Much of the desert Southwest region of the United States was settled in the past century following a century of historically high rainfall. Towns and cities grew, and the Bureau of Land Management diverted water from melting snows, rivers, and underground aquifers to meet the needs of growing cities. Some of the country's largest and newest cities, including Phoenix, Tucson, Denver, Las Vegas, Los Angeles, San Diego, and Albuquerque, have grown out of the desert using water from the Colorado River system. Even though the temperatures can be high, the air is good, and many people have chosen to move to these regions to escape crowded, polluted, or allergen-rich cities and air elsewhere. The surge in population has been met with increases in the water diverted to these cities, and fountains, swimming pools, resorts, golf courses, and green lawns have sprung up all over. In general the life can be comfortable.
In the past decade the water seems to be diminishing. Lake Powell in Arizona has shrunk to half its capacity, and the Colorado River flow shrunk to a quarter of its typical rates. The Colorado River is used to supply 30 million people with water and irrigates four million acres of fertile farmland, producing billions of dollars worth of crops. The massive waterworks systems across seven states in the southwest were all built using river flow data for the Colorado River based on 20th century flow records. Now, studies of the ancient climate history in the region going back thousands of years indicate that the 20th century may have been one of the wettest on record for the region. The Hoover Dam, the California aqueduct, and cities across the region were all built during this high flow stage of the Colorado River, and water budgets for the region were calculated assuming these flows would continue. Now, precipitation is decreasing, and the historical records show that the region regularly experiences droughts where the flow decreases to 80 percent and even 50 percent of the 20th century values used for building the civilization in the desert Southwest. Now that more than 80 percent of the water from the river is used for human consumption, droughts of this magnitude have severe implications for any community, and the water wars of the Southwest may eventually start again. Historical records show that past civilizations such as the Anasazi in the region disappeared at the end of the 13th century during a similar drought period, and similar trends are expected by climate modelers for the future in the region.
Climate change models released by the National Ocean and Atmospheric Administration show that the flow of the Colorado River may decrease to half of its 20th century values by the middle of this century and that these lower flow values will persist into the foreseeable future. The region is already experiencing rapid changes, with wildfires burning huge tracts of vegetation and occasional storms initiating mudflows and other desert processes. Climate models predict a likely descent of the region into dust bowl conditions and that these changes have already begun. The region saw many mega-droughts in medieval times and throughout history, and states of the region need to prepare for the likelihood of many years of water shortage and increasing drought conditions. \_/
systems typically lose moisture as they rise over mountains. These remote areas therefore have little chance of receiving significant rainfalls. The most significant desert in this category is the Taklimakan-Gobi region of China, resting south of the Mongolian steppe on the Alashan plateau, and the Karakum of western Asia. The Gobi is the world's northernmost large desert, and it is characterized by 1,000-foot-high sand dunes made of coarser than normal sand and gravel, built up layer by layer by material moved and deposited by the wind. It is a desolate region, conquered successively by Genghis Khan, warriors of the Ming dynasty, then the People's Army of China. The sands are still
littered with remains of many of these battles, such as the abandoned city of Khara Khoto. In 1372, Ming dynasty warriors conquered this walled city by cutting off its water supply consisting of the Black River, making a blockade and siege of the city, then massacring all remaining people in the city.
A third type of desert is found on the leeward (or back) side of some large mountain ranges, such as the sub-Andean Patagonian Gran Chaco and Pampas of Argentina, Paraguay, and Bolivia. A similar effect is partly responsible for the formation of the Mojave and Sono-ran Deserts of the United States (although these are also in the belt of global 15-30° latitude downwelling dry air). These deserts form because as moist air masses move toward the mountain ranges they must rise to move over the ranges. As the air rises it cools, and cold air can hold less moisture than warm air. The clouds thus drop much of their moisture on the windward side of the mountains, explaining why places like the western Cascades and western Sierras of the United States are extremely wet, as are the western Andes in Peru. However the eastern lee sides (or back sides) of these mountains are extremely dry. The reason for this is that as the air rose over the fronts or windward sides of the mountains, it dropped its moisture as it rose. As the same air descends on the lee side of the mountains it
Warm moist air
Rainstorm expansion and cooling
Evaporation from compression and cooling
Warm moist air
Rainstorm expansion and cooling
Evaporation from compression and cooling
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Windward Sierra Nevada Leeward Great Basin (wet) range (dry)
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Windward Sierra Nevada Leeward Great Basin (wet) range (dry)
Diagram illustrating formation of rainshadow deserts gets warmer, and is able to hold more moisture than it has left in the clouds. The result it that the air is dry and it rarely rains. This explains why places like the eastern sub-Andean region of South America and the Sonoran and Mojave Deserts of the western United States are extremely dry.
