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In this lesson we are going to discuss the action of water on arid areas

Answer Text:
Action of water in arid areas
- Receives short occasional rains causing flash floods which erode, transport and deposit large loads of materials
produced by weathering.
- Water action is short lived.
Resultant Features of Water Action in Arid Areas
a) Wadis

- Wide deep steep sided dry valley in a desert
- Strong surface runoff and flash floods form rills.
- Rills are enlarged into gullies.
- Flash floods flow into gullies widening and deepening them to form wadis.
Characteristics
- Wide and deep
- Steep with cliff like walls
- Flat floor
- Dry (lack permanent drainage)
b) Dry River Valleys
- Valleys in arid areas through which streams flow during the wet season and dry up in dry season e.g. in Turkana, Wajir and Mandera.
c) Mesas and Buttes

Mesas - Extensive table like residual hills found in arid areas.
Buttes - Smaller blocks of table like residual hills found in arid areas.
Formation
- First there are sedimentary rocks occurring in layers with a resistant one on top and a less resistant below.
- Weathering breaks the hard cap.
- Then sheet floods break the surface and carries materials away.
- Large outstanding blocks are left which are called mesas.
- Mesas may be eroded farther to form smaller blocks called buttes.
Features in an Inland Drainage Basin

- Gently sloping rock surface formed at the edges of desert uplands.
- A steep/scarp slope of a highland is eroded by sheet flooding reducing its height.
- The process continues causing the scarp slope to shift its position upwards.
- The gently sloping surface of 6-7◦results at the foot of the upland.
d) Playas/sebkha
- Extensive inland drainage basin in a desert formed by deflation or crustal warping or a small fluctuating salty lake contained in an inland drainage
basin in a desert formed when water from torrential outpours flows into the basin by multiple temporary streams e.g. Chemchane sebkha in
Mauritania.
e) Peripediment
Materials dry up leaving a hard salty crustal surface called Salina/salar e.g. in Arizona desert in U.S.A.
f) Pediment

g) Peneplain
Low level plain formed when pediments are eroded to form a low level plain.
h) Pediplain

- Extensive low and gently sloping lands common in deserts.
- Pediments surrounding a highland are extended by sheet erosion.
- With time the highland is reduced to a residual hill like Inselbergs.
- The hill is eventually eroded forming a continuous plain(Pediplain)
i) Inselbergs
- Prominent residual rocks in a desert.
- Formed by extension of pediments into upland areas.
Types
Bonhardt - Steep isolated round topped mass of rock rising steeply from desert surface.
- Dissection of plateau by streams producing steep sided valleys.
- The plateau is further eroded forming remnant hills e.g. Sugar Loaf Mountain in Rio de Janeiro in Brazil. Castle kopje –
Residual rocks in a desert found in groups.
- Formed from break down of Bonhardt with closely spaced joints.
- Or deep withering of a plateau edges.
- Weathered rocks are then removed by water reducing plateau into Inselbergs e.g. Nzambani rock in Kitui.