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Wadi ( ar, وَادِي, wādī), alternatively ''wād'' ( ar, وَاد),
North African Arabic Maghrebi Arabic (, Western Arabic; as opposed to Eastern or Mashriqi Arabic) is a vernacular Arabic dialect continuum spoken in the Maghreb region, in Morocco, Algeria, Tunisia, Libya, Western Sahara, and Mauritania. It includes Moroccan, Alg ...
Oued, is the Arabic term traditionally referring to a
valley A valley is an elongated low area often running between Hill, hills or Mountain, 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 ...
. In some instances, it may refer to 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 stream bed or streambed is the bottom of a stream or river (bathymetry) or the physical confine of the normal water flow (channel). The lateral confines or channel margins are known as the stream banks or river banks, during all but flood st ...
that contains water only when heavy
rain Rain is water droplets that have condensed from atmospheric water vapor and then fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water f ...
occurs.


Etymology

The term ' is very widely found in Arabic
toponym Toponymy, toponymics, or toponomastics is the study of '' 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 a proper name of ...
s. Some Spanish toponyms are derived from Andalusian Arabic 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 south-western Pacific, northeast of Australia. It is the largest island in the Solomon Islands by area, and the seco ...
from ''wādī al-qanāl'' ( ar, وَادِي الْقَنَال, "river of refreshment stalls"),
Guadalajara Guadalajara ( , ) is a metropolis in western Mexico and the capital of the list of states of Mexico, state of Jalisco. According to the 2020 census, the city has a population of 1,385,629 people, making it the 7th largest city by population in Me ...
from ''wādī al-ḥijārah'' ( ar, وَادِي الْحِجَارَة, "river of stones"), or 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 fans and extend to inland sabkhas 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 exceeds recharge. If the floor of a dry lake is covered by deposits of alkaline c ...
. In
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. Permanent channels do not exist, due to lack of continual water flow. They have
braided stream A braided river, or braided channel, consists of a network of river channels separated by small, often temporary, islands called braid bars or, in English usage, ''aits'' or ''eyots''. Braided streams tend to occur in rivers with high sediment lo ...
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 transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand an ...
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: 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 or snow flowing o ...
. 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, predominantly silt-sized sediment that is formed by the accumulation of wind-blown dust. Ten percent of Earth's land area is covered by loess or similar deposits. Loess is a periglacial or aeolian ...
may also be present.


Hydrological action

Modern English usage differentiates wadis from canyons or 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: 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 or snow flowing o ...
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 , photo = Sahara real color.jpg , photo_caption = The Sahara taken by Apollo 17 astronauts, 1972 , map = , map_image = , location = , country = , country1 = , ...
, as they travel in complex
transhumance Transhumance is a type of pastoralism or 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 lower vall ...
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 Velocity is the directional speed of an object in motion as an indication of its rate of change in position as observed from a particular frame of reference and as measured by a particular standard of time (e.g. northbound). Velocity is a ...
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


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 The International Hydrological Programme (IHP) is UNESCO’s international scientific cooperative program in water research, water resource management, water education, and capacity- building, and the only broadly based science program of the UN s ...
,
UNESCO The United Nations Educational, Scientific and Cultural Organization is a specialized agency of the United Nations (UN) aimed at promoting world peace and security through international cooperation in education, arts, sciences and culture. It ...
.
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