When water soaks into the ground, it moves at dif-ferent rates through the difdif-ferent materials that make up Earth’s surface.. The Water Cycle Earth’s water supply is recycled in a
Trang 1Waterfall
Surface Water
BIG Idea Surface water
moves materials produced by
weathering and shapes the
surface of Earth.
9.1 Surface Water
Movement
MAIN Idea Running water is
an agent of erosion, carrying
sediments in streams and rivers
and depositing them
downstream.
9.2 Stream Development
MAIN Idea Streams erode
paths through sediment and
rock, forming V-shaped stream
valleys.
9.3 Lakes and
Freshwater Wetlands
MAIN Idea As the amount of
water changes and the amount
of sediments increases, lakes can
be transformed into wetlands
and eventually into dry land.
GeoFacts
• The United States has
approximately 5,600,000 km of
rivers.
• The Missouri River is about
4087 km long, making it the
longest river in North America.
• The Mississippi River Basin
drains 41 percent of the
United States.
Slow-moving water
Trang 2Section 1 • XXXXXXXXXXXXXXXXXX 223
Start-Up Activities
L
Chapter 9 • Surface Water 223
How does water infiltrate?
When water soaks into the ground, it moves at
dif-ferent rates through the difdif-ferent materials that make
up Earth’s surface
Procedure
1 Read and complete the lab safety form.
2 Place a small window screen on each of
two clear plastic shoe boxes.
3 Place an 8-cm × 16-cm clump of grass or
sod on one screen.
4 Place an 8-cm × 16-cm clump of barren
soil on the other screen.
5 Lightly sprinkle 500 mL of water on each
clump.
6 Observe the clumps for 5 min.
7 Measure the amount of water in each box.
Analysis
1 Describe what happens to the water after
5 min.
2 Infer the reason for any differences in the
amount of water collected in each box.
Stream Development Make
this Foldable that features the steps in stream development.
STEP 1 Fold three sheets of notebook paper
in half horizontally to find the middle Holding two of the sheets together, make
a 3-cm cut at the fold line
on each side of the paper.
STEP 2 On the third sheet, cut along the fold line to within 3-cm of each edge.
STEP 3 Slip the first two sheets through the cut in the third sheet to make a six-page book.
STEP 4 Label your
book Stream
Development.
As you read this section, use the pages of your Foldable to describe and illustrate the steps in stream development.
review content with the Interactive Tutor and take Self-Check Quizzes.
Stream Development
Trang 3◗ Describe how surface water can
move weathered materials.
◗ Explain how a stream carries its
load.
◗ Describe how a floodplain
develops.
Review Vocabulary
solution: a homogeneous mixture in
which the component particles cannot
Surface Water Movement
MAIN Idea Running water is an agent of erosion, carrying sediments in streams and rivers and depositing them downstream
Real-World Reading Link Have you ever noticed that sometimes a river is muddy but other times it is clear? In floods, rivers can carry greater amounts of materials, which makes them muddy Under normal conditions, they often carry less sediment, which makes them clearer.
The Water Cycle
Earth’s water supply is recycled in a continuous process called the water cycle, shown in Figure 9.1 Water molecules move
continuously through the water cycle following many pathways:
they evaporate from a body of water or the surface of Earth, dense into cloud droplets, fall as precipitation back to Earth’s sur-face, and infiltrate the ground As part of a continuous cycle, the water molecules eventually evaporate back to the atmosphere, form clouds, fall as precipitation, and the cycle repeats Understanding the mechanics of the water cycle will help you understand the rea-sons for variations in the amount of water that is available
con-throughout the world
Often, a water molecule’s pathway involves time spent within a living organism or as part of a snowfield, glacier, lake, or ocean
Although water molecules might follow a number of different pathways, the overall process is one of repeated evaporation and condensation powered by the Sun’s energy
Reading Check Explain What happens once water reaches Earth’s surface?
