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Section 6.2
Page 164
6.2 The Work of Streams
1 FOCUS
Section Objectives
6.6
6.7
6.8
6.9
6.10
Explain how streams erode
their channels and transport
sediments.
Explain how stream deposition
occurs.
Identify the two general types
of stream valleys.
Predict the causes of floods
and describe major flood
control measures.
Explain the relationship
between streams and drainage
basins.
Key Concepts
How do streams erode
their channels and
transport sediment?
◆
What causes floods, and
what are the major flood
control measures?
◆
◆
◆
Reading Strategy
Monitoring Your Understanding Preview
the Key Concepts, topic headings, vocabulary,
and figures in this section. List two things you
expect to learn about each. After reading, state
what you learned about each item you listed.
What I Expect to Learn
What I Learned
S
treams are Earth’s most important agents of erosion. They can
downcut or erode their channels. They can also transport enormous
amounts of sediment. Most of the sediment a stream carries comes
from weathering. Weathering produces huge amounts of material that
are delivered to the stream by sheet flow, mass movements, and
groundwater. Eventually, streams drop much of this material to create
many different depositional features.
L2
Erosion
Streams generally erode their channels lifting loose particles by
abrasion, grinding, and by dissolving soluble material. When the
flow of water is turbulent enough, it can dislodge loose particles from
the channel and lift them into the moving water. In this manner, the
force of running water rapidly erodes some streambeds and banks.
The stronger the current is, the more erosional power it has and the
more effectively the water will pick up particles.
Sand and gravel carried in a stream can erode solid rock channels
like sandpaper grinds down wood. Moreover, pebbles caught in
swirling stream currents can act like cutting tools and bore circular
“potholes” into the channel floor.
L2
What are three ways that streams erode their
channels?
Erosion
L1
Refer to p. 334D in Chapter 12, which
provides guidelines for outlining.
Outline Have students outline the
section, leaving room for notes. Then
have students scan through each heading
and find the main idea. Allow students
to refer to their outlines when answering
the questions in Section 6.2 Assessment.
Logical, Verbal
Chapter 6
◆
bed load
capacity
alluvium
delta
natural levee
floodplain
flood
drainage basin
divide
What is the relationship
between a stream and
a drainage basin?
2 INSTRUCT
164
◆
What are the two types
of stream valleys?
Answers will vary. Sample answer:
What I Expect to Learn—how erosion
happens and what features it can form
What I Learned—streams erode by
lifting loose particles and by abrasion
meanders
Build Reading Literacy
◆
◆
Paraphrase Ask students to write the
vocabulary terms on a sheet of paper.
Instruct students to write a definition, in
their own words, for each term as they
encounter the term while going through
the chapter. After writing their own
definition, encourage students to write
the term in a complete sentence.
Reading Strategy
◆
How does stream
deposition occur?
Reading Focus
Build Vocabulary
Vocabulary
164 Chapter 6
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Sediment Transport
Sediment Transport
Integrate Chemistry
Streams transport sediment in three ways.
1. in solution (dissolved load)
2. in suspension (suspended load)
3. scooting or rolling along the bottom (bed load)
Solutions and Suspensions Streams
transport sediment in solution and in
suspension. In chemistry, a solution is
a homogeneous mixture of dissolved
substances. A suspension is a
heterogeneous mixture that separates
into layers over time. Ask students to
write a paragraph explaining how
streams are both solutions and
suspensions. (Streams are composed,
in part, of groundwater that contains
dissolved substances, and in this way
streams are solutions. Streams also
transport fine sand, silt, and clay that are
not dissolved, but rather suspended in
moving water. In this way, streams are
suspensions.)
Verbal
Dissolved Load Most of the dissolved load enters streams
through groundwater. Some of this load also enters by dissolving rock
along the stream’s course. The amount of material the stream carries
in solution changes depending on climate and the geologic setting.
Usually the dissolved load is expressed as parts of dissolved material
per million parts of water (parts per million, or ppm). Some rivers may
have a dissolved load of 1000 ppm or more. However, the average
figure for the world’s rivers is estimated at 115 to 120 ppm. Streams
supply almost 4 billion metric tons of dissolved substances to the
oceans each year.
Suspended Load Most streams carry
the largest part of their load in suspension.
The visible cloud of sediment suspended in
the water is the most obvious portion of a
stream’s load. Streams usually carry only
sand, silt, and clay this way. However, streams
also transport larger particles during a flood
because water velocity increases. The total
amount of material a stream carries in suspension increases dramatically during floods,
as shown in Figure 8.
