Geography- Deserts

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Introduction

  • Deserts are arid areas with low precipitation and high rates of evapo-transpiration.  They receive less than 250mm of rainfall a year
  • Desert margins are semi-arid areas on the fringes of arid deserts. They receive between 250mm and 500mm of rainfall a year
  • Hyper arid desert areas receive less than 100mm of rain a year
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Hot deserts and Margins

Hot deserts and their margins are:

  •  Mostly located within and around the tropics of Cancer and Capricorn, at around 30 degrees north and south of the equator
  •  not found at the Equator, apart from a small part of East Africa
  •  on the western sides of continents, especially North and South America, Australia and southern Africa

The semi-arid areas are generally on the edges of the arid ones. They are most extensive in North Africa, Asia and Australia.

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Temperature

Hot desert and their margins have extreme temperature ranges, both annually and diurnally.

Annual range:Hot desert areas that are closer to the equator can get extremely hot, with air temperatures reaching 50ºC in the shade, but they have lower annual ranges of around 10ºC.   Example: Central Sahara. The sun is overhead throughout the year.

 Sub-tropical deserts and semi-arid areas have colder winters and therefore a wider annual temperature range, often between 25-30ºC   Example:   Northern Sahara

Higher latitudes mean the sun is at an angle in winter. This means more radiation is reflected and a larger area of land has to be heated, therefore less efficient insolation. Also the days are shorter in winter meaning less time for insolation to happen.

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Key words

Diurnal range- The difference between day time and night time temperatures. This is usually large in deserts because of a lack of cloud cover to trap heat at night.

Sirocco- This is a wind which starts in the Sahara but often blows across the Mediterranean bringing hot dry weather – and sand!

Hyper Arid- Some deserts are extremely dry and only receive up to 100mm of rain a year.

Erratic- Very unpredictable – this is what precipitation is like in deserts. Often a yearly average will fall in just a few hours.

Annual range- The difference between the highest and lowest temperatures in a year. This is usually quite big in deserts.

Flash floods- These are torrents of water that often occur after heavy rain.

Insolation- The solar radiation that reaches the earth's surface, warming it up.

Solar radiation- This is energy emitted by the sun.

Surface run-off- Where precipitation cannot soak into the ground either because it is hard, baked earth or because it is already saturated, it travels across the surface.

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Key words continued

Water deficit- Deserts usually do not have enough water – there is more ‘taken out’ by ……………………………than there is put in by precipitation.

Trade winds- These are wind patterns that blow from areas of high pressure (across deserts) to areas of low pressure (towards the equator).

Evapotranspiration- This is a combination of where water is evaporated from the earth’s surface or bodies of water (like lakes) and where water moves from the leaves of plants into the air.

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Diurnal range

The range between daytime and night temperatures is high because there is usually no cloud cover, often ranging between 15 -20ºC

  • Daytime- In the daytime the lack of clouds means high levels of solar radiation reach the desert surface. The ground soaks up much of this energy (insolation). Heat transfer from conduction then warms the air above the surface to give high daytime temperatures, as high as 50 ºC in the central Sahara.
  • Night- At night the lack of clouds means that the warm surface and air is rapidly cooled as energy (long wave terrestrial radiation) is not trapped and escapes to the atmosphere. It is not uncommon for temperatures to fall below freezing at night in the northern Sahara.

However deserts on the west coast of continents like the Atacama and Namib generally have lower diurnal ranges. This is because ocean breezes travelling across the land keep daytime temperatures lower, usually around 22 ºC,  thus reducing the day and night range.

Precipitation-  Rainfall in hot deserts is:Low - usually less than 250mm. Hyper-arid areas such as the central Sahara can be as low as 10mm a year. (800mm in Wigan)Annual averages are generally calculated over a 30 year period. Erratic  - unreliable – often areas with the lowest rainfall (hyper-arid) have the most unpredictable patterns. Some parts of the Atacama desert in Peru and Chile for don’t see rain decades, leading to drought.

 

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Precipitation

However - when rainfall does arrive in hot deserts it can be very extreme.

It is not uncommon for a yearly average to fall in just a few hours and often single storms can exceed annual averages.