Land Rivers
Groundwater
Infiltration
■ Figure 9.1 The water cycle, also
referred to as the hydrologic cycle, is a
never-ending, natural circulation of
water through Earth’s systems.
Identify the driving force for
the water cycle.
Interactive Figure To see an
animation of the water cycle, visit
glencoe.com.
Trang 4Water flowing downslope along Earth’s surface is called runoff
Runoff might reach a stream, river, or lake, it might evaporate, or
it might accumulate as puddles in small depressions and infiltrate
the ground During and after heavy rains, you can observe these
processes in your yard or local park Water that infiltrates Earth’s
surface becomes groundwater
A number of conditions determine whether water on Earth’s
surface will infiltrate the ground or become runoff For water to
enter the ground, there must be large enough pores or spaces in the
soil and rock to accommodate the water’s volume, as in the loose
soil illustrated in Figure 9.2 If the pores already contain water,
the newly fallen precipitation will either remain in puddles on top
of the ground or, if the area has a slope, run downhill Water
stand-ing on the surface of Earth eventually evaporates, flows away, or
slowly enters the groundwater
Soil composition The physical and chemical composition of
soil affects its water-holding capacity Soil consists of decayed
organic matter, called humus, and minerals Humus creates pores in
the soil, thereby increasing a soil’s ability to retain water The
min-erals in soil have different particle sizes, which are classified as sand,
silt, or clay As you learned in Chapter 7, the percentages of particles
of each size vary from soil to soil Soil with a high percentage of
coarse particles, such as sand, has relatively large pores between its
particles that allow water to enter and pass through the soil quickly
In contrast, soil with a high percentage of fine particles, such as
clay, clumps together and has few or no spaces between the
parti-cles Small pores restrict both the amount of water that can enter
the ground and the ease of movement of water through the soil
Rate of precipitation Light, gentle precipitation can infiltrate
dry ground However, the rate of precipitation might temporarily
exceed the rate of infiltration For example, during heavy
precipita-tion, water falls too quickly to infiltrate the ground and becomes
runoff Thus, a gentle, long-lasting rainfall is more beneficial to
plants and causes less erosion by runoff than a torrential
down-pour If you have a garden, remember that more water will enter
the ground if you water your plants slowly and gently
Section 1 • Surface Water Movement 225
■ Figure 9.2 Soil that has open surface
pores allows water to infiltrate The particle size that makes up a soil helps determine the pore space of the soil.
Silt grains
Pore spaces
Fine grain size
Sand
grains
Pore spaces
Large grain size
Pore spaces
Sand grains
Silt grains
Mixed grain size
Trang 5Vegetation Soils that contain grasses or other vegetation allow more water to enter the ground than do soils with no vegetation
Precipitation falling on vegetation slowly flows down leaves and branches and eventually drops gently to the ground, where the plants’ root systems help maintain the pore space needed to hold water, as shown in Figure 9.3 In contrast, precipitation falls with far more force onto barren land In such areas, soil particles clump together and form dense aggregates with little space between them
The force of falling rain can then push the soil clumps together, thereby closing pores and allowing less water to enter
Slope The slope of a land area plays a significant role in mining the ability of water to enter the ground Water from precip-itation falling on slopes flows to areas of lower elevation The steeper the slope, the faster the water flows There is also greater potential for erosion on steep slopes In areas with steep slopes, much of the precipitation is carried away as runoff
deter-Stream Systems
Precipitation that does not enter the ground usually runs off the surface quickly Some surface water flows in thin sheets and even-tually collects in small channels, which are the physical areas where streams flow As the amount of runoff increases, the channels widen, deepen, and become longer Although these small channels often dry up after precipitation stops, the channels fill with water each time it rains and become larger and longer
Tributaries All streams flow downslope to lower elevations
However, the path of a stream can vary considerably, depending on the slope and the type of material through which the stream flows
Some streams flow into lakes, while others flow directly into the ocean Rivers that flow into other streams are called tributaries For example, as shown in Figure 9.4, the Missouri River is a tributary
of the Mississippi River
226 Chapter 9 • Surface Water
Grasses slow the movement of runoff water.