Build Science Skills
Bed Load Bed load is that part of a
stream’s load of solid material that is made up
of sediment too large to be carried in suspension. These larger, coarser particles move along the bottom, or bed, of
the stream channel. The suspended and dissolved loads are always
moving. But the bed load moves only when the force of the water is
great enough to move the larger particles. The grinding action of the
bed load is very important in eroding the stream channel.
L2
Figure 8 During this 1997 flood,
the suspended load in the muddy
Ohio River is clearly visible. The
greatest erosion and sediment
transport occur during floods.
Applying Concepts What other
types of load might account for
the muddiness of the river?
L2
Using Models Some students will have
difficulty comprehending the tiny scale
of one part per million. To help build
number sense and awareness of how
small a part per million is, ask students
to imagine they have a budget of one
million dollars, and have them think of
the cost of their lunch in terms of parts
per million. A four-dollar lunch would be
equal to 4 ppm. This represents only a
tiny fraction of their total budget.
Logical
Competence and Capacity The ability of streams to carry a
load is determined by two factors: the stream’s competence and its
capacity. Competence of a stream measures the largest particles it can
transport. A stream’s competence increases with its velocity. In fact, the
competence of a stream increases four times when the velocity doubles.
The capacity of a stream is the maximum load it can carry. Capacity
is directly related to a stream’s discharge. The greater the volume of
water in a stream is, the greater its capacity is for carrying sediment.
Running Water and Groundwater
165
Customize for English Language Learners
Students who are learning English can benefit
from real-life examples that relate to science
content. Encourage students to think of actual
flooding events that may have occurred in
your area or that they have heard about on the
news. Have them discuss the type of damage
done by the flood and some of the amazing
pictures of rescues and houses floating
downstream. Encourage students to share
their knowledge and examples with the class.
Answer to . . .
Figure 8 Dissolved load might
account for the muddiness of the river.
Streams erode their
channels lifting loose
particles by abrasion, grinding, and
by dissolving soluble material.
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Page 166
Section 6.2 (continued)
Mississippi Delta Region
fal
Atcha
Deposition
Figure 9
Expected Outcome Sediment should
settle out in layers, with pebbles on the
bottom and clay-sized particles at the
top. The water sorts the material as the
energy from the shaking dissipates, the
heavier material drops out sequentially.
If this is not illustrated, try using a larger
jar, which will allow more room for the
materials to settle out.
ss
ippi Rive
New Orleans
r
Procedure Fill the jar about one-third
full with the mixed sediment. Pour water
into the jar and tightly close the lid.
Shake the jar and allow sediment to
settle out.
si
is
Materials mixed sediment (pebbles,
sand, silt, and clay-sized particles), clear
glass jar with lid, water
M
Purpose Students learn how
deposition occurs in a stream.
ver
L2
a
Ri
Alluvium
shows the growth of the
Mississippi River delta over
the past 5,000 to 6,000 years.
As you can see, the river has
built a series of sub-deltas,
one after the other. The
numbers indicate the order in
which they were deposited.
Locating In which overall
direction has the Mississippi
River built its delta over the
past few thousand years?
Locating How has the
growth of the delta changed
the location of the mouth of
the Mississippi River in
relation to New Orleans?
Baton Rouge
ay
Movement This map
2
4
6
1
3
5
7
Gulf of Mexico
Deposition
Whenever a stream slows down, the situation reverses. As a stream’s
velocity decreases, its competence decreases and sediment begins to drop
out, largest particles first. Each particle size has a critical settling velocity.
Deposition occurs as streamflow drops below the critical
settling velocity of a certain particle size. The sediment in that category begins to settle out. Stream transport separates solid particles of
various sizes, large to small. This process is called sorting. It explains why
particles of similar size are deposited together.
The sorted material deposited by as stream is called alluvium.
Many different depositional features are made of alluvium. Some occur
within stream channels. Some occur on the valley floor next to the
channel. And others occur at the mouth of a stream.
Answers
Locating southeast
Locating New Orleans gets farther
from the mouth as the delta is built.
Deltas When a stream enters the relatively still waters of an ocean
or lake, its velocity drops. As a result, the stream deposits sediment and
forms a delta. A delta is an accumulation of sediment formed where a
stream enters a lake or ocean. As a delta grows outward, the stream’s
gradient lessens and the water slows down. The channel becomes
choked with sediment settling out of the slow-moving water. As a
result, the river changes direction as it seeks a shorter route to base
level. The main channel often divides into several smaller channels
called distributaries as shown in sub-delta 7 in Figure 9. These shifting
channels act in the opposite way of tributaries.