Rainshadow deserts tend to be mountainous because of the way they form, and they are associated with a number of mass-wasting hazards such as landslides, debris flows, and avalanches. Occasional rain storms that make it over the blocking mountain ranges can drop moisture in the highlands, leading to flash floods coming out of mountain canyons into the plains or intermountain basins on the lee side of the mountains.
There are some deserts that are located along coastlines, where intuition would seem to indicate that moisture should be plentiful. However, the driest place on Earth is the Atacama Desert, located along the coast of Peru and Chile. The Namib Desert of southern Africa is another coastal desert, which is known legendarily as the Skeleton Coast, because it is so dry that many of the large animals that roam out of the more humid interior climate zones in search of food perish there, leaving their bones sticking out of the blowing sands. The waters off the Skeleton Coast are also known as particularly treacherous, with strong currents and rogue waves leading to many shipwrecks.
These coastal deserts form adjacent to such large bodies of water where ocean currents place cold upwelling water from the deep ocean next to the coast, which cools the atmosphere. The effect is similar to rainshadow deserts, where cold air can hold less moisture, and the result is no rain.
In some places on the planet, seasonal variations in wind systems bring alternating dry and wet seasons. The Indian Ocean is famous for its monsoonal rains in the summer as the southeast trade winds bring moist air on shore. However, as the moisture moves across India it loses moisture and must rise to cross the Aravalli Mountain Range. The Thar Desert of Pakistan and the Rajasthan Desert of India are located on the back or lee side of these mountains and do not generally receive this seasonal moisture.
A final class of deserts is the polar desert, found principally in the Dry Valleys and other parts of Antarctica, parts of Greenland, northern Canada, and Nunavut. Approximately 3 million square miles (7,770,000 km2) on Earth consists of polar desert environments. In these places, cold downwelling air lacks moisture, and the air is so dry that the evaporation potential is much greater than the precipitation. Temperatures do not exceed 50°F (10°C) in the warmest months, and precipitation is less than one inch per year. There are places in the Dry Valleys of Antarctica that have not been covered in ice for thousands of years.
Polar deserts are generally not covered in sand dunes, but are marked by gravel plains or barren bedrock. Hazards to travelers in polar deserts include the effects of extreme cold such as hypothermia, frostbite, and dehydration. Polar deserts may also have landforms shaped by frost wedging, where alternating freeze-thaw cycles allow small amounts of water to seep into cracks and other openings in rocks. When the water freezes it expands, pushing large blocks of rock away from the main mountain mass. In polar deserts and other regions affected by frost wedging large talus slopes may form adjacent to mountain fronts, and these are prone to frequent rock falls from frost wedging.
Some desert areas seem out of place; for instance, a large area of sand dunes in north-central Alaska looks like it belongs in the Sahara of North Africa. Many deserts like this are actually paleo-deserts, remnants of past dry climates, that have not yet converted into more fertile land. The sand dune fields in central Alaska are surrounded by pine forest and tundra, gradually replacing the old desert environment.
Desert landforms are some of the most beautiful on Earth, often presenting bizarre sculpted mountains, steep walled canyons, and regional gravel plains. They can also be some of the most hazardous landscapes on the planet. The regolith in deserts is thin, discontinuous, and much coarser-grained than in moist regions and is produced predominantly by mechanical weathering. Chemical weathering is of only minor importance, because of the rare moisture. Also, the coarse size of particles produced by mechanical weathering produces steep slopes, eroded from steep cliffs and escarpments.