 Lima in Peru has an annual mean of 46mm. A storm in 1925 put down 1524mm in one day.These storms lead to rapid surface run-off and flash floods.

 Water Deficit in the water budget- Hot deserts almost always have a water deficit in the water budget because of low precipitation and high temperatures which give a high potential evapo-transpiration.

Semi-arid areas have more predictable seasonal patterns of rainfall.

 Wind- Both large scale and local wind patterns vary between and within deserts regions. Large scale trade winds blow towards the equator.

Winds are generally stronger during the day because solar radiation causes convection. These can sometimes lead to sandstorms and dust devils.

The sirocco is a Sahara wind that blows across the Mediterranean into southern Europe, bringing hot, dry weather and sand!

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Soils in the desert

Description- Soils in hot deserts are called aridisols and they are often coarse, dry and shallow. They have mineral content but low levels of organic matter. They are usually alkaline, with pH value between 7 and 8.5. They are also salty (saline) on the surface. All of this means most soils are infertile. They range from yellowy red to grey-brown in colour. In semi-arid regions solentz soils are less salty on the surface but contain sodium salts below, which limits farming.

 Explanation- Desert soils are restricted by:

  • Lack of moisture
  • High levels of evapotranspiration
  • Low or no vegetation cover

 Aridsols in hot deserts are dry because of low precipitation and high evaporation. They are sometimes salty (saline) soils called solonchaks because of capillary action, caused by high surface temperatures drawing water from the water table.  This water contains minerals such as carbonates and chlorides. The water quickly evaporates to leave behind the salty, alkaline minerals. They lack organic content because of sparse vegetation. This means very little decaying matter is available to create humus. Even in semi-arid regions with more vegetation the lack of rainfall for most of the year slows down biological decomposition in the solonetze soils. Soils are thin because of the lack of ground cover, meaning there are no roots to bind the soil together. This makes the soil easily exposed to wind and water erosion.

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Formation of soils in deserts

Soils in hot deserts are called aridisols and they are often coarse, dry and shallow. They do have mineral content but low levels of organic matter. They are usually alkaline.The soils are dry because of low precipitation and high evaporation; this is because most deserts are located in areas of high pressure where the sun is over head for most of the year; for example central Sahara. These high temperatures can make desert soils salty because of capillary action, caused by high surface temperatures drawing water from the water table.  This water contains minerals such as carbonates and chlorides. The water quickly evaporates to leave behind the salty, alkaline minerals.They lack organic content because of sparse vegetation. This means very little decaying matter is available to create humus. Therefore soils are infertile and only specially adapted vegetation is able to grow. Even in semi-arid regions with more vegetation the lack of rainfall for most of the year slows down biological decomposition in the solonetze soils.Soils are thin because of the lack of ground cover, meaning there are no roots to bind the soil together. This makes the soil easily exposed to aeolian and water erosion.

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Vegetation- Xerophytes

Desert plants have many special adaptations that allow them to survive low precipitation

Xerophytes   - withstand drought- These are plants that have adapted to cope with long periods of drought. Two types are:

1. Succulents:          - store water in stems, leaves and roots E.g. Saguaro Cactus- Cacti, found in North and South America, are the best known succulents. They store water in fleshy stems. They have shallow but far-reaching roots, adapted to soak up maximum moisture during rainfall.They can also store water in root bulbs, allowing them to survive for years without rain. Some succulents have also adapted to reduce transpiration. They do this by:

  •  having thick, waxy leaves which reduce moisture loss and reflect radiation from the sun.
  • having spikes or thorns instead of leaves
  • having pores (stomata) that close during the day and open at night when it is cooler.

 2. Phreatophytes:  -  have long roots to soak up water deep below the surface  E.g Mesquite- These plants are often found where the water table is closer to the surface, often around wadis. Mesquite plants, found in Arizona, can tap water 20 metres below the surface.

