3/19/2010
Landforms Made by
Running Water
Chapter 15
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Chapter Outline
1. Slope Erosion
Canyons
from Space
2. The Work of Streams and
Stream Gradation
3. Fluvial Landscapes
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Canyons from Space
Canyons
• Deep canyons, carved by powerful rivers from Space
crossing high terrain, are among the most
dramatic features of the landscape.
• The Grand Canyon of the Colorado River is
among the most famous in the world,
spanning a length of 450 km (about 280 mi)
with vertical drops up to about 1500 m
(about 5000 ft).
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Canyons from Space –
Grand Canyon
Canyons
from Space
Grand Canyon
perspective view
from ASTER
Grand Canyon,
Arizona, imaged
by MISR
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Canyons from Space –
Canyons
from Space
Canyons of the Andes as seen
by MISR
Three Gorges region of the
Yangtze River imaged by
ASTER
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LANDFORMS & RUNNING
WATER
Fluvial landforms - the world’s land surface sculpted by
running water.
Waves, glacial ice, and wind also carve out landforms, but running water is the
most important because landforms made by glacial ice, wind, and waves are
restricted to specific areas
Fluvial landforms are shaped by the fluvial processes of overland flow and
streamflow. Wherever rain falls, these processes act to create landforms. Two
major groups of landforms - erosional landforms and depositional landforms
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1. Slope Erosion
ACCELERATED SOIL EROSION
SLOPE EROSION IN SEMIARID
AND ARID ENVIRONMENTS
1. Slope Erosion
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1. Slope Erosion
The ravine, canyon, peak, spur, and col
are erosional landforms.
Fan of rock fragments below the mouth
of the ravine, and the floodplain, built of
material transported by a stream, are
depositional landforms.
1. Slope Erosion
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1. Slope Erosion
This narrow valley, located in a
biosphere reserve on the Kamchatka
Peninsula, is an example of an
erosional landform
1. Slope Erosion
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ACCELERATED SOIL
EROSION
Human activities can upset that
natural balance and produce
accelerated erosion. Erosion
occurs when overland flow
moves soil particles downslope
• Greatest on bare slopes of fine
particles.
• Rills and gullies form.
• Vegetation cover greatly
reduces soil erosion.
1. Slope Erosion
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ACCELERATED SOIL
EROSION
Runoff and sediment
yield - much greater for
open land than for land
covered by shrubs and
forest.
Sediment yield – sediment removal in metric
tons per hectare per year
Example - sediment yield from cultivated land
undergoing accelerated erosion
- 10 times greater than pasture
- 1,000 times greater than pine plantation.
1. Slope Erosion
(Uniform climate, soil, and
topography for upland surfaces
in northern Mississippi.)
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ACCELERATED SOIL
EROSION
A large raindrop lands on a wet
soil surface, producing a
miniature crater. Grains of clay
and silt are thrown into the air,
and the soil surface is disturbed.
1. Slope Erosion
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SLOPE EROSION IN SEMIARID
AND ARID ENVIRONMENTS
In semiarid and arid lands
erosion produces badlands.
• Badlands are underlain by
clay formations, which are
easily eroded by overland
flow.
• Erosion rates are too rapid
for vegetation, so no soil
can develop.
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1. Slope Erosion
2. The Work of Streams and
Stream Gradation
STREAM EROSION
STREAM TRANSPORTATION
STREAM GRADATION
LANDSCAPE EVOLUTION OF A
GRADED STREAM
STREAM ORDER
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STREAM EROSION
Streams erode their beds and banks by hydraulic action,
abrasion, and corrosion.
Abrasion by stones on a bedrock river
bed can create deep depressions
known as potholes.
A - Dissolved matter is
transported invisibly in
the form of chemical
ions. All streams carry
some dissolved ions
created by mineral
alteration.
C - Clay and silt are
carried in suspension,
they are held within the
water by turbulent
eddies in the stream.
B - Sand, gravel, and larger particles
move as bed load, rolling or sliding close
to the channel floor.
2. The Work of Streams and Stream Gradation
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STREAM TRANSPORTATION
Streams transport material in a variety of ways:
• dissolved matter
• sediment in suspension
• bed load of larger particles bumped and rolled along
the bottom
A stream’s capacity to carry sediment increases sharply
with its velocity.
