11-726: Wadi Howar ( Wadi Howa or Yellow Nile ) is a wadi in Sudan and Chad . Originating in the Ennedi Region of Chad Wadi Howar runs through the Sudanese states of North Darfur and Northern to join the Nile north of the great bend opposite Old Dongola . Stretching over 1100 km in west–east direction across the southern fringes of the Libyan Desert , it ordinarily receives 25 mm of rainfall per year. Wadi Howar
22-688: A chain of freshwater lakes and marshes, as shown by Ptolemy's world map , then it became extinct about 2000 years ago. Abundant prehistoric sites certify Wadi Howar as a once ecologically favoured area of settlement and a communication route between the inner regions of Africa and the Nile Valley. The hitherto most thoroughly investigated archaeological site in the wadi is Gala Abu Ahmed . Wadi Wadi ( Arabic : وَادِي , romanized : wādī , alternatively wād ; Arabic : وَاد , Maghrebi Arabic oued , Hebrew : וָאדִי , romanized : vadi , lit. 'wadi')
33-491: 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
44-709: 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 (4–12 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 be present. Modern English usage differentiates wadis from canyons or washes by
55-504: Is the Arabic term traditionally referring to a river valley . In some instances, it may refer to a wet ( ephemeral ) riverbed that contains water only when heavy rain occurs. Arroyo ( Spanish ) is used in the Americas for similar landforms. The term wādī is very widely found in Arabic toponyms . Some Spanish toponyms are derived from Andalusian Arabic where wādī
66-579: Is the remnant of the ancient Yellow Nile , a tributary of the Nile during the African humid period from about 9500 to 4500 years ago. At that time, savanna fauna and cattle herders occupied this region and the southern edge of the Sahara was some 500 kilometres (310 mi) further north than it is today. When the Sahara underwent desertification between 6000 and 4000 years ago, the wadi first became
77-541: 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 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. Imbrication (sedimentology) In sedimentology , imbrication
88-425: 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
99-504: 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. They 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
110-485: 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 diverse of all desert environments. Flash floods result from severe energy conditions and can result in
121-428: 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"), 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
SECTION 10
#1732773312977#976023