166 Chapter 6
Facts and Figures
The city of New Orleans, Louisiana, is built on
a delta at the mouth of the Mississippi River.
As is expected, the water table in this area is
very high due to the fact that the delta is built
right into the ocean. This high water table
leaves New Orleans with a troubling
problem—how do they bury their dead?
Early settlers were forced to bury their dead
in shallow graves due to the high water table.
166
Chapter 6
If they dug down only a few feet, the grave
filled with water and caused the casket to
float.
Finally, settlers adopted another method
of burial. They built above-ground vaults.
Today many of the cemeteries in New Orleans
have tombs arranged in a street-like fashion.
In fact, the cemeteries are often referred to
as “cities of the dead.”
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Stream Valleys
Rather than carrying water into the main channel like tributaries,
distributaries carry water away. After many shifts of the channel, a delta
may grow into a triangular shape, like the Greek letter delta (⌬).
However, not all deltas have this idealized shape. Differences in the
shapes of shorelines and variations in the strength of waves and currents result in different shapes of deltas.
Integrate Language Arts L2
Natural Levees Some rivers occupy valleys with broad, flat floors.
Successive floods over many years can build natural levees along them.
A natural levee is a landform that parallels some streams. They form
when a stream overflows its banks. When it overflows, its velocity rapidly decreases and leaves coarse sediment deposits in strips that border
the channel. As the water spreads out over the valley, less sediment is
deposited. This uneven distribution of material produces the gentle
slope of the natural levee.
Stream Valleys
Figure 10 The Yellowstone River
is an example of a V-shaped
valley. The rapids and waterfall
show that the river is vigorously
downcutting the channel.
Narrow Valleys The Yellowstone River, shown in
Figure 10, is an excellent example of a narrow valley.
A
narrow V-shaped valley shows that the stream’s primary
work has been downcutting toward base level. Rapids and
waterfalls are the most prominent features of a narrow valley.
Both rapids and waterfalls occur where the stream profile
drops rapidly. The variations in the erosion of the underlying
bedrock cause these rapid drops.
Prefixes Remind students that a
tributary is a stream that empties into
another stream (p. 162). The text
provides a contextual definition of
distributary, but tell students that even
without this context, they could deduce
that a distributary is the opposite of a
tributary by knowing the prefix dismeans, among other things, “opposite
of.” Encourage students to make a list of
common word parts as they read. Have
them look up each one in a dictionary.
Doing so will help them with new
vocabulary and verbal portions of
standardized tests.
Verbal
Build Science Skills
L2
Designing
Experiments Divide
students into groups
and ask them to model
a delta using a sloped paint tray (the
type used with rollers), sand, and a
constant supply of water (such as from a
hose or faucet). (First, dampen the sand.
Then distribute the sand evenly in
a thin layer over the sloped part of the
tray. Supply a gentle but constant flow of
water to the top of the slope, and observe
the channel the water makes in the sand.
Next, observe how some sand is eroded
and transported to the mouth of the
channel, where it settles into the pool of
water at the flat part of the tray.)
Kinesthetic, Visual
Wide Valleys Once a stream has cut its channel closer to
base level, downward erosion becomes less dominant. More
of the stream’s energy is directed from side to side. The result
is a widening of the valley as the river cuts away first at one
bank and then at the other.
The side-to-side cutting of a stream eventually produces a
flat valley floor, or floodplain. A floodplain is appropriately
named because during a flood the river overflows its banks
and floods the plain.
Streams that flow on floodplains move in meanders. Once
a bend in a channel begins to form, it grows larger. Most of
the erosion occurs on the outside of the meander—often
called the cut bank—where velocity and turbulence are greatest. Much of the debris the stream removes at the cut bank
moves downstream where it is deposited as point bars. Point
bars form in zones of decreased velocity on the insides of
meanders. In this way, meanders move side to side by eroding
the outside of bends and depositing on the inside.
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167
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Oxbow lake
Section 6.2 (continued)
Build Science Skills
L2
Inferring Draw on the board a cross
section of the river valley. (a broad
relatively flat-bottomed shape, similar to a
horizontal bracket) Then have students
draw cross sections representing at least
three earlier stages in the development
of the valley. Tell them the first stage
should be of a time when waterfalls and
rapids were common along the river’s
course. (Cross sections should show a
narrowing valley with the earliest cross
section showing a V-shaped valley.)
Logical
Use Visuals
Figure 11 A One
meander has overtaken
the next, forming a
ring of water on the
floodplain. B After
deposits of sediment
cut off the ring, an
oxbow lake forms.