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Vegetation- Ephemerals and Halophytes

Ephemerals – short periods of growth or short life cycles- E.g Desert Lily

There are two types:

  •  Perennial ephemerals have short periods of growth and can stay dormant for long periods before ‘coming to life’ when it rains. The Desert Lily in Arizona does this by storing energy in its roots and bulbs. In summer the whole of the exposed plant dries out making it appear to be dead.
  •  Annual ephemerals exist as seeds until it rains. They then germinate, flower, seed and die within weeks or even days. This can produce a brief explosion of colour in the desert after the rains. The seeds then lie dormant until the next rains.

 Halophytes    -Adapted to salt (saline tolerant) E.g Salt bush- These plants have adapted to salty conditions, especially around salt pans. The saltbush is often the only species growing in the region as no other plant can cope with the salt levels in the soil.

Some of these plants can actually ‘compartmentalise’ the salt; storing it in separate tissues away from growing cells

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Causes of Aridity

  • Global Atmospheric Circulation
  • Continentality
  • Cold Ocean Currents
  • Relief

Global atmospheric Circulation-

  • At the equator air rises rapidly as it is heated by the sun, leaving low pressure at ground level.
  •  As this dry air then travels north and south from the equator it begins to cool causing it to sink at 20 - 30º latitude (the sub-tropics).
  •  This descending air creates high pressure and becomes warmer due to compression, allowing it to hold more moisture.
  •  High pressure prevents any evaporation rising and therefore clouds don’t form meaning no rain for most of the time, e.g. in the Sahara.

 When the air reaches the surface it is then sucked back to the equator to fill the low pressure gap. This movement creates trade winds.

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Causes of aridity

Continentality- The central Sahara, Simpson and Colorado deserts are all in the middle of continents a long way from the sea. This means that moist coastal winds have already dropped their rainfall before they reach these areas. They also pick up low moisture as they travel overland because less evaporation takes place on land than sea. These are called continental isolation deserts

Relief- Trade winds blow from the south east- The winds are forced to rise over the Andes mountains- This causes condensation and precipitation on the windward side of the mountain-This leaves a dry wind descending the leeward side of the Andes

Cold Ocean Currents-Deserts that are located on the western edge of continents (Atacama, Namib, Mojave and Western Desert in Australia) have cold ocean currents offshore

The cold water cools the air above to dew point causing condensation. Small amounts of moisture remain which creates a fog which is then blown inland. But, descending warm air from above prevents the fog from rising to form clouds; this is called a temperature inversion. Instead the fog is ‘burnt off’ by the strong solar radiation and usually disappears by midday. This process results in very little cloud formation and so very low precipitation levels.

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Mechanical weathering

Exfoliation (onion skin weathering) - This is when the rock is weakened by the high diurnal temperature range. During the day high levels of insolation cause the rock to expand. At night the rock cools down and contracts. This repeated process eventually causes layers of rock to peel off, exfoliation.

Granular disintegration- This is also caused by the high diurnal and annual temperature range. This time different minerals within the rock expand and contract at different rates, causing stresses in the rock structure. In granite this eventually causes the rock to breakdown.

Shattering - Repeated expansion and contraction cause some rocks such as basalt to shatter as a result of the stresses.

Block separation - Repeated heating and cooling of sedimentary rocks such as limestone causes the rock to break away from the horizontal and vertical joints. However geologists now believe that these processes of mechanical weathering also rely on small amounts of water from fog and dew to act as a trigger. Once the rock has been broken down by weathering the loose rock is easily eroded by wind and water.

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Erosion

Erosion is when broken down rock is transported somewhere else, usually by wind or water.

  • Wind Action (Aeolian Erosion)- In the desert weathered broken down rock particles (regolith) lie loose (unconsolidated) on the surface. They are easily picked up by wind and water and used for erosion.
  • Deflation  - This is when the wind removes the finest, loosest material to leave behind larger stones. This happens a lot because the sand and dust particles are dry and not bound together by vegetation roots. This creates a reg, sometimes called a desert pavement
  • Abrasion - This is when sharp particles of sand carried by the wind rub against exposed rock to wear away the surface, like sandpaper action or sandblasting, and create new shapes and features. This mostly happens no more than 2m above ground because the most erosive sand particles are heaviest. The rate of erosion is much slower than deflation and will depend on the wind frequency, speed and direction and the hardness of rocks and regolith in the area. It will often take 100 years to erode 1mm of exposed rock. Both of these erosion processes occur during transportation of particles.
  • Transportation- Wind carries particles in three ways. This movement will depend on: Wind frequency- speed and direction, Turbulence – forces of drag and lift, Type of surface rock, Size of regolith, Amount of surface vegetation


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Erosion

Suspension- The finest silt, clay and sand (less than 0.15mm diameter) can be carried within the atmosphere over long distances. Particles from the Sahara are occasionally blown as far as the U.K. where they fall as ‘red rain’. If winds are strong enough sandstorms can occur.