■ Figure 9.3 Vegetation can slow the
rate of runoff of surface water Raindrops
are slowed when they strike the leaves of
trees or blades of grass, and they trickle
down slowly.
Trang 6Watersheds and divides All of the land area whose water
drains into a stream system is called the system’s watershed.
Watersheds can be relatively small or extremely large in area
A divide is a high land area that separates one watershed from
another In a watershed, the water flows away from the divide,
as this is the high point of the watershed
Each tributary in a stream system has its own watershed and
divides, but they are all part of the larger stream system to which
the tributary belongs The watershed of the Mississippi River,
shown in Figure 9.4, is the largest in North America
Reading Check Describe what a divide is and what role it plays in
a watershed.
Section 1 • Surface Water Movement 227
500 km
Mississippi Delta
Continental Divide
Drainage basin of the Mississippi River
0
Ohio R
iver
M issouriRiv er
Mississip
piR
■ Figure 9.4 The watershed of the
Mississippi River includes many stream systems, including the Mississippi, Missouri and Ohio Rivers The Continental Divide marks the west- ern boundary of the watershed.
Identify what portion of the continental United States eventually drains into the Mississippi River.
PROBLEM-SOLVING Lab
Interpret the Graph
How do sediments move in a stream? The critical velocity of water determines the size
of particles that can be moved The higher the stream velocity, the larger the particles that can be transported.
100.0 10.0 1.0 0.1 0.01
0.001 0.0001 0.0004 cm
0.006 cm Silt
Sand
Pebbles Cobbles Boulders
Clay
0.2 cm
6.4 cm 25.6 cm
0.00001
100 200 300 400
Stream velocity (cm/s)
500 600 700 800 0
Stream Velocity and Particle Size
Trang 7■ Figure 9.6
Floods in Focus
Floods have shaped the landscape and
affected human lives.
1958 Following a flood that claimed almost 2000 lives, Holland begins creating a vast network of dams, dikes, and barriers, shortening its coastline by 700 km.
1927 Heavy rains flood the Mississippi River from Illinois
to Louisiana leaving more than 600,000 people home- less
1931 China’s Yellow River floods when heavy rain causes the river’s large silt deposits
to shift and block the channel
1902 In Egypt, the Aswan Dam is built to stabilize the flow of annual flood waters that create the fertile Nile Delta
trans-Bed load Sediment that is too large or heavy to be held up by bulent water is transported by streams in another manner A stream’s
tur-bed load consists of sand, pebbles, and cobbles that the stream’s water can roll or push along the bed of the stream The faster the water moves, the larger the particles it can carry As the particles move, they rub against one another or the solid rock of the stream-bed, which can erode the surface of the streambed, as shown in
Figure 9.5.
Materials in solution Solution is the method of transport for materials that are dissolved in a stream’s water When water runs through or over rocks with soluble minerals, it dissolves small amounts
of the minerals and carries them away in the solution Groundwater adds the majority of the dissolved load to streams The amount of dis-solved material that water carries is often expressed in parts per mil-lion (ppm) For example, a measurement of 10 ppm means that there are 10 parts of dissolved material for every 1 million parts of water
The total concentration of materials in solution in streams averages 115–120 ppm, although some streams carry as little dissolved material
as 10 ppm Values greater than 10,000 ppm have been observed for streams draining desert basins
228 Chapter 9 • Surface Water
■ Figure 9.5 Particles rub, scrape,
and grind against one another in a
streambed, which can create potholes.