Plugs with
silt and clay
A
Erosion is more effective on the downstream side of a meander
because of the slope of the channel. The bends gradually travel down the
valley. Sometimes the downstream movement of a meander slows when
it reaches a more resistant portion of the floodplain. This resistance allows
the next meander upstream to overtake it, as shown in Figure 11.
Gradually the neck of land between the meanders is narrowed.
Eventually the river may erode through the narrow neck of land to the
next loop. The new, shorter channel segment is called a cutoff and,
because of its shape, the abandoned bend is called an oxbow lake. Such
a situation is shown in the bottom portion of Figure 6 on page 163.
L1
Mississippi River Flooding
July 18, 1993
Figure 12
iss
pi
er
ip
Riv
Riv
er
Chapter 6
iss
pi
Missouri River
Missouri River
168 Chapter 6
Facts and Figures
On May 31, 1889, residents of Johnstown,
Pennsylvania, heard what sounded like a roar
of thunder. Their worst fears were realized.
The South Fork Dam, located 22.4 km
upstream along the Little Conemaugh River,
broke after a night of heavy rain. Twenty
168
M
ip
Region These satellite
images show the
confluence of the Missouri
and Mississippi rivers. The
first photo shows the rivers
during normal flow.
Interpreting
Photographs What does
the second satellite image
show? How do you know?
July 4, 1988
iss
Answer
Interpreting Photographs The
second satellite image shows the rivers
during flood stage. You can tell the
rivers are at flood stage because the
area covered by water is much wider.
A flood occurs when the discharge of a stream becomes so great that it
exceeds the capacity of its channel and overflows its banks. Floods are
the most common and most destructive of all natural geologic hazards.
Most floods are caused by rapid spring snow melt or storms
that bring heavy rains over a large region. Heavy rains caused the
devastating floods in the upper Mississippi River Valley during the
summer of 1993, as shown in Figure 12.
Unlike far-reaching regional floods, flash floods are more limited
in extent. However, flash floods occur with little warning, and they can
be deadly as walls of water sweep through river valleys. Several factors
iss
Floods and Flood
Control
Floods and Flood Control
M
Figure 11 Have students study the
streams shown in the figures. Ask: Are
other oxbow lakes shown on these
diagrams? (Yes, the remnants of two
other crescent-shaped lakes are shown.)
What does this show you about the
river? (The oxbow lakes indicate previous
positions of the river.) Where might the
next oxbow lake form along this
section of river? (It will most likely form
along the course at the lower part of the
diagrams, where a neck is forming at the
base of a meander that loops to the left.
Some students might think the course of
the river at the top of the diagrams
represents an even narrower neck that is
not shown but would pinch off the large
meander to the right, forming an oxbow
lake.)
Visual, Logical
B
million tons of water crashed down the
river valley made narrower by the growing
community. Over 2200 people died as a result
of the flood and the aftermath of fires that
followed.
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influence flash floods: rainfall intensity and duration, surface conditions, and topography. As you have learned, many urban areas are
susceptible to flash floods. Mountainous areas are also susceptible
because steep slopes can send runoff into narrow canyons.
Human interference with the stream system can worsen or even cause
floods. A prime example is the failure of a dam or an artificial levee. These
structures are designed to contain floods of a certain size. If that size is
exceeded, water can then spill over or break through a dam or levee and
rush downstream causing a disastrous flood.
There are several flood control strategies.
Measures to control
flooding include artificial levees, flood control dams, and placing
limits on floodplain development.
Artificial Levees Artificial levees are earthen mounds built on
the banks of a river. These levees increase the volume of water a
channel can hold. When levees confine a river during periods of high
water, the river often deposits material in its channel as the discharge
diminishes. This discharge is sediment that would have been dropped
on the floodplain. Because the stream cannot deposit material outside
of its channel the bottom of the channel is gradually built up. When
the channel is built up, it takes less water to overflow the levee. As a
result, people may have to raise the height of the levee periodically to
protect the floodplain behind it. Moreover, many artificial levees are
not built to withstand periods of extreme flooding. For example, there
were many levee failures in the Midwest during the summer of 1993
when the upper Mississippi experienced record flooding.
L2
Q Sometimes a major flood is
described as a 100-year flood.
What does that mean?
A The phrase “100-year flood”
is misleading because it makes
people believe that such an
event happens only once every
100 years. In truth, a huge
flood can happen any year. The
phrase “100-year flood” is really
a statistical designation. It indicates that there is a 1-in-100
chance that a flood this size will
happen during any year.
