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Chapter 18: Arid Region Landforms and Eolian Processes
Physical Physical GeographyGeographyNinth EditionNinth Edition
Robert E. Gabler
James. F. Petersen
L. Michael Trapasso
Dorothy Sack
Arid Region Landforms and Eolian Processes
Arid Region Landforms and Eolian Processes
• Running water does more geomorphic work than wind– Eolian geomorphic processes are those caused
by wind– Arid regions provide a unique opportunity to
study geomorphology as there is little vegetation
18.1 Surface Runoff in the Desert
• Desert Characteristics– Lack of precipitation– High evaporation rates– Sparse vegetation– Low weathering rates and
insufficient vegetation result in little moisture-retention soil
– Running water very effective in shaping land
Q: Why do you think the drainage density is so high here?
18.1 Surface Runoff in the Desert
• Paleogeography– Most deserts have
not always been arid– Evidence:
• Deposits• Wave-cut shorelines
of extinct lakes• Immense canyons
occupied by streams far too small to have eroded such a large valley
18.1 Surface Runoff in the Desert
• Running Water in Deserts– Ephemeral channels– Lose water through
infiltration– Abundance of coarse
sediments– Braided channels– Discharge rates
• Downstream decrease:– Infiltration– Evaporation
18.1 Surface Runoff in the Desert
• Many desert streams terminate before sea– Cause
• Diminishing discharge• Mountains block
stream– Terminate in
depressions and form shallow, ephemeral lakes
– Interior drainage
18.1 Surface Runoff in the Desert
• Regional base level– Stream that terminates in interior– Tectonic Activity can change regional base level
• Floors below sea level– Death Valley, CA– Dead Sea, Middle East– Turfan Basin, China– Lake Eyre, Australia
18.1 Surface Runoff in the Desert
• Many desert streams terminate before sea– Streams that
originate in humid climates
• Humboldt River, Nevada
Q: Was the gorge eroded by the streams with this amount of flow?
18.1 Surface Runoff in the Desert
• Exotic Streams– Rivers that successfully
traverse the desert and make it to the sea
• Nile (Egypt and Sudan)• Tigris-Euphrates (Iraq)• Indus (Pakistan)• Murray (Australia)
– Colorado River• Usually does not make
due to human use
18.2 Water as a Geomorphic Agent in Arid Lands
• Arid Region Landforms of Fluvial Erosion– Channels of ephemeral
streams• Washes (arroyos)• Barrancas (Latin America)• Wadis (N. Africa and SE
Asia)– Braided channels– Prone to flash floods
18.2 Water as a Geomorphic Agent in Arid Lands
• Arid Region Landforms of Fluvial Erosion– Dense network of
barren slopes (Badlands)
• High drainage density• Dakotas• Death Valley, CA• Big Bend, TX• S. Alberta
18.2 Water as a Geomorphic Agent in Arid Lands
• Arid Region Landforms of Fluvial Erosion– Plateau
• Extensive, elevated region with fairly flat top
• Horizontal rock layers• Colorado Plateau
– Grand Canyon • Exposes the horizontal
rock layers• Rim of Grand Canyon
called caprock
18.2 Water as a Geomorphic Agent in Arid Lands
• Arid Region Landforms of Fluvial Erosion– Mesas– Buttes– Monument Valley (Utah
and Arizona)
18.2 Water as a Geomorphic Agent in Arid Lands
• Arid Region Landforms of Fluvial Erosion– Pediment
• Gentle retreat of steep mountain front
– Inselbergs
18.2 Water as a Geomorphic Agent in Arid Lands
• Arid Region Landforms of Fluvial Deposition– As flow of stream
decreases, capacity decreases, which increases deposition
– Alluvial Fans• Channels may flare out
onto open plains• Sediment deposited
along base of highlands• Fan apex
18.2 Water as a Geomorphic Agent in Arid Lands
• Arid Region Landforms of Fluvial Erosion– Alluvial fan
• Course sediment (boulders and cobbles) near fan apex
• Steepness decreases downslope
• Debris flow fans
18.2 Water as a Geomorphic Agent in Arid Lands
• Arid Region Landforms of Fluvial Erosion– Bajada
• Alluvial fans joined together
Q: Why would a series of alluvial fans have a tendency to eventually join to form a bajada?