2. The Work of Streams and Stream Gradation
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STREAM TRANSPORTATION –
Increased Sediment
Accumulation - If more
sediment accumulates in
the stream channel than
can be carried away, the
channel surface builds up,
increasing stream's slope.
Increased velocity Increased slope causes
increased velocity and a
greater capacity to carry
sediment.
2. The Work of Streams and Stream Gradation
Equilibrium - Slope will
stabilize, stream just carries
away the sediment it
receives.
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STREAM TRANSPORTATION –
Decreased Sediment
Reduced sediment Sediment flow reduced,
stream will gradually
erode channel downward,
reducing its slope.
Decreased velocity Reducing slope reduces
stream's velocity and its
capacity to carry sediment.
2. The Work of Streams and Stream Gradation
Equilibrium – Slope
stabilizes, stream just
carries away sediment that
it receives.
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STREAM GRADATION
Over time, a stream develops a graded profile in which the gradient is just
sufficient to carry the average annual load of water and sediment produced
by its drainage basin.
1 - channel consists of a
succession of lakes, falls,
and rapids
2 - landscape is slowly
eroded by fluvial action
Each stream segment
adjusts to its own average
load, and the stream profile
is smoothed out into a
uniform curve
4 – 6 – profile now graded. Graded
profile is steadily lowered in elevation
toward the base level
2. The Work of Streams and Stream Gradation
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LANDSCAPE EVOLUTION OF A
GRADED STREAM
Stream Gradation Process –
1) Downcutting produces canyons and
gorges.
2) Stream becomes graded and begins to
build a floodplain.
3) River moves freely from one side of the
valley to the other.
4) Floodplain and alluvial meanders form.
(diagrams next slide)
2. The Work of Streams and Stream Gradation
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LANDSCAPE EVOLUTION OF A
GRADED STREAM
Ungraded stream - waterfalls, rapids, and
lakes and ponds. Flow faster at the
waterfalls and rapids, abrasion of bedrock
is intense, cutting back the falls and
trenching the rapids. Ponds and lakes fill,
lakes disappear, falls are transformed into
rapids.
Rapids eroded until gradient is closer to
stream’s average gradient. Main stream
branches into higher parts of the original
land mass, carving out many new small
drainage basins.
2. The Work of Streams and Stream Gradation
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LANDSCAPE EVOLUTION OF A
GRADED STREAM
Stream graded, floodplains develop. River
begins to wander sideward, cutting into
the side slopes, creating a curving path.
Alluvium accumulates on the inside of
each bend.
Cutting continues, floodplain widens,
channel develops bends, or alluvial
meanders. Floodplain becomes a
continuous belt of flat land between valley
walls.
2. The Work of Streams and Stream Gradation
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STREAM ORDER
• Smallest tributaries are
first-order (overland flow
from first-order basin)
• Two first-order streams
join, result in second-order
Stream properties are related
to stream order:
- Slope
- Drainage area
- Discharge.
Third-order drainage basin showing stream
channel system and drainage divide network.
2. The Work of Streams and Stream Gradation
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3. Fluvial Landscapes
GREAT WATERFALLS
AGGRADATION AND ALLUVIAL
TERRACES
ALLUVIAL RIVERS AND THEIR
FLOODPLAINS
ENTRENCHED MEANDERS
FLUVIAL PROCESSES IN AN ARID
CLIMATE
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3. Fluvial Landscapes
ALLUVIAL FANS
THE LANDSCAPE OF
MOUNTAINOUS DESERTS
THE GEOGRAPHIC CYCLE
EQUILIBRIUM APPROACH TO
LANDFORMS IN AN ARID CLIMATE
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Victoria Falls, on the Zambezi River.
GREAT WATERFALLS
Large waterfalls - comparatively rare
since stream gradation drains lakes
and removes falls and rapids.
Great waterfall formed by :
1. Fracturing by tectonic activity and crustal
dislocation. Great waterfalls on large rivers in the
African Rift Valley region, such as Victoria Falls,
on the Zambezi River.
2. River channels resulting from glacial activity in
the Ice Age. Large ice sheets eroded and
deposited sediment, creating lakes and shifting
river courses in northern continental regions.
Niagara Falls is a prime example (next slide).
3. Fluvial Landscapes
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GREAT WATERFALLS
Niagara River as it plunges over the Canadian (Horseshoe) Falls (foreground, right) and
the American Falls (center).
