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