18 Wadi Ghuweir Trail to Feynan - In the Oasis - panoramio

Wadi ( ; ) is a

river valley A valley is an elongated low area often running between hills or mountains and typically containing a river or stream running from one end to the other. Most valleys are formed by erosion of the land surface by rivers or streams over a ...

or a wet (

ephemeral Ephemerality (from the Greek word , meaning 'lasting only one day') is the concept of things being transitory, existing only briefly. Academically, the term ephemeral constitutionally describes a diverse assortment of things and experiences, fr ...

)

riverbed A streambed or stream bed is the bottom of a stream or river and is confined within a Stream channel, channel or the Bank (geography), banks of the waterway. Usually, the bed does not contain terrestrial (land) vegetation and instead supports d ...

that contains water only when heavy

rain Rain is a form of precipitation where water drop (liquid), droplets that have condensation, condensed from Water vapor#In Earth's atmosphere, atmospheric water vapor fall under gravity. Rain is a major component of the water cycle and is res ...

occurs. Wadis are located on gently sloping, nearly flat parts of deserts; commonly they begin on the distal portions of

alluvial fan An alluvial fan is an accumulation of sediments that fans outwards from a concentrated source of sediments, such as a narrow canyon emerging from an escarpment. They are characteristic of mountainous terrain in arid to Semi-arid climate, semiar ...

s and extend to inland

sabkha A sabkha () is a predominately coastal, supratidal mudflat or sandflat in which evaporite-saline minerals accumulate as the result of a semiarid to arid climate. Sabkhas are gradational between land and intertidal zone within restricted coast ...

s or

dry lakes A dry lake bed, also known as a playa (), is a basin or depression that formerly contained a standing surface water body, which disappears when evaporation processes exceed recharge. If the floor of a dry lake is covered by deposits of alkalin ...

. Permanent channels do not exist, due to lack of continual water flow. Water percolates down into the stream bed, causing an abrupt loss of energy and resulting in vast deposition. Wadis may develop dams of sediment that change the stream patterns in the next

flash flood A flash flood is a rapid flooding of low-lying areas: washes, rivers, dry lakes and depressions. It may be caused by heavy rain associated with a severe thunderstorm, hurricane, or tropical storm, or by meltwater from ice and snow. Flash f ...

. Wadis tend to be associated with centers of human population because sub-surface water is sometimes available in them. Nomadic and pastoral desert peoples will rely on seasonal vegetation found in wadis, even in regions as dry as the

Sahara The Sahara (, ) is a desert spanning across North Africa. With an area of , it is the largest hot desert in the world and the list of deserts by area, third-largest desert overall, smaller only than the deserts of Antarctica and the northern Ar ...

, as they travel in complex

transhumance Transhumance is a type of pastoralism or Nomad, nomadism, a seasonal movement of livestock between fixed summer and winter pastures. In montane regions (''vertical transhumance''), it implies movement between higher pastures in summer and low ...

routes. The centrality of wadis to water – and human life – in desert environments gave birth to the distinct sub-field of wadi hydrology in the 1990s.

Etymology

The word 'wadi' is very widely found in Arabic

toponym Toponymy, toponymics, or toponomastics is the study of ''wikt:toponym, toponyms'' (proper names of places, also known as place names and geographic names), including their origins, meanings, usage, and types. ''Toponym'' is the general term for ...

s. Some Spanish toponyms are derived from

Andalusian Arabic Andalusi Arabic or Andalusian Arabic () was a variety or varieties of Arabic spoken mainly from the 8th to the 15th century in Al-Andalus, the regions of the Iberian Peninsula under the Muslim rule. Arabic spread gradually over the centuries ...

where wadi was used to mean a permanent river, for example:

Guadalcanal Guadalcanal (; indigenous name: ''Isatabu'') is the principal island in Guadalcanal Province of Solomon Islands, located in the southwestern Pacific Ocean, northeast of Australia. It is the largest island in the Solomons by area and the second- ...

from ''wādī al-qanāl'' (, "river of refreshment stalls"),

Guadalajara Guadalajara ( ; ) is the capital and the most populous city in the western Mexican List of states of Mexico, state of Jalisco, as well as the most densely populated municipality in Jalisco. According to the 2020 census, the city has a population ...

from ''wādī al-ḥijārah'' (, "river of stones"), or

Guadalquivir The Guadalquivir (, also , , ) is the fifth-longest river in the Iberian Peninsula and the second-longest river with its entire length in Spain. The Guadalquivir is the only major navigable river in Spain. Currently it is navigable from Seville ...

, from ''al-wādī al-kabīr'' (, "the great river").