Much of the regolith that sits in deserts is coated with a dark coating of manganese and iron oxides, known as desert varnish, produced by a combination of microorganism activity and chemical reactions with fine manganese dust that settles from the wind.
Most streams in deserts evaporate before they reach the sea. Most are dry for long periods of time, and subject to flash floods during brief but intense rains. These flash floods transport most of the sediment in deserts and form fan-shaped deposits of sand, gravel, and boulders found at the bases of many mountains in desert regions. These flash floods also erode deep steep-walled canyons through the upstream mountain regions, which is the source of the boulders and cobbles found on the mountain fronts. Intermountain areas in deserts typically have finer-grained material, deposited by slower moving currents that represent the waning stages of floods as they expand into open areas between mountains after they escape out of mountain canyons.
Flash floods can be particularly hazardous in desert environments, especially when the floods are the result of distant rains. More people die in deserts from drowning in flash floods than die from thirst or dehydration. In many cases rain in far away mountains may occur, without people in downstream areas even aware that it is raining upstream. Rain in deserts is typically a brief but intense thunderstorm, which can drop a couple of inches (> 5 cm) of rain in a short time. The water may then quickly
move downstream as a wall of water in mountain canyons, sweeping away all loose material in its path. Any people or vehicles caught in such a flood are likely to be lost, swept away by the swiftly moving torrent.
Dry lake beds in low-lying flat areas, which may have water in them only once every few years, characterize many deserts. These are known as playas, or hardpans, and they typically have deposits of white salts, which formed when water from storms evaporated leaving the lakes dry. There are more than 100 playas in the American Southwest, including Lake Bonneville, which formed during the last ice age and now covers parts of Utah, Nevada, and Idaho. When there is water in these basins, they are known as playa lakes. Playas are very flat surfaces that make excellent race tracks and runways. The U.S. space shuttles commonly land on Rogers Lake playa at Edwards Air Force Base in California.
Alluvialfans are coarse-grained deposits of alluvium that accumulate at the fronts of mountain canyons. Alluvial fans are very common in deserts, where they are composed of both alluvium and debris flow deposits. Alluvial fans are quite important for people in deserts, because they are porous and permeable and they contain large deposits of groundwater. In many places, alluvial fans so dominate the land surface that they form a bajada (or slope) along the base of the mountain range, formed by fans that have coalesced to form a continuous broad alluvial apron.
Pediments represent different kinds of desert surfaces. They are surfaces sloping away from the base of a highland and covered by a thin or discontinuous layer of alluvium and rock fragments. These are ero-sional features, formed by running water, and are typically cut by shallow channels. Pediments grow as mountains are eroded.
Inselbergs are steep-sided mountains or ridges that rise abruptly out of adjacent monotonously flat plains in deserts. Ayres Rock in central Australia is perhaps the world's best known inselberg. These are produced by differential erosion, leaving behind as a mountain rocks that for some reason are more resistant to erosion.
Wind plays a significant role in the evolution of desert landscapes. Wind erodes in two basic ways. Deflation is a process whereby wind picks up and removes material from an area, resulting in a reduction in the land surface. The process is akin to deflating a balloon below the surface, hence its name. Abrasion is a different process that occurs when particles of sand and other sizes are blown by the wind and impact each other. Exposed surfaces in deserts are subjected to frequent abrasion, which is similar to sandblasting.
Yardangs are elongate streamlined wind-eroded ridges, which resemble an overturned ship's hull sticking out of the water. These unusual features are formed by abrasion, by the long-term sandblasting along specific corridors. The sandblasting leaves erosionally resistant ridges, but removes the softer material which itself will contribute to sandblasting in the downwind direction, and eventually contribute to the formation of sand, silt, and dust deposits.
Deflation is important on a large scale in places where there is no vegetation, and in some places the wind has excavated large basins known as deflation basins. Deflation basins are common in the United States from Texas to Canada as elongate (several miles/kilometers long) depressions, typically only 3-10 feet (0.9-3 m) deep. However, in some places like in the Sahara, deflation basins may be as much as several hundred feet deep.