The river, which connects Lake Erie to Lake Ontario, provides water for domestic and
industrial uses as well as hydroelectric power
3. Fluvial Landscapes
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AGGRADATION AND
ALLUVIAL TERRACES
Alluvial terraces form when an aggrading river loses its sediment
input and begins degrading its bed, leaving terraces behind as it
cuts deeper into its sediment-filled valley.
3. Fluvial Landscapes
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ALLUVIAL RIVERS AND
THEIR FLOODPLAINS
Alluvial river (low gradient and broad floodplain)
has characteristic landforms, including:
• bluffs
• meanders
• cutoffs
• ox-bow lakes
• natural levees
Alluvial river - large river of very low channel
gradient, flowing on a thick floodplain of alluvium.
An alluvial river in a humid environment normally
experiences overbank floods each year or two
3. Fluvial Landscapes
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ALLUVIAL RIVERS AND
THEIR FLOODPLAINS
Ox-bow lake - silt and sand
deposited across the ends of
the abandoned channel. Oxbows fill with fine sediment
and organic matter and
eventually turn into swamps.
Bluff
Backswamp –
lower ground
between the
levees and the
bluffs
Natural levees - created during overbank flooding, when
sand and silt are deposited next to the channel creating
belts of higher land on either side of the channel.
Deposition is heavier closest to the channel, so levee
surface slopes away from the channel.
3. Fluvial Landscapes
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ENTRENCHED MEANDERS
Entrenched meanders form where rapid uplift causes
meandering rivers to cut deeply into bedrock
• Uplift increases the river’s gradient
• Gradient increases causes increased
velocity
• Water cuts downward into the bedrock.
• Forms a steep-walled inner gorge.
• Meandering gorge pattern distinguishes
entrenched meanders from floodplain
meanders of an alluvial river.
3. Fluvial Landscapes
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FLUVIAL PROCESSES IN AN
ARID CLIMATE
Although rain is infrequent in desert environments, running
water shapes desert landforms with great effectiveness
because of the lack of vegetation cover.
Humid regions (a), stream
channel receives ground water
through seepage.
Arid regions (b), stream water seeps
out of channel and into water table
3. Fluvial Landscapes
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3. Fluvial Landscapes
ALLUVIAL FANS
THE LANDSCAPE OF
MOUNTAINOUS DESERTS
THE GEOGRAPHIC CYCLE
EQUILIBRIUM APPROACH TO
LANDFORMS IN AN ARID CLIMATE
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ALLUVIAL FANS
Alluvial fans are common features of arid landscapes. They occur where
streams discharge water and sediment from a narrow canyon or gorge onto
an adjacent plain.
Alluvial fans are primary sites of
ground water reservoirs in the
southwestern United States.
A cross section shows
mudflow layers interbedded
with sand layers, providing
water (arrows) for a well in
the fan.
3. Fluvial Landscapes
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THE LANDSCAPE OF
MOUNTAINOUS DESERTS
Landforms of mountainous
deserts include alluvial fans,
dry lakes or playas, and
pediments - rock platforms
veneered with alluvium.
Where tectonic activity produces block
faulting in continental desert, fluvial
forms such as those of the basin-andrange region of the western United
States are produced.
3. Fluvial Landscapes
THE GEOGRAPHIC
CYCLE
3. Fluvial Landscapes
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The geographic cycle traces the fate
of rivers and fluvial landforms from
an initial uplift creating steep slopes
and canyons to a final low, gently
rolling surface called a peneplain.
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THE GEOGRAPHIC CYCLE
3. Fluvial Landscapes
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EQUILIBRIUM APPROACH TO
LANDFORMS IN AN ARID CLIMATE
The equilibrium approach sees fluvial
landforms as reflecting a balance between
processes of uplift and denudation acting on
rocks of varying resistance to erosion.
Equilibrium of landforms and landscapes is more
widely accepted today.
• Davis’ Geographic Cycle applies only where the land
surface is stable over time.
• Characteristics of the underlying rock material will
produce landscapes that appear to be part of the
geographical cycle, but are a function of resistance,
not time.
3. Fluvial Landscapes
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Chapter Review
1. Slope Erosion
Canyons
from Space
2. The Work of Streams and
Stream Gradation
3. Fluvial Landscapes
39
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