Saltation- Particles measuring between 0.15 – 0.25 mm in diameter are lifted by strong gusts of wind. They then hop along the surface close to the ground (around 1m), as they drop they often loosen other grains, causing them to hop.This is the main form of transportation in deserts.

Surface Creep- Heavier grains of sand (over 0.25mm diameter) are too heavy to be lifted and are usually rolled along the surface by wind or pushed along by other grains. Some will be dislodged by saltation and then rolled.

Deposition- When the wind speed drops particles are dropped. This often happens on the downward (leeward)side of an obstacle such as vegetation or a rock.

Erosion, Transportation and Deposition working together. All of these processes work together in a cycle:

Erosion will pick up (entrain) loose particles in the regolith- Transportation carries this material- The particles are used for abrasion as they are transported- Deposition drops the particles when the wind drops - When the wind increases the same particles will be entrained again.

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Landforms

From wind erosion- Regs- This is when the wind removes the finest, loosest material to leave behind larger stones. This happens a lot because the sand and dust particles are dry and not bound together by vegetation roots.                         Deflation Hollows- Sometimes massive volumes of sand can be eroded to create a deflation hollow.They are low-lying rounded basins

From abrasion- abrasion takes place no more than 1.5m above ground because the most erosive sand particles are heaviest. The process is effectively sandblasting.

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Dunes

  • Dunes are formed by the transportation of large quantities of sand particles in suspension, saltation and surface creep.
  • When the wind velocity drops these particles are deposited to form the dunes in an Erg. They tend to form where vegetation, other dunes or hollows can trap the sand.
  • The size and shape of dunes depends on wind direction, duration and strength, the availability of sand and the amount and type of vegetation.
  • Dunes are mobile and constantly move downwind as transportation continues with the prevailing wind.
  • Most dunes are asymmetrical. The windward gently sloping side is formed by saltation and surface creep, creating ripples.
  • The steep leeward side is formed as the sand slips and slumps due to eddying.
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Effect of water in hot deserts

Exogenous- These are perennial rivers (they flow all year long). They have their source outside of the desert, then flow through the desert until they reach the sea. Nile: Has its main source, the Blue Nile, in the mountains of Ethiopia. It then flows through the Sahara in Egypt until reaching the Mediterranean at Alexandria. Colorado: Rises in the Rocky Mountains, flows through the arid south east USA until reaching the Pacific Ocean. The level of discharge varies thoughout the year and because they flow through arid areas they are usually heavily used for water supplies and irrigation.

Endoreic-

These are rivers that flow into an inland lake in the desert rather than the sea. These lakes often dry up to leave salt pans. This happens because of high levels of water loss due to evaporation and infiltration. In other words the river never make it to the sea.

The river Jordan flows into the Dead Sea at 422m below sea level.

 

 

 

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River erosion in deserts

Rapid surface run-off and high river discharge levels in desert storms results in powerful erosion. This is due to;

 

  • High intensity rainfall

  • Low interception because of sparse vegetation cover

  • Dry sun-baked soil that limits infiltration

  • Shallow soils meaning low soil moisture storage and rapid saturation

Erosion features are common in the high mountain areas, whereas…..Deposition features are dominant in the low lying areas

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Landforms- WADIS

Wadis are steep sided dry river channels. They have a wide channel floor covered in sediment. They range from small channels a few metres long to networks hundreds of km long. Smaller wadis are called gullies or rills. The channels only hold water after heavy rainfalls.