– Piedmont alluvial plain
18.2 Water as a Geomorphic Agent in Arid Lands
• Playas– Bolsons
• Desert basins of interior drainage surrounded by mountains
– Playa• Lowest part of bolson• Fine-grained bed of
ephemeral lake• Pan
18.2 Water as a Geomorphic Agent in Arid Lands
• Playas– Salt crust playas (salt flats or salinas)– Bonneville Salt Flat, UT
• Speed record– Edwards Air Force Base, CA
• Space shuttle landings
18.3 Wind as a Geomorphic Agent
• Eolian– Landform created by
wind– Less effective than water– Wind Erosion and
Transportation• Deflation• Suspension• Surface creep• Ripples
18.3 Wind as a Geomorphic Agent
• Wind Erosion & Transportation– Deflation– Surface creep– Ripples– Abrasion
18.3 Wind as a Geomorphic Agent
• Wind Erosion & Transportation– Dust Storms– Sandstorms– Deflation hollows
Q: Can you suggest a continent that might be a source of major dust storms today?
18.3 Wind as a Geomorphic Agent
• Wind Erosion & Transportation– Desert pavement (gibber)
Q: Is desert pavement a surface indestructible by human activities? Why?
– Ventifacts• Individual wind-fashioned
rocks
18.3 Wind as a Geomorphic Agent
• Wind Erosion & Transportation– Pedestaled, or balanced
rock• Forms as a result of
physical and chemical weathering
Q: What other processes or rock factors could account for such an unusual shape?
18.3 Wind as a Geomorphic Agent
• Wind Erosion & Transportation– Yardang
• Wind sculpted remnant ridge, often of easily eroded rock
Q: Which is the upwind side of the yardang?
18.3 Wind as a Geomorphic Agent
• Wind Deposition– Sand dunes– Loess
• Fine grained sediment (e.g. silt) can be transported in suspension long distances before blanketing and modifying the existing topography
18.3 Wind as a Geomorphic Agent
• Sand Dunes– Result of wind deposition– Sand seas
• Seemingly endless dune regions
– Small dune fields (coast)– Dune topography
• Sand sheets
Q: Why are coastlines such good locations for dune formation?
18.3 Wind as a Geomorphic Agent
• Sand Dune Classification– Active
• Slip face• Angle of repose (35o)
– Stabilized
Q: These stabilized dunes are crossed by vehicle trails. How might these trails affect the stabilized dunes?
18.3 Wind as a Geomorphic Agent
• Sand Dune– When wind and velocity
are constant, a dune can move forward by downward transfer of sediment
– Blowout
18.3 Wind as a Geomorphic Agent
Q: Explain how plants can stabilize dunes.
18.3 Wind as a Geomorphic Agent
• Types of Sand Dunes– Classified according to
shape and relationship to the wind
– Barchans (crescent-shaped dune)
18.3 Wind as a Geomorphic Agent
• Types of Sand Dunes– Parabolic dunes– Transverse dunes– Longitudinal dunes– Star dunes
18.3 Wind as a Geomorphic Agent
• Longitudinal Dune, Sahara DesertQ: Estimate the ground length of the dunes in this satellite image.
18.3 Wind as a Geomorphic Agent
• Dune Protection– Recreation– Fragile environment
• White Sands, NM• Great Sand Dunes, CO• Cape Cod, MA
Q: Why should some dune driving need to be protected from human activities such as driving dune buggies and other recreational vehicles?
18.3 Wind as a Geomorphic Agent
• Loess Deposits– Wind can carry in
suspension dust-sized particles for thousands of km before depositing them
• Gobi Desert, China (30-90m thick)
• American Midwest
• Q: Where is the origins of these loess deposits?
18.3 Wind as a Geomorphic Agent
• Loess Deposits
Q: Why might the instability of loess cliffs be a problem?
18.4 Landscape Development in Deserts
• Geomorphic differences between arid and humid climates– Expanse of bedrock– Lack of continuous water flow– Eolian process plays a greater role– Fault-block mountains in the Great Basin range
• Orographic• Warner mountains• Panamint range
18.4 Landscape Development in Deserts
• A False-color satellite image of Death Valley, CA
• Panamint range to SW
Q: Why do you think the white areas are in the center of the valley?
18.4 Landscape Development in Deserts
• Alluvial fans and playas (interior drainage)• Pediments and inselbergs (tectonically stable since a
distant period of mountain formation
18.4 Landscape Development in Deserts
• Inselbergs– Erosional
remnant– Uluru (Ayers
Rock), Australia
18.4 Landscape Development in Deserts
• Green River Overlook, Utah• Q: What aspects of this environment make this an
attractive landscape?
Physical Geography
End of Chapter 18: Arid Region Landforms and Eolian Processes