(tl)Salvatore Vasapolli/Animals Animals, (bl)Lloyd Cluff/CORBIS, (br)Anthony Cooper/Ecoscene/CORBIS
Trang 82005 Category 5 Hurricane Katrina slams into Louisiana, Mississippi, and Alabama, devastating New Orleans
1996 Volcanic eruptions in Iceland release meltwater from under the Vatnajökull glacier that washes away power lines, major roads, and bridges.
Interactive Time Line To learn more about these discoveries and others, visit
glencoe.com.
1988 Monsoon rains
in Bangladesh flood two-thirds of the country, affecting 45 million people.
1974 The United Kingdom
begins building the Thames Barrier to protect London from rising tide levels as the city sinks and sea levels rise.
Stream Carrying Capacity
The ability of a stream to transport material, referred to as its
carrying capacity, depends on both the velocity and the amount
of water moving in the stream The channel’s slope, depth, and
width all affect the speed and direction the water moves within it
A stream’s water moves more quickly where there is less friction;
consequently, smooth-sided channels with great slope and depth
allow water to move the most rapidly The total volume of moving
water also affects a stream’s carrying capacity Discharge, shown in
Figure 9.7, is the measure of the volume of stream water that flows
past a particular location within a given period of time Discharge is
commonly expressed in cubic meters per second (m3/s) The
fol-lowing formula is used to calculate the discharge of a stream
discharge = average width × average depth × average velocity
The largest river in North America, the Mississippi River, has a
huge average discharge of about 17,000 m3/s The Amazon River,
the largest river in the world, has a discharge of about ten times
that amount The discharge from the Amazon River over a
two-hour period would supply New York City’s water needs for an
entire year!
As a stream’s discharge increases, its capacity also increases
Both water velocity and volume increase during times of heavy
pre-cipitation, rapid melting of snow, and flooding In addition to
increasing a stream’s carrying capacity, these conditions heighten a
stream’s ability to erode the land over which it passes As a result of
an increase in erosional power, a streambed can widen and deepen,
adding to the stream’s carrying capacity Streams shape the
land-scape both during periods of normal flow and during floods, as
highlighted in Figure 9.6
Section 1 • Surface Water Movement 229
Average depth
Average width
Average velocity
■ Figure 9.7 Stream discharge is
the product of a stream’s average width, average depth, and the velocity of the water.
Jerry Grayson/Helifilms Australia PTY Ltd/Getty Images
Trang 9The amount of water being transported in a particular stream at any given time varies with weather conditions Sometimes, more water pours into a stream than the banks of the stream channel can
hold A flood occurs when water spills over the sides of a stream’s
banks onto the adjacent land The broad, flat area that extends out from a stream’s bank and is covered by excess water during times
of flooding is known as the stream’s floodplain
Floodwater carries along with it a great amount of sediment eroded from Earth’s surface and the sides of the stream channel
As floodwater recedes and its volume and speed decrease, the water drops its sediment load onto the stream’s floodplain After repeated floods over time, sediments deposited by floods tend to accumulate along the banks of the stream These develop into continuous ridges along the sides of a river, called natural levees, as shown in
Figure 9.8. Floodplains develop highly fertile soils as more
sedi-ment is deposited with each subsequent flood The fertile soils of floodplains make some of the best croplands in the world
Reading Check Describe what happens when floodwaters recede.
Flood stages Floods are a natural occurrence After a rain event or snowmelt, it takes time for runoff water to reach the streams As water enters the streams, the water level continues to rise and might reach its highest point, called its crest, days after precipitation ends When the water level in a stream rises higher than its banks, the river is said to be at flood stage The resulting flooding might occur over localized areas or across large regions
The flooding of a small area is known as an upstream flood
Heavy accumulation of excess water from large regional age systems results in downstream floods Such floods occur dur-ing or after long-lasting, intense storms or spring thaws of large snowpacks The tremendous volume of water involved in a down-stream flood can result in extensive damage The effects of flood-ing on the landscape are shown in Figure 9.9
drain-230 Chapter 9 • Surface Water
Flood plain Sand
Clay
■ Figure 9.8 When rivers overflow their banks, the floodwater deposits sediment
Over time, sediment accumulates along the edges of a river, resulting in natural levees.
