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Wadi (Arabic: وادي wādī) is the Arabic term traditionally referring to a valley. In some cases, it may refer to a dry (ephemeral) riverbed that contains water only during times of heavy rain or simply an intermittent stream.
In North Africa the transcription Oued, pronounced as Wad, is used. The term kouri is used in Hausa speaking and surrounding areas of West Africa. The Hebrew term nahal (נחל) and Hindi/Urdu term "Nala" are synonymous in meaning and usage.
Some names of Spanish locations are derived from Andalusian Arabic toponyms where wādī was used to mean a permanent river, for example: Guadalcanal from wādī al-Qanal = "river of refreshment stalls", Guadalajara from wādī al-hidjārah = "river of stones", or Guadalquivir from al-wādī al-kabīr = "the great river". Seasonal streams, frequent in south-east Spain, are called rambla instead.
In Maghreb, the term wadi (wad in Maghrebi Arabic) is applied to all rivers including regular ones of the Northern Mediterranean coast (as opposed to the southern Saharan areas), such as Oued-el-Kebir in Algeria. Both modern Spain and Portugal having been part of the Maghrebi Arabic-speaking world for several centuries, among many other examples, the Spanish river Guadalquivir comes from the Arabic al-wādi al-kabīr (الوادي الكبير), 'The Great River', and the 8th-century Spanish town of Guadalajara from Wadi-al-Hejara (وادي الحجارة), or Wad-al-Hayara, meaning "Valley of Stones".
Modern English usage differentiates a wadi from another canyon or wash 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 floods. Crossing wadis at certain times of the year can be dangerous as a result. Such flash floods cause several deaths each year in North America and many Middle Eastern countries.
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.
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 poorly sorted gravels and sands. These sediments are often reworked by eolian processes.
Over time, wadi deposits may become "Inverted Wadis" where the presence at one time of underground water caused vegetation and sediment to fill in the Wadi's eroded channel to the point that previous washes appear as ridges running through desert regions.
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