Physical Geography Final - Chapter 18: Topography of Arid Lands

deserts: a specialized environment

~ 1/3rd of Earth's land surface is desert = arid land with meager rainfall that supports only sparse vegetation and a limited population of people and animals.
• Stark - severe and bare
• These arid regions are called deserts because they are dry.
• They may be hot, they may be cold.
• They may be regions of sand or vast areas of rocks and gravel peppered with occasional plants.
• But deserts are always dry.

weathering

mechanical weathering dominant; slower weathering and angular particle formation

soil and regolith

soil is thin or absent, exposing bedrock

soil creep

minor due to lack of soil and lubricating effects of water

impermeable surfaces

caprocks and hardpans; high water runoff

sand

some deserts have sand abundance, allows for water input into the ground, easily moved by rain and wind

rainfall

most streams are ephemeral, effective agents of erosion; alluvium unusually common in deserts

wind

wind action shifts particles

basins of interior drainage

most watersheds do not drain into any ocean, water transferred to basin or valley with no external outlet

vegetation

lack of continuous vegetative cover

role of water

• Running water = most important external landform agent
• Erosion tremendously effective with little plant cover
• Intensity of rain + impermeable land surfaces = intense runoff
• Unpredictable imbalance between erosion and deposition
• Surface water in the desert

exotic streams

permanent streams that originate outside the arid land (Nile River)

ephemeral streams

periodically flow - intense erosion, transportation, & deposition

desert lakes

playas and salinas (dry salt lake beds) saline lakes

nile river

The River Nile is the longest river in the world ~6,695 km (4,160 miles) long with average discharge is 3.1 million liters (680,000 gal) per second.
• Its origin although not clear (it is either in Burundi, or in Victoria Falls on the Zambezi River at the border of Zambia and Zimbabwe) is in the south of the equator, and it flows northward through northeastern Africa cutting a deep channel through the Sahara in Eritrea, northern Sudan, and in Egypt, finally discharging into the Mediterranean Sea.
• Three rivers feed the Nile: the Blue Nile, the White Nile and the Arbara.
• The Nile river basin comprises ten countries: Burundi, Democratic Republic of Congo, Egypt, Eritrea, Ethiopia, Kenya, Rwanda, Sudan, Tanzania, and Uganda.

fluvial erosion in arid lands

- Occurs during small portion of the year, flash floods
- Differential erosion - variations in slope and shape of
landform from rock type variations
- Residual erosional surfaces - inselbergs (e.g. bornhardts), pediments
- Desert stream channels - ephemeral stream beds

fluvial deposition in arid lands

- Talus accumulations at the foot of steep slopes
- Piedmont - zone at the foot of a mountain range
- Piedmont angle
- Basins of interior drainage covered with fine particles since flow volume & speed are low

role of wind - erosion

•Wind as a sculptor is a relatively limited effect
•Air right near surface has zero wind
•Wind speed increases with distance above ground
•Effects of wind shear
•Aeolian processes (Aeolian = relating to or arising
from the action of the wind)

Erosion - Two effects:

Deflation - shifting of loose particles via the wind, blowouts
- Abrasion - requires tools such as airborne sand and dust, sculpts landforms already in existence, ventifacts

aeolian transportation

- only finest particles are carried into suspension as dust
- dust storms
- large particles moved by saltation (curved trajectory) and traction (rolled or pushed)
- creep by saltation

aeolian depostion

- Fine sand laid as thin coating; no landform significance
- Coarser sand deposited locally; sand plains or sand dunes

desert sand dunes

some dune fields composed of unanchored sand, moved by local winds, slip face

coastal dunes

- Ocean waves deposit sand on beaches
- Prominent onshore winds move sands inland

loess

- Wind deposited silt
- Lacks horizontal stratification
- Great vertical durability
- Formation not well understood

ergs

seas of sand
- Large area covered with sand in dune formation from wind
- Hypothesized to have originated in a more humid climate
- Drying of climate combined with wind created formations seen today
- Sahara and Arabian deserts

regs

stony deserts
- Tight covering of coarse gravel, pebbles, and/or boulders
- Desert pavement or desert armor
- Desert varnish - dark shiny coating consisting of iron and manganese oxides
- Desert varnish - a useful dating tool

hamada

barren bedrock
- Barren surface of consolidated material
- Exposed bedrock or cemented sedimentary material
- Regs and hamadas extremely flat

basin-and-range terrain (death valley)

- Largely without external drainage
- Numerous fault-block mountain ranges
- Three principle features: basins, ranges & piedmont zones.

the basin

-Flattish floor, very gentle slope on all sides towards a
low point
-Shallow, ill-defined drainage channels
-Playa lakes - Salt accumulations
-Basin floor covered in fine grain material

ranges

- Surface features shaped by weathering, mass wasting, & fluvial processes
- Long, narrow ranges of different elevations

piedmont zone

- Sharp break in slope that marks change from range to piedmont
- Underlain by erosional pediment
- Alluvial fan - channels on piedmont break into distributaries, deposit new material on old material
- Coalescing alluvial fans = Piedmont alluvial plain aka bajada

Mesa-and-Scarp Terrain

Associated with horizontal strata
• Variable resistance to erosion in strata
• Plateaus and stripped plains
• Sapping - groundwater seeps out of the scarp
face and erodes soluble material
• Buttes - small surface areas and cliffs that rise
above surroundings
• Pinnacles
• Buttes, mesas, and pinnacles typically found
near a retreating escarpment face

badlands

- Overland flows from occasional rains develop tiny
rills that expand into ravines or gullies
- Characterized by maze of ravines and gullies,
lifeless and nearly impassable
e.g. Badlands National Park, South Dakota

arches and natural bridges

- Arch formation
- Natural bridge formation
- Pillar formation
e.g. Arches National Park, Utah

the aral sea

• The Aral Sea (45°N, 60°E) - situated in Central Asia between Southern Kazakhstan and Northern Uzbekistan - is a natural, permanent, saline, endorheic (completely contained within with no outlet or exit to ocean) lake.
• The Aral Sea is tectonic in origin and is one of less than 20 ancient lakes in the world, estimated to be more than 5 million years old.
• The Aral Sea has two major inflow rivers, the Amu Darya and the Syr Darya, and its catchment basin spans six countries within Central Asia.
• Until late 20th century it was the world's 4th largest saline lake, containing 10g salt/ liter.
• The Aral Sea is synonymous with environmental catastrophe. In the Soviet Union era massive amounts of water were diverted for irrigation of cotton and the lake began to shrink dramatically. At the same time, pesticides were being applied to fields in the watershed by airplane.
• It has now lost 90% of its source waters, 2/3rd of its surface area, and its salinity has increased fourfold.
• Changes in the lake have caused local climates to change and desertification has increased, along with rates of respiratory diseases and cancers from salt and toxic-laden dusts.