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Wadi ( ar, وَادِي, wādī), alternatively ''wād'' ( ar, وَاد), North African Arabic Oued, is the Arabic term traditionally referring to a
valley A valley is an elongated low area often running between hills or mountains, which will typically contain 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 ...

valley
. In some instances, it may refer to a dry (
ephemeral Ephemerality (from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is ...
) riverbed that contains water only when heavy
rain Rain is liquid water in the form of droplet Rain water flux from a canopy. Among the forces that govern drop formation: cohesion, Van der Waals force">Cohesion_(chemistry).html" ;"title="surface tension, Cohesion (chemistry)">cohesion, ...

rain
occurs.


Etymology

The term ' is very widely found in Arabic
toponym Toponymy, toponymics, or toponomastics (from grc, τόπος / , 'place', and / , 'name') is the study of ''toponyms Toponymy, also toponymics or toponomastics (from grc, τόπος / , 'place', and / , 'name') is the study of ''wikt: ...
s. Some Spanish toponyms are derived from
Andalusian Arabic Andalusian Arabic, also known as Andalusi Arabic, was a variety or varieties of Arabic spoken in Al-Andalus, the regions of the Iberian Peninsula The Iberian Peninsula , ** * Aragonese language, Aragonese and Occitan language, Occitan: ...
where ' 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 Pacific Ocean, south-western Pacific, northeast of Australia. It is the largest island in the Solomon Islands by ar ...
from ''wādī al-qanāl'' ( ar, وَادِي الْقَنَال, "river of refreshment stalls"),
Guadalajara Guadalajara ( , ) is a metropolis A metropolis () is a large city or conurbation A conurbation is a region comprising a number of metropolis in the background A metropolis () is a large city or conurbation which is a significan ...
from ''wādī al-ḥijārah'' ( ar, وَادِي الْحِجَارَة, "river of stones"), or
Guadalquivir The Guadalquivir (, also , , ) is the fifth-longest river A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry ...

Guadalquivir
, from ''al-wādī al-kabīr'' ( ar, اَلْوَادِي الْكَبِير, "the great river").


General morphology and processes

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 shaped like a section of a shallow cone, with its apex at a point source of sediments, such as a narrow canyon emerging from an escarpment. They are characteristic of mountainous terrain in arid to ...

alluvial fan
s and extend to inland
sabkha A term typically used by Earth scientists, a sabkha ( ar, سبخة) is a coastal, supratidal mudflat or sandflat in which evaporite An evaporite () is a water-soluble Solubility is the property of a solid, liquid or gaseous chemical s ...
s or
dry lakes A dry lake, also known as a playa, is either a basin or depression that formerly contained a standing surface water body, which disappeared when evaporation processes exceeded recharge. If the floor of a dry lake is covered by deposits of alkali ...
. In
basin and range topography Basin and range topography is an alternating landscape of parallel mountain ranges and valleys. It is a result of crustal extension/stretching (extensional tectonics) of the lithosphere (crust and upper mantle (Earth), upper mantle) due to mantle u ...
, wadis trend along basin axes at the terminus of fans. Permanent channels do not exist, due to lack of continual water flow. Wadis have
braided stream A braided river, or braided channel, consists of a network of river channel Channel, channels, channeling, etc., may refer to: Geography * Channel (geography), in physical geography, a landform consisting of the outline (banks) of the path of ...
patterns because of the deficiency of water and the abundance of
sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently sediment transport, transported by the action of wind, water, or ice or by the force of gravity acting on the particles. ...

sediment
s. 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: Washland, washes, rivers, dry lakes and Depression (geology), depressions. It may be caused by heavy rain associated with a severe thunderstorm, hurricane, tropical storm, or meltwater from ...
. 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.


Sediments and sedimentary structures

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 Loess (, ; from german: Löss ) is a clastic Clastic rocks are composed of fragments, or clasts, of pre-existing minerals In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fair ...
may also be present.


Hydrological action

Modern English usage differentiates wadis from
canyons A canyon (; archaic British English British English (BrE) is the standard dialect A standard language (also standard variety, standard dialect, and standard) is a language variety that has undergone substantial codification of grammar a ...

canyons
or Arroyo (creek), washes by the action and prevalence of water. Wadis, as drainage courses, are formed by water, but 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
flash flood A flash flood is a rapid flooding of low-lying areas: Washland, washes, rivers, dry lakes and Depression (geology), depressions. It may be caused by heavy rain associated with a severe thunderstorm, hurricane, tropical storm, or meltwater from ...
s. Crossing wadis at certain times of the year can be dangerous as a result. 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, as they travel in complex transhumance 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.


Deposits

Deposition in a wadi is rapid because of the sudden loss of stream velocity and seepage of water into the porous sediment. Wadi deposits are thus usually mixed gravels and sands. These sediments are often altered by eolianite, 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

File:Wadi Bani Khalid East RB.jpg, Wadi Bani Khalid in the Ash Sharqiyah North Governorate, Northern Governorate of Ash-Sharqiyyah Region, Oman, Arabian peninsula File:NachalParan1.jpg, Wadi in Nahal Paran, Negev, Israel File:Oued Tissint.jpg, Oued Tissint, Morocco, North Africa File:Dry riverbed and basalt columns (Namibia).jpg, Dry fluvial channel cutting through columnar basalt in Namibia, southern Africa File:Al Bithnah Fort viewed from the Wadi Ham.jpg, The Al Bithnah Fort in the Wadi Ham, United Arab Emirates. File:Wadi Shawka looking North.jpg, The Wadi Shawkah in the United Arab Emirates.


See also

* * * * * * * * in the Sinai peninsula, holy Muslim site


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