.
What’s different about these landscapes?
Both of these rivers run through Yellowstone National Park. The Firehole River is a tributary of the Madison. In this
photo, it’s flowing over flat ground. The Yellowstone River on the right is cascading over Yellowstone Falls. Which
river is doing more erosion? In what direction is the stream eroding?
Erosion by Surface Water
Water that flows over Earth’s surface includes runoff, streams, and rivers. All these types of flowing water can cause
erosion and deposition.
Erosion by Runoff
When a lot of rain falls in a short period of time, much of the water is unable to soak into the ground. Instead, it
runs over the land. Gravity causes the water to flow from higher to lower ground. As the runoff flows, it may pick
up loose bits of soil and sand.
Runoff causes more erosion if the land is bare. Plants help hold the soil in place. The runoff water pictured below
( Figure 1.1) is brown because it eroded soil from a bare, sloping field. Can you find evidence of erosion by runoff
where you live? What should you look for?
Much of the material eroded by runoff is carried into bodies of water, such as streams, rivers, ponds, lakes, or oceans.
Runoff is an important cause of erosion. That’s because it occurs over so much of Earth’s surface.
Erosion by Streams
Streams erode sediment from their banks. They pick up and transport sediments.
As a stream erodes its banks, it creates a V-shaped valley ( Figure 1.2). This contrasts with the U-shaped valleys
created by glaciers.
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FIGURE 1.1
Runoff
has
eroded
small
channels
through this bare field.
FIGURE 1.2
A stream in the desert rushes past its
banks. The power of the water erodes the
cliff face.
Erosion and Water Speed
Erosion by a stream depends on the velocity of the water. Fast water erodes more material than slow water.
Eventually, the water deposits the materials. As water slows, larger particles are deposited first. As the water
slows even more, smaller particles are deposited. The graph pictured below ( Figure 1.3) shows how water velocity
and particle size influence erosion and deposition.
Erosion in the Mountains
Streams often start in mountains, where the land is very steep ( Figure 1.4). A mountain stream flows very quickly
because of the steep slope. This causes a lot of erosion and very little deposition. The rapidly falling water digs
down into the stream bed and makes it deeper. It carves a narrow, V-shaped channel.
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Chapter 1. Erosion by Streams
FIGURE 1.3
Flowing water erodes or deposits particles depending on how fast the water is
moving. It also depends on how big the
particles are.
FIGURE 1.4
This mountain stream is in Whitney Portal in the Sierra Nevada of
California. The slope is so steep that water cascades down in a waterfall.
How a Waterfall Forms
Mountain streams may erode waterfalls. A waterfall forms where a stream flows from an area of harder to softer
rock ( Figure 1.5). The water erodes the softer rock faster than the harder rock. This causes the stream bed to drop
down, like a step. This creates a waterfall. As erosion continues, the waterfall gradually moves upstream.
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FIGURE 1.5
How a waterfall forms and moves. Why
does a waterfall keep moving upstream?
Erosion by Slow-Flowing Rivers
Streams eventually run onto flatter ground. Rivers flowing over gentle slopes erode the sides of their channels more
than the bottom. Large curves, called meanders, form because of erosion and deposition by the moving water. The
curves are called meanders because they slowly “wander,” or meander, over the land. Below, you can see how this
happens ( Figure 1.6).
As meanders erode from side to side, they create a floodplain. This is a broad, flat area on both sides of a river.
Eventually, a meander may become cut off from the rest of the river. This forms an oxbow lake ( Figure 1.7).
Summary
•
•
•
•
Faster water carries more and larger sediment.
Streams erode their banks to create V-shaped valleys.
A river on flat ground meanders. When a meander is cut off it may become an oxbow lake.
A floodplain is where the extra water goes when the river is in flood.
Explore More
Use the resource below to answer the questions that follow.
• Meanders and Oxbow Lakes at http://www.youtube.com/watch?v=4qKS_Nk7UmY (2:17)
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/58882
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Chapter 1. Erosion by Streams
FIGURE 1.6
Meanders form because water erodes the
outside of curves and deposits eroded
material on the inside.
Over time, the
curves shift position.
FIGURE 1.7
An oxbow lake forms in the Mackenzie
River Delta, Canada.
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1.
2.
3.
4.
5.
6.
7.
What are meanders?
Where do meanders form?
How do meanders form?
What does the fast flowing water carry? Where does that material go?
What happens to the meanders over time?
What is an oxbow lake?
Explain how oxbow lakes form.
Review
1.
2.
3.
4.
Where along a meander does a river erode its banks? Where does it deposit sediment? Why?
How does a waterfall form?
How does stream erosion in the high mountains differ from that on flatter ground?
How does erosion by runoff differ from stream erosion?
References
1. Courtesy of the USDA Natural Resources Conservation Service. Runoff has eroded small channels through
this bare field . Public Domain
2. Image copyright Gary Whitton, 2013. A stream in the desert erodes the cliff face . Used under license from
Shutterstock.com
3. Christopher Auyeung. Graph of how particle size and water velocity affect erosion and deposition . CC
BY-NC 3.0
4. Flickr:jar. Stream flowing over a waterfall . CC BY 2.0
5. Jodi So. Diagram of how a waterfall forms . CC BY-NC 3.0
6. Christopher Auyeung. Diagram of how meanders form and move . CC BY-NC 3.0
7. Courtesy of GSFC/METI/ERSDAC/JAROS, and the US/Japan ASTER Science Team. On oxbow lake along
a river . Public Domain
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