Perhaps a better term would be
the “1-in-100 chance flood.”
Some students may have heard the term
“100-year flood” and assume that this
means that floods only occur in the area
every 100 years. This is not true and is
often misleading. A major flood can occur
along a river during any year. What the
term does suggest, however, is that
statistically speaking there is a 1 in 100
chance that in any given year, a major
flood will occur. The chances of flooding
along any stream are reevaluated
periodically. It has been found that the
flooding likelihood changes and that
major flooding may occur more
frequently than every 100 years. Ask
students to predict what sort of human
activity may increase the likelihood of
flooding. (Urban development and the
building of dams may increase the amount
of serious flooding in an area.)
Logical
Flood-Control Dam Flood-control dams store floodwater and
then let it out slowly. Since the 1920s, thousands of dams have been
built on nearly every major river in the United States. Many dams have
other non-flood related functions, such as providing water for irrigation and for hydroelectric power generation.
Although dams may reduce flooding and provide other benefits,
building dams has consequences. For example, dams trap sediment.
Deltas and floodplains downstream can erode because silt no longer
replenishes them during floods. Built up sediment behind a dam means
the volume of the stored water will gradually diminish. This build-up
reduces the effectiveness of the dam for flood control. Large dams also
cause ecological damage to river environments.
Limiting Development Today many scientists and engineers
advocate sound floodplain management instead of building structures.
That often means preserving floodplains in their natural state.
Minimizing development on floodplains allows them to absorb floodwaters with little harm to homes and businesses.
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Section 6.2 (continued)
Drainage Basins
Every stream has a drainage basin.
A drainage basin is the land
area that contributes water to a stream. An imaginary line called a
divide separates the drainage basins of one stream from another.
Divides range in scale from a ridge separating two small gullies on a
hillside to a continental divide, which splits continents into enormous
drainage basins. The Mississippi River has the largest drainage basin in
North America. See Figure 13. The river and its tributaries collect water
from more than 3.2 million square kilometers of the continent.
Drainage Basins
L1
Missou
Divide
iver
Mississippi River
Drainage Basin
L2
pp
issi i River
ss
Mi
Use Community
Resources
Figure 13 Mississippi River
Drainage Basin Divides are the
boundaries that separate
drainage basins from each other.
ri R
Figure 13 Point out the Mississippi River
drainage basin. Ask: Do other drainage
basins exist within this one? (Yes, every
stream, regardless of size, has its own
drainage basin. A larger river, such as the
Mississippi, will have a drainage basin that
includes those of all of its tributaries.)
Where is the divide that is commonly
called the Continental Divide? (This is
the western portion of the Mississippi River
drainage basin divide that runs through
the Rocky Mountains.)
Drainage Basins Invite a hydrologist
to speak to the class about a drainage
basin in your area. Have students trace
out the drainage basin of a local stream
or river and discuss their findings with
the scientist.
Interpersonal
ver
Ri
Use Visuals
Ohio
Divide
Divide
3 ASSESS
Evaluate
Understanding
L2
To assess students’ knowledge of section
content, have them create a visual
showing a narrow and a wide stream
valley.
Reteach
L1
Have students make a chart summarizing
the differences between erosion and
deposition.
Section 6.2 Assessment
Reviewing Concepts
1.
2.
3.
How do streams erode their channels?
What causes floods?
What is the relationship between a stream
and a drainage basin?
4.
How do streams transport sediments?
Critical Thinking
5. Analyzing Concepts How does urban
development interfere with the natural
function of floodplains?
Student paragraphs should describe
accurately researched floods and their
causes and effects.
6. Summarizing Explain the formation of one
of the landforms that streams create by
deposition.
Descriptive Paragraph Use library
sources or the Internet to research the
causes of a recent major flood. Write a
paragraph that tells the name of the flood,
when it happened, where it happened, and
the conditions that led to the flood itself.
170 Chapter 6
Section 6.2 Assessment
1. Streams erode their channels by lifting
loose particles by abrasion, grinding, and by
dissolving soluble material.
2. Floods occur when the discharge of a
stream exceeds the capacity of the channel.
Most floods are caused by rapid spring snow
melt and storms.
3. A drainage basin is the land area that
contributes water to a stream.
170
Chapter 6
4. Streams transport sediment in solution, in
suspension, and by rolling along the bottom.
5. Urban development can decrease the effectiveness of floodplains by replacing waterabsorbing vegetation with concrete and
asphalt and increasing flooding.
6. Sample answer: Deltas are formed as accumulating sediment is deposited where a
stream or river enters a lake or ocean.