  • Wadis are caused by ephemeral rivers that only flow in seasonal rainfalls or flash floods after storms.
  • However discharge can be very high due to rapid surface run-off and steep long profile gradients.
  • This makes erosion from abrasion and hydraulic action powerful and carves out steep sided, wide channels. When the rivers dry up deposition often forms braiding on the river bed.
  • Some larger wadis were formed in previous climates when there was more rainfall.
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Pediments

Description

Pediments are the gently sloping (less than 7º) bases of mountains, cliffs or steep hills in deserts. Sometimes they are exposed bare rock but usually they are covered with debris from rock falls or alluvial fans. This sediment gets finer with distance from the foot of the cliff.

 Formation

  •  The origin of pediments is uncertain. One main theory is:
  • Firstly weathering of the steep slope above the pediment, often mesas or buttes, causes thin layers of rock to peel away. This is called parallel retreat. The debris falls on to the pediment below.
  • Then when storms occur flash flooding creates extensive surface run-off known as sheetflow. This water washes the debris down the pediment and scratches the surface via abrasion.
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Mesa and Butte

These are examples of Relict Hills. They are the remains of large plateaus that have been left behind after extensive water erosion around them over millions of years. Some of this erosion took place when the desert area had a wetter climate.  These features continue to be worn away by water action, weathering and wind erosion.

 Mesas and Buttes- Description- Mesas (Spanish for table) are large, flat topped and steep sided hills or mountains (plateaus) that are isolated from each other. They usually have gentle sloping pediments at their base covered in sediment (scree). They are formed in areas of sedimentary rock and horizontal layers are often visible in their structure.

 Buttes (Spanish for tree trunk) are the same as mesas but further water, wind erosion and weathering has reduced their size until they are no longer wider than tall.They resemble pillars and are often close to mesas because they may once have been part of the mesa.

 Formation- Mesas and Buttes are Relict Hills. They are the remains of large plateaus that have been left behind after extensive water erosion around them over millions of years. Some of this erosion took place when the desert area had a wetter climate. They are formed in areas of sedimentary rock and have remained because they have more resistant cap rocks which are harder to erode.

They continue to be worn away by water action, weathering; such as exfoliation, and wind erosion. This often happens in parallel retreat. Water erosion happens from surrounding wadis and sheetwash during flash floods.

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Inselberg

Inselbergs are also isolated relict hills but they are more rounded than mesas and buttes. They are dome shaped. They are more common in semi-arid regions and are often granite.

Formation

They were formed when precipitation was much higher than today. This caused chemical weathering of the crystalline rock such as granite. As water washed away the weathered deposits the rock became exposed. They are isolated as surrounding areas of less resistant rock have been eroded away by water.

They can be several km’s long.

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Landforms of river deposition

Salt lakes- Description- Salt lakes are hollows or depressions in the desert which are sometimes partly covered by shallow saline lakes. However they often dry up when precipitation is absent to leave behind a thick crust which cracks to form a polygonal pattern. This is called a salt flat or salt pan. They are often bordered by saline resistant halophytes such as salt bushes. Examples include the Dead Sea and the Great Salt Lake in Utah, North America which cover several km².

  • Salt lakes are formed when precipitation collects in hollows from surface run off and endoreic rivers in inland drainage basins. They can also form when deflation hollows have been eroded down to the water table.
  • However lack of precipitation and high rates of evaporation leave many salt lakes dry for most of the year to leave behind salt pans.
  • This leaves behind deposits of soluble salts that were previously transported by desert streams. This dried out crust often cracks to form a polygonal surface pattern.
  • The most common salts are sodium chloride and calcium sulphate (gypsum) which are often commercially extracted from the area
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Alluvial Fans

Description- Alluvial Fans are cone shaped deposits of sediment that form where ephemeral wadis emerge from mountains onto the pediments or plains below.

The sediment is graded with the coarsest material closest to the mountain and finer silts washed further down the fan.

Alluvial fans vary from a few metres to several kilometers in length and they sometimes join with adjacent fans to form a bahada.  These are common in Death valley in California.

