Water Action

Water Action

1. Types of rivers

   • Exogenous
     • Perennial
       • flow all year
       • source outside desert
         • flow through desert until sea
     • E.g. Nile & Colarado
     • Level of discharge varies
       • Heavily used for water supplies & irrigation
   • Ephemeral
     • Don't flow all year
       • Seasonal
         • after heavy rainstorms
           • rivers quickly fill up
             • short lag time
             • low interception
               • baked ground
                 • impermeable surface
     • can have sources in/out arid area
     • E.g. Atlas Mountains Morocco - Guigou Wadi
   • Endoreic
     • flow into inland lake
       • dry up
         • leave salt pans
           • high level of water loss
             • evaporation
             • infiltration
     • E.g. River Jordan into dead sea
   • Other sources of water in deserts include precipitation, ground water, aquifers, fog in coastal deserts/dea

2. Landforms of River Erosion

   • Rapid surface run off & high river disacharge
     • powerful erosion
   • Erosion features - high mountain areas Deposition feature - low lying areas
   • Wadis
     • steep sided dry river channels
       • wide channel floor
         • small to networks of 100's km
           • smaller ones called gullies/rills
             • channels only hold water after heavy rainfalls
         • covered by sediment
     • Caused by ephemeral rivers
       • Only flow in seasonal rainfalls or flash floods post storms
       • Discharge high due to rapid surface run-off & steep long profile gradients
         • Erosion from abrasion and hydraulic action powerful
           • Makes steep sided & wide channels
             • dry up deposition forms braiding on river bed
               • larger wadis formed in previous climates with more rainfall
   • Pediments
     • gently sloping
       • less than 7 degrees
       • bases of cliffs, steep hills or mountains
         • SOMETIMES exposed bare rock
           • USUALLY covered with debris
             • from rock fall
             • sediment gets finer
               • with distance from foot of cliff
     • Main theory of formation
       • weathering of steep slope
         • thin layers of rock peel away
           • 'parallel retreat'
           • storms & flash flooding
             • lots of surface run off
               • 'sheet flow'
               • water washes debris down pediment
   • 'Relict hills'
     • remains of large plateaus
       • been left behind after extensive water erosion
         • over millions of years
           • when area had wtter climate
             • continue to be worn away by
               • water action
               • weathering
               • wind erosion
             • Formation of them
               • sedimentary rock due to
                 • more resistant cap rocks
                   • harder to erode
                 • worn away by factors on left
                   • parallel retreat
                     • water erosion from wadis & sheetwash
     • Mesas
       • large, flat topped
         • steep sided hills
           • isolated from each other
             • usually have gently sloping pediments
               • base covered in sediment (scree)
       • sedimentary rock - horizontal layers visable in structure
     • Buttes
       • same as Mesas
       • more erosion made them smaller
         • no longer wider than tall
           • look like pillars
     • Inselbergs
       • isolated relict hill
       • but more rounded
       • dome shaped
       • common in semi-arid regions
       • often granite
       • Formed when preciptiation was much higher than today
         • caused chemical weathering of crystalline rock
           • e.g. granite
           • water washes away
             • weathered deposits of rock become exposed
               • isolated because surrouding areas are less resistant rock
                 • can be several km's long - eg Ayers rock

3. Landforms of River Deposition

   • Salt Lakes
     • hollows/depressions
     • can be partially covered by shallow saline lakes
       • but are dry when precipitation is lacking
         • leaving thick crust with cracks
           • polygonal pattern
           • salt flat/pan
     • bordered by saline resistant halophytes
       • E.g. salt bushes
     • Dead sea - several Km
     • Formation
       • precitipation collects in hollows
         • from surface run off & endoreic rivers
           • in inland drainage basins
           • lack of precipitation & high rates of evaporation
             • salt lakes dry for most of the year - hence salt pans
               • leaves behind deposits of soluble salts
                 • dried out crust forms cracks so polygonal surface
                   • common salts are sodium chloride and calcium sulphate that are commercially extracted from the area
         • also when deflation hollows have been eroded down to water table
   • Alluvial Fans
     • cone shaped deposits of sediment
     • where ephemeral wadis emerge from mountains onto pediments or plains
     • sediment graded with coarest material closest to mountain
     • finer silts washed further down the fan
     • Few metres to km's and sometimes join other fans to form bahada
       • like in Death Valley
     • Formation
       • wadis streams fill with water in fresh floods
         • heavy preciptation
         • high discharge erodes steep sided wadis
           • provide sediment