Wadi

The term wādī is very widely found in Arabic toponyms. Some Spanish toponyms are derived from Andalusian Arabic where wādī was used to mean a permanent river, for example: Guadalcanal from wādī al-qanāl (Arabic: وَادِي الْقَنَال‎, "river of refreshment stalls"), Guadalajara from wādī al-ḥijārah (Arabic: وَادِي الْحِجَارَة‎, "river of stones"),[2] or Guadalquivir, from al-wādī al-kabīr (Arabic: اَلْوَادِي الْكَبِير‎, "the great river").

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. In basin and range topography, 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 patterns because of the deficiency of water and the abundance of sediments. 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.

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.

Wadi sediments may contain a range of material, from gravel to mud, and the sedimentary structures vary widely. Thus, wadi sediments are the most diagnostic 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 10–30 cm (3.9–11.8 in). 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 may also present.

Triassic wadi deposit near Ogmore-By-Sea, Wales. Clasts are Carboniferous Limestone.

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 floods. 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.[3]

• 
  Wadi Bani Khalid in the Northern Governorate of Ash-Sharqiyyah Region, Oman,[5][6] Arabian peninsula
• 
  Wadi in Nahal Paran, Negev, Israel
• 
  Oued Tissint, Morocco, North Africa
• 
  Dry fluvial channel cutting through columnar basalt in Namibia, southern Africa

Wikimedia Commons has media related to Wadis.

Look up wadi in Wiktionary, the free dictionary.

• IHP REGIONAL WADI HYDROLOGY NETWORK, International Hydrological Programme, UNESCO.
• Arab Center for Studies of Arid Zones and Dry lands (ACSAD): Water resources division.