Sediment deposited during flood Natural levees
■ Figure 9.9 This flood was caused
by heavy rainfall upstream Notice the
farm fields that have been covered in
floodwater.
Analyze What long-term effects
might this flood have on the crops
grown in this area?
Barrie Rokeach/Getty Images
Trang 10Self-Check Quiz glencoe.com
Flood Monitoring and Warning Systems
In order to provide warnings for people at risk, government
agencies, such as the National Weather Service, monitor potential
flood conditions Earth-orbiting weather satellites photograph Earth
and collect and transmit information about weather conditions,
storms, and streams In addition, the U.S Geological Survey (USGS)
has established approximately 7300 gaging stations in the United
States to provide a continuous record of the water level in each
stream as shown in Figure 9.10 These gaging systems often
transmit data to satellites and telephone lines where the information
is then sent to the local monitoring office
In areas that are prone to severe flooding, warning systems are
the first step in implementing emergency management plans Flood
warnings and emergency plans often allow people to safely
evacuate an area in advance of a flood
Section 1 • Surface Water Movement 231
Section Summary
◗◗ Infiltration of water into the ground
depends on the number of open
pores.
◗
◗ All the land area that drains into
a stream system is the system’s
watershed.
◗
◗ Elevated land areas called divides
separate one watershed from
◗ Flooding occurs in small, localized
areas as upstream floods or in large
downstream floods.
Understand Main Ideas
1 MAIN Idea Analyze ways in which moving water can carve a landscape.
2 Describe the three ways in which a stream carries its load.
3 Analyze the relationship between the carrying capacity of a stream and its
discharge and velocity.
4 Determine why little water from runoff infiltrates the ground in areas with steep
slopes.
Think Critically
5 Determine how a floodplain forms and why people live on floodplains.
6 Analyze how levees prevent flood damage.
Earth Science MATH in
7 Design a data table that compares how silt, clay, sand, and large pebbles settle
to the bottom of a stream as the velocity of water decreases.
■ Figure 9.10 Gaging stations, like
this one, can send data to meteorologic stations There, scientists can process the information and alert the public to poten- tial floods.
Trang 11◗ Describe some of the physical
fea-tures of stream development.
◗ Describe the relationship between
meanders and stream flow.
◗ Explain the process of rejuvenation
in the stream development.
Review Vocabulary
abrasion: process of erosion in
which windblown or waterborne
parti-cles, such as sand, scrape against rock
surfaces or other materials and wear
Stream A
Stream capture Stream B
■ Figure 9.11 The headward
ero-sion of Stream A cuts into Stream B and draws away from its water into one stream.
Supply of Water
Stream formation relies on an adequate water supply Precipitation provides water for the beginnings of stream formation Streams can also be fed by underground deposits of water As a stream develops,
it changes width and size, and shapes the land over which it flows
Stream channels The region where water first accumulates to supply a stream is called the headwaters It is common for a stream’s headwaters to be high in the mountains Falling precipitation accumulates in small gullies at these higher elevations and forms briskly moving streams As surface water begins its flow, its path might not be well defined In time, the moving water carves a narrow
pathway into the sediment or rock called the stream channel The
channel widens and deepens as more water accumulates and cuts into
Earth’s surface Stream banks hold the moving water within them.