Sediments and sedimentary structures

basin and range topography Basin and range topography is characterized by alternating parallel mountain ranges and valleys. It is a result of crustal extension due to mantle upwelling, gravitational collapse, crustal thickening, or relaxation of confining stresses. The e ...

, wadis trend along basin axes at the terminus of fans. They have

braided stream A braided river (also called braided channel or braided stream) consists of a network of river channels separated by small, often temporary, islands called '' braid bars'' or, in British English usage, '' aits'' or ''eyots''. Braided streams t ...

patterns because of the deficiency of water and the abundance of

sediment Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...

s. Wadi sediments may contain a range of material, from gravel to mud, and the sedimentary structures vary widely. Thus, wadi sediments are the most diverse of all desert environments. Flash floods result from severe energy conditions and can result in a wide range of sedimentary structures, including ripples and common plane beds. Gravels commonly display imbrications, and mud drapes show desiccation cracks. Wind activity also generates sedimentary structures, including large-scale cross-stratification and wedge-shaped cross-sets. A typical wadi sequence consists of alternating units of wind and water sediments; each unit ranging from about . Sediment laid by water shows complete fining upward sequence. Gravels show imbrication. Wind deposits are cross-stratified and covered with mud-cracked deposits. Some horizontal

loess A loess (, ; from ) is a clastic rock, clastic, predominantly silt-sized sediment that is formed by the accumulation of wind-blown dust. Ten percent of Earth's land area is covered by loesses or similar deposition (geology), deposits. A loess ...

may also be present. Wind also causes sediment deposition. When wadi sediments are underwater or moist, wind sediments are deposited over them. Thus, wadi sediments contain both wind and water sediments.

Hydrological action

Modern English usage differentiates wadis from

canyons A canyon (; archaic British English spelling: ''cañon''), gorge or chasm, is a deep cleft between escarpments or cliffs resulting from weathering and the erosive activity of a river over geologic time scales. Rivers have a natural tendency t ...

or '' arroyo'' (

Spanish Spanish might refer to: * Items from or related to Spain: **Spaniards are a nation and ethnic group indigenous to Spain **Spanish language, spoken in Spain and many countries in the Americas **Spanish cuisine **Spanish history **Spanish culture ...

, used in the

Americas The Americas, sometimes collectively called America, are a landmass comprising the totality of North America and South America.''Webster's New World College Dictionary'', 2010 by Wiley Publishing, Inc., Cleveland, Ohio. When viewed as a sing ...

for similar landforms) by the action and prevalence of water. Wadis, as drainage courses, are formed by water and are distinguished from river valleys or gullies in that surface water is intermittent or ephemeral. Wadis are generally dry year round, except after a rain. The desert environment is characterized by sudden but infrequent heavy rainfall, often resulting in

s. Crossing wadis at certain times of the year can be dangerous as a result.

Deposits

Deposition in a wadi is rapid because of the sudden loss of stream

velocity Velocity is a measurement of speed in a certain direction of motion. It is a fundamental concept in kinematics, the branch of classical mechanics that describes the motion of physical objects. Velocity is a vector (geometry), vector Physical q ...

and seepage of water into the porous sediment. Wadi deposits are thus usually mixed gravels and sands. These sediments are often altered by eolian processes. Over time, wadi deposits may become "inverted wadis", where former underground water caused vegetation and sediment to fill in the eroded channel, turning previous washes into ridges running through desert regions.

Gallery

References

Bibliography

Summary: Drainage Courses, Wadis
United States Army Corps of Engineers. Desert Processes Working Group; Knowledge Sciences, Inc. (n.d), retrieved 2008-08-26.

United States Army Corps of Engineers. Desert Processes Working Group; Knowledge Sciences, Inc. (n.d), retrieved 2008-08-2onments * Gelennie, K. W., 1970 Deserts sedimentary Environments. Developments in Sedimentology, v.14. Elsevier, Amsterdam, 222p.

External links

IHP REGIONAL WADI HYDROLOGY NETWORK

International Hydrological Programme The Intergovernmental Hydrological Programme, formerly the International Hydrological Programme (IHP), is UNESCO’s international scientific cooperative program in water research, water resource management, water education, and capacity- buildi ...

UNESCO The United Nations Educational, Scientific and Cultural Organization (UNESCO ) is a List of specialized agencies of the United Nations, specialized agency of the United Nations (UN) with the aim of promoting world peace and International secur ...

.
Arab Center for Studies of Arid Zones and Dry lands (ACSAD)
Water resources division. {{Authority control 01 Dry or seasonal streams Canyons and gorges Fluvial landforms Valleys Arabic words and phrases