  • Alluvial fans are formed when wadi streams fill with water during flash foods in times of heavy precipitation.
  • High discharge erodes the steep sided wadis to provide the sediment which is transported in the steep mountain valleys.
  • When the river emerges onto the lowland the sudden decrease in gradient slows the water down as it loses energy. Also, as the water is no longer trapped by the steep sided wadi walls, it as able to spread out across a wide area; creating friction and rapid deposition.
  • As this happens the water spreads out into numerous distributaries which creates the cone shaped deposition. The sediment is graded with the coarsest material deposited first closest to the mountain and finer silts washed to the foot of the fan.
  • When two or more fans from parallel wadis join together they create a bahada.
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Badlands

Badlands are found in some semi-arid desert margins. Water and wind erosion has carved out a dramatic landscape leaving erosion and deposition features; including wadis, gullies, alluvial fans, pipes and natural arches, and hoodoos.

Badlands usually have more vegetation than arid areas but it still tends to be sparse.

Examples include the Badlands National Park in South Dakota and Matmata in Tunisia.

  • Badlands are semi-arid (between 250 and 500mm rainfall a year) but because rainfall is erratic vegetation is sparse; meaning the dry soils and regolith are not stabilised by roots and are therefore exposed to water and wind erosion.
  • However when rain does arrive it is often heavy leading to rapid surface run off and replenishment of ephemeral streams in wadis.
  • The combination of intensive rainstorms, sparse vegetation cover and soft sediments from sedimentary bedrock results in high rates of erosion.
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Deposition landforms

Erosion features

Wadis –   steep sided, beds covered in debris, dry most of the time

Gullies –  small channels that cut into the hillsides contributing to their collapse from slumping (slope failure)

Pipes  -These are formed when water gets into the cracks as clay dries out. The water flows underground and erodes passageways. Sometimes pipes are just a few mm in diameter (micro-piping) but increased discharge after heavy storms can carve out pipes several metres in diameter (meso-piping)

Micro-piping often creates a honeycomb appearance on the surface above.

Sometimes the roof of pipes collapse to expose gullies.

Caves and natural arches- Caves are formed when the pipes flow through a particularly soft part of the bedrock. This results in faster erosion which carves out a cave. Over time caves can be eroded backwards to form natural arches.

 Hoodoos- These are relict hills formed in sedimentary rock. They are pillar shaped and formed when resistant cap rock has protected the softer column of rock below from being washed away during sheetflow.

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Role of water in desert landforms

Although precipitation is low and erratic in deserts, such as the Sonoran, when it does arrive it is often heavy leading to rapid surface run off and high discharge in desert rivers. This means that water erosion is powerful. Water has been important in creating certain erosion landforms. Wadis are caused by ephemeral rivers that only flow in seasonal rainfalls or flash floods after storms. However discharge can be very high due to rapid surface run-off and steep long profile gradients. This makes erosion from abrasion and hydraulic action powerful and carves out steep sided, wide channels. When the rivers dry up deposition often forms braiding on the river bed. Other erosion features include mesas and buttes. These are relic hills formed when a resistant cap rock has protected softer layers of rock below. Over time water has washed away the surrounding rock to leave behind the isolated flat topped hills. These features continue to be shaped by the action of water, however much of the erosion involved in creating them happened in previous climatic periods when there was more precipitation.Water has also created deposition features- Alluvial fans are formed when wadis emerges onto the lowland. The sudden decrease in gradient slows the water down as it loses energy. Also, as the water is no longer trapped by the steep sided wadi walls, it as able to spread out across a wide area; creating friction and rapid deposition. As this happens the water spreads out into numerous distributaries which creates the cone shaped deposition.  ,Salt lakes are formed when precipitation collects in hollows from surface run off and endoreic rivers in inland drainage basins. However lack of regular precipitation and high rates of evaporation leave many salt lakes dry for most of the year to leave behind salt pans. In semi arid areas such as the Badlands National Park in South Dakota water plays a more important role in carving out features such as gullies, arches and pipes.In conclusion then past and present actions of water are important in forming desert landforms. However weathering and wind erosion are also important in changing some of these features, particularly in hyper-arid areas.For example mesas continue to wear away due to parallel retreat by exfoliation and wadis get undercut by abrasion as the wind carries sand in suspension.Water and wind work together; fluvial deposition in wadis is picked up by the wind in dry periods and used for abrasion.

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