LANDFORMS DUE TO WATER ACTION IN THE DESERTS

Wind erosion, transportation and deposition are most effective and widespread in desert regions, but few deserts in the world are entirely without rain or water. In some deserts, there are perennial rivers such as the Nile in Egypt which has its source in more humid regions outside the Sahara Desert. Otherwise desert streams or wadis are not permanent, their flow regime is flashy and temporary as they flow only during heavy rainfall occurrences and dry up shortly after the rains. Apart from rain and rivers, there is also dew in deserts. These moisture sources make water action in deserts important. It is also evident that parts of the present day sand deserts were at some period, during the Quaternary, lacustrine and alluvial plains with inherited deltas, high lake terraces, and washed out and winnowed lacustrine depressions. The landforms discussed here which are due to water action in desert landscapes include Mesas and Inselbergs which are also common in the Savanna regions. Their presence in deserts suggest a more humid climate in the past. Other landforms which are due to water action in deserts are gullies and alluvial fans, wadis and depression basins.

1) Gullies and Alluvial Fans: In desert areas, there are occasional heavy thunderstorms which fall within one to three hours. The sparse vegetation cover and the dry soil make the generation of large volume of surface runoff possible (rain water flowing on the ground surface). The surface runoff soon turns into torrents, sweeping loose gravel, sand and fine dust down the hill side. Such torrents are called flash floods. The flash floods cut deep into gullies and ravines thereby forming badland topography. Subsequent down pours widen and deepen the gullies by detaching and transporting more materials or sediment. When there is so much sediment in the flash flood, a mud flow is formed. When the flash flow leaves the mountains and goes on to the plains, it deposits the transported sediment due to sudden change in gradient which reduces the competence and the capacity of flow. The material or sediment deposited at the foot of the hill is termed an alluvial fan. It is a cone-like deposit of alluvium with the apex pointing uphill. A number of alluvial fans may merge to form an alluvial plain or bajaha, as the streams discharge through several channels depositing more sediment. The size of alluvial fans depends on the supply of sediment which is higher on easily erodible material. Prominent alluvial fans are found in the basin and range province in the U.S.A and around the Himalayan Mountains.

 

2) Wadis and Depression Basins: Wadis are large desert stream channels or valleys which originate in highlands and terminate in depressions or at the coast. Wadis are also the basic form of ancient erosional relief. The wadis may be associated with high or low terraces which mark phases of stream incision during periods of increased aridity. In the western parts of the Sahara, the wadis have preserved their morphology, but in other areas such as the central and eastern parts of the Sahara, the beds are filled with thick layers of sand. In the Libyan desert, it is difficult to establish the presence of ancient wadis due to sand deposition in the bed. Examples of prominent wadis are the Tilemsi valley and its tributary valleys in Mali, and the Sahara El-Arabiya in eastern Egypt.

Wadis may terminate in closed depressions. The floor of a depression is made up of two features, the bajada and the pediment. The bajada is a depositional feature made up of alluvium deposited by the intermittent streams. The pediment is an erosional plain formed at the base of the surrounding mountain scarps.

The water collected in a depression or desert basin may not completely disappear by evaporation or seepage, and a temporary lake is formed. Such lakes contain a high concentration of salts, because of high evaporation, and are glistening white when they dry up. The lakes and the alluvial plains formed by them are called playas, salinas or salars in the United States and Mexico, and Shotts in northern Africa.

 

3) Mesas and Inselbergs: Mesa is a Spanish word meaning table. It is a landform with a flat, table-like cap of duricrust (laterite or calcrete) and very steep sides. The resistant cap may be broken into boulders as a result of seepage erosion, this could reduce the size of a mesa and a smaller landform termed a butte is formed. The steep sides of a mesa may consist of duricrust boulders which are remnants of the broken resistant cap transported from the top to the sides. Mesas are common features of the savanna and desert landscapes; for example, in the Jos Plateau and Northern-western parts of Kano State where they form residual hills on the basement complex (igneous and metamorphic rocks) plains.

They are also found in the sandstone region of eastern Mauritania, and in eastern Mali, extending into the neighbouring Niger Republic. Mesas may be formed in canyon regions e.g. Arizona, or on fault blocks e.g. the Table Mountain of Cape Town, South Africa. Many of them in arid countries are separated by deep gorges or canyons.

The word Inselberg is a German word meaning Island mountain. They are isolated residual hills rising abruptly from the plains. Inselbergs are often composed of granite and gneiss and are characterized by steep slopes which may have boulders and stones eroded and transported down the hillslope.

Two models illustrated in Fig. 7.15 are suggested to explain the origin of inselbergs. The first theory explains the origin of inselbergs by stream erosion and the retreat of slopes to form residual hills. The second theory attributes the formation of an inselberg of deep weathered regolith, leaving behind isolated hills. Inselbergs are typical of savanna and desert landscapes e.g. those of northern Nigeria, western Australia, Sahara and Kalahari deserts. The plain at the base of an inselberg is called a pediplain or an etchplain when the inselberg originates due to deep weathering and stripping of material.

 

Deserts and Human Activities

Because of the excessive heat and the acute shortage of rainfall and surface water, the desert constitutes a very harsh environment for both plant and animal life, hence, the very sparse vegetation and the limited range and population of animals. Thus, the productivity of desert environments is very low so that they can not support dense concentrations of human population.

Population and human activities are concentrated in oases, river valleys, deltas and coastal plains. However, population has been attracted even to the most dry and difficult desert environments wherever valuable minerals have been discovered. In modern times, crude oil mining is the major factor attracting people to the deserts; for example, in the Middle East, Libya, Algeria, Mexico and southwest United States. Other mineral resources of the deserts include phosphates (e.g. northwest Africa, Mauritania), salts and gold.

Although surface water is very meagre in the deserts, there are as yet untapped vast reserves of groundwater. The future development of the deserts may depend on the exploitation of these groundwater resources.