Deflation by wind can move only small particles away from the source, since the size of the particle that can be lifted is limited by the strength of the wind, which rarely exceeds a few tens of miles per hour (~ 50 km/h). Deflation therefore leaves boulders, cobbles and other large particles behind. These get concentrated on the surface of deflation basins and other surfaces in deserts, leaving a surface concentrated in boulders known as desert pavement.
Desert pavements represent a long-term stable desert surface, and they are not particularly hazardous. However, when the desert pavement is broken, for instance, by being driven across, the coarse cobbles and pebbles get pushed beneath the surface and the underlying sands get exposed to wind action again. Driving across a desert pavement can raise a considerable amount of sand and dust, and if many vehicles drive across the surface then it can be destroyed and the whole surface becomes active.
A very striking large-scale example of this process was provided by events in the Gulf War of 1991. After Iraq invaded Kuwait in 1990, U.S. and Allied Forces massed hundreds of thousands of troops on the Saudi Arabian side of the border with Iraq and Kuwait, and eventually mounted
a multi-pronged counterattack on Kuwait City that led to its liberation. Several of the prongs circled far to the north then turned around and came back south to Kuwait City. These prongs took many thousands of heavy tanks, artillery, and vehicles across a region of stable desert pavement, and the weight of these military vehicles destroyed the pavement in order to free Kuwait. Since the liberation, the steady winds from the northwest have continued, and this area that was once stable desert pavement and stable dune surfaces (covered with desert pavement and minor vegetation) has been remobilized. Large sand dunes have formed from the sand previously trapped under the pavement. Other dunes that were stable have been reactivated. Now, the Kuwait City residents are bracing for what they call the second invasion of Kuwait, but this time the invading force is sand and dust, not a foreign army.
Several things have been considered to try to stabilize the newly-migrating dunes. One consideration is to try to re-establish the desert pavement by spreading cobbles across the surface, but this is unrealistic because of the large area involved. Another proposition, being tested, is to spray petroleum on the migrating dunes to effectively create a blacktop or tarred surface that would be stable in the wind. This is feasible in oil-rich Kuwait, but not particularly environmentally friendly.
In China's Gobi and Taklimakan Deserts, a different technique to stabilize dunes has proven rather successful. Bales of hay are initially placed in a grid pattern near the base of the windward side of dunes, which decreases the velocity of the air flowing over the dune and reduces the transportation of sand grains over the slip surface. Drought-resistant vegetation is planted between the several-foot-wide grid of hay bales, and then when the dune is more stabilized, vegetation is planted along the dune crest. China is applying this technique across much of the Gobi and Taklimakan Deserts, protecting railways and roads. In northeastern China, this technique is being applied in an attempt to reclaim some lands that became desert through human activity, and they are constructing a 5,700-mile- (9,173.3-km-) long line of hay bales and drought-resistant vegetation. China is said to be building a new "Green Wall" which will be longer than the famous Great Wall of China and it is hoped will prove more effective at keeping out invading forces (in this case, sand) from Mongolia.
Windblown Sand and dust
Most people think of deserts as areas with lots of big sand dunes and continual swirling winds of dust storms. Really, dunes and dust storms
are not as common as depicted in popular movies, and rocky deserts are more common than sandy deserts. For instance, only about 20 percent of the Sahara Desert is covered by sand, and the rest is covered by rocky, pebbly, or gravel surfaces. However, sand dunes are locally very important in deserts, and wind is one of the most important processes in shaping deserts worldwide. Shifting sands are one of the most severe geologic hazards of deserts. In many deserts and desert border areas, the sands are moving into inhabited areas, covering farmlands, villages, and other useful land with thick accumulations of sand. This is a global problem, as deserts are currently expanding worldwide. The Desert Research Institute in China has recently estimated that in China alone, 950 square miles (2,460 km2) are encroached on by migrating sand dunes from the Gobi Desert each year, costing the country 6.7 billion dollars per year and affecting the lives of 400 million people.
Wind moves sand by saltation, in arced paths, in a series of bounces or jumps. The surface of dunes on beaches or deserts is typically covered by a thin moving layer of sand particles that are bouncing and rolling along the surface in this process of saltation.
Wind typically sorts different sizes of sedimentary particles, forming elongate small ridges known as sand ripples, very similar to ripples found in streams. Sand dunes are larger than ripples, up to 1,500 feet (ca. 450 m) high, made of mounds or ridges of sand deposited by wind.
These may form where an obstacle distorts or obstructs the flow of air, or they may move freely across much of a desert surface. Dunes have many different forms, but all are asymmetrical. They have a gentle slope that faces into the wind and a steep face that faces away from the wind. Sand particles move by saltation up the windward side, and fall out near the top where the pocket of low-velocity air can not hold the sand anymore. The sand avalanches, or slips down, the leeward slope, known as the slip face. This keeps the slope at the angle of repose, 30-34°. The asymmetry of old dunes is used to tell the directions ancient winds blew.
The steady movement of sand from one side of the dune to the other causes the whole dune to migrate slowly downwind, typically about 80-100 feet (28-30 m) per year, burying houses, farmlands, temples, and towns. Rates of dune migration of up to 350 feet (107 m) per year have been measured in the Western Desert of Egypt and the Ningxia Province of China.
A combination of many different factors leads to the formation of very different types of dunes, each with a distinctive shape, potential for movement, and hazards. The main variables that determine a dune's shape are the amount of sand that is available for transportation, the strength (and directional uniformity) of the wind, and the amount of vegetation that covers the surface. If there is a lot of vegetation and little wind, no dunes will form. In contrast, if there is very little vegetation, a lot of sand, and moderate wind strength (conditions that might be found on a beach), then a group of dunes known as transverse dunes forms, with the dune crests aligned at right angles to the dominant wind.
Barchan dunes have crescent-shapes and have horns pointing downwind and form on flat deserts with steady winds and a limited sand supply. Parabolic dunes have a U-shape with the U facing upwind. These form where there is significant vegetation that pins the tails of migrating transverse dunes, with the dune being warped into a wide U-shape. These dunes look broadly similar to barchans, except the tails point in the opposite direction. They can be distinguished because in both cases, the steep side of the dune points away from the dominant winds direction. Linear dunes are long, straight ridge-shaped dunes elongate parallel to the wind direction. These occur in deserts with little sand supply and strong, slightly variable winds, and they are elongate
(Opposite) Sand dune types and classification based on relative strength of wind, sand supply, and vegetation. A, barchan dunes; B, transverse dunes; C, barchanoid dunes; D, longitudinal dunes; E, parabolic dunes; F, star dunes
parallel to the wind direction. Star dunes form isolated or irregular hills formed where the wind directions are irregular.
Strong winds that blow across desert regions sometimes pick up dust made of silt and clay particles and transport it thousands of kilometers from its source. For instance, dust from China is found in Hawaii, and the Sahara Desert commonly drops dust in Europe. This dust is a nuisance, has a significant influence on global climate, and has at times, as in the dust bowl days of the 1930s, been known to nearly block out the Sun.
Loess is a name for silt and clay deposited by wind. It forms a uniform blanket that covers hills and valleys at many altitudes, which distinguishes it from deposits of streams. In Shaanxi Province, China, an earthquake that killed 830,000 people in 1556 had such a high death toll in part because the people in the region built their homes out of loess. The loess formed an easily excavated material that hundreds of thousands of villagers cut homes into, essentially living in caves. When the earthquake struck, the loess proved to be a poor building material and large-scale collapse of the fine-grained loess was directly responsible for most of the high death toll.
Recently, it has been recognized that wind-blown dust contributes significantly to global climate. Dust storms that come out of the Sahara can be carried around the world and can partially block out some of the Sun's radiation. The dust particles may also act as small nuclei for raindrops to form around, perhaps acting as a natural cloud-seeding phenomenon. One interesting point to ponder is that as global warming increases global temperatures, the amount and intensity of storms increase, and some of the world's deserts expand. Dust storms may serve to counter this effect, reduce global temperatures, and increase precipitation.
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