When small streams erode away the rock or soil at the head of
a stream, it is known as headward erosion These streams move swiftly over rough terrain and often form waterfalls and rapids as they flow over steep inclines Sometimes, a stream erodes the high area separating two drainage basins and joins another stream, and then draws water away from the other stream This process is called stream capture, shown in Figure 9.11
Trang 12Formation of Stream Valleys
The driving force of a stream is the force of gravity on water This
means that the energy of a stream comes from the movement of
water down a slope called a stream gradient When the gradient
of a stream is steep, water in the stream moves downhill rapidly,
cutting steep valleys The gradient of the stream depends on its
base level, which is the elevation at which it enters another stream
or body of water The lowest base level possible for any stream is
sea level, the point at which the stream enters the ocean, as shown
in Figure 9.12
Far from its base level, a stream actively erodes a path through
the sediment or rock, and a V-shaped channel develops V-shaped
channels have steep sides and sometimes form canyons or gorges
The Yellowstone River in Wyoming flows through an impressive
example of this type of narrow, deep gorge carved by a stream
Figure 9.13 shows the classic V-shaped valley As a stream
approaches its base level, it has less energy for downward erosion
Instead, streams that are near their base level tend to erode at the
sides of the stream channel, and over time result in broader valleys
with gentle slopes, as shown in Figure 9.13
Section 2 • Stream Development 233
Maximum energy for downward erosion
Minimum energy for downward erosion
Base level of streams
Sea level
■ Figure 9.12 The height of a stream
above its base level determines how much downcutting energy the stream will have.
■ Figure 9.13 A V-shaped valley is formed
by the downcutting of a stream A wide, broad valley is a result of stream erosion over a long period of time.
Identify which river is closer to its base level.
Incorporate information from this section into your Foldable.
(l)Mike Norton/Animals Animals, (r)Tom Bean/CORBIS
Trang 13S CIENCE USAGE V C OMMON USAGE
Meander
Science usage: a bend or curve in
a stream channel caused by
or wind A bend or curve in a stream channel caused by moving
water is called a meander, shown in Figure 9.14.
Water in the straight parts of a stream flows at different ties, depending on its location in the channel In a straight length
veloci-of a stream, water in the center veloci-of the channel flows at the mum velocity Water along the bottom and sides of the channel flows more slowly because it experiences friction as it moves against the land
maxi-In contrast, the water moving along the outside of a meander curve experiences the greatest velocity within the meander The water that flows along this outside part of the curve continues to erode away the sides of the streambed, thus making the meander larger Along the inside of the meander, the water moves more slowly and deposition is dominant These differences in the veloc-ity within meanders cause the meanders to become more accentu-ated over time This process is illustrated in Figure 9.15
Oxbow lakes Stream meanders continue to develop and become larger and wider over time After enough winding, however, it is common for a stream to cut off a meander and once again flow along a straighter path The stream then deposits material along the adjoining meander and eventually blocks off its water supply,
as shown in Figure 9.14 The blocked-off meander becomes an oxbow lake, which eventually dries up
As a stream approaches a larger body of water or its endpoint, the ocean, the streambed’s gradient flattens out and its channel becomes very wide The area of the stream that leads into the ocean
or another large body of water is called the mouth
234 Chapter 9 • Surface Water
■ Figure 9.14 As the path of the
stream bends and winds, it creates
meanders and eventually oxbow lakes.
Interactive Figure To see an animation of
meander formation, visit glencoe.com.
S.J Krasemann/Peter Arnold, Inc.
Trang 14Section 2 • Stream Development 235
Visualizing Erosion and
Deposition in a Meander
Figure 9.15 As the water travels down a meander, the area of maximum velocity changes As shown in cross-section A, when the
meander is straight, the maximum velocity is located near the center When the meander curves, the maximum velocity shifts to the outside
of the curve, as shown in cross-section B As the meander travels around to cross-section C, the maximum velocity shifts again to the
out-side of the curve Erosion occurs around curves in the meander in areas of high velocity The high velocity of the water carries the sediment
downstream and deposits it where the velocity decreases, on the inside of a curve The area where the erosion occurs is called a cutbank
and the area where the deposition occurs is called a point bar.
Erosion and cutbank
Maximum velocity
To explore more about erosion and deposition, visit glencoe.com.