Awadi (/ˈwɒ.di/WOD-ee;Arabic:وَادِي) is ariver valley or a wet (ephemeral)riverbed that contains water only when heavyrain occurs. Wadis are located on gently sloping, nearly flat parts of deserts; commonly they begin on the lowest portions ofalluvial fans and extend to inlandsabkhas ordry lakes. 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 nextflash flood.
Wadis tend to be associated withcenters 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 theSahara, as they travel in complextranshumance 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.[1]
The word 'wadi' is very widely found in Arabictoponyms. Some Spanish toponyms are derived fromAndalusian Arabic where wadi was used to mean a permanent river,[citation needed] for example:Guadalcanal fromwādī al-qanāl (Arabic:وَادِي الْقَنَال, "river of refreshment stalls"),Guadalajara fromwādī al-ḥijārah (Arabic:وَادِي الْحِجَارَة, "river of stones"),[2] orGuadalquivir, fromal-wādī al-kabīr (Arabic:اَلْوَادِي الْكَبِير, "the great river").
Inbasin and range topography, wadis trend along basin axes at the terminus of fans. They havebraided stream patterns because of the deficiency of water and the abundance ofsediments. 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 displayimbrications, 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 10–30 cm (4–12 in). Sediment laid by water shows a complete fining upward sequence. Gravels show imbrication. Wind deposits are cross-stratified and covered with mud-cracked deposits. Some horizontalloess 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.
Modern English usage differentiates wadis fromcanyons orarroyo (Spanish, used in theAmericas for similar landforms)[3] 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 inflash floods. Crossing wadis at certain times of the year can be dangerous as a result.
Deposition in a wadi is rapid because of the sudden loss of streamvelocity and seepage of water into the porous sediment. Wadi deposits are thus usually mixed gravels and sands. These sediments are often altered byeolian processes.[4]
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.
"Summary: Drainage Courses, Wadis". United States Army Corps of Engineers. Desert Processes Working Group; Knowledge Sciences, Inc. Archived fromthe original on 9 May 2003. Retrieved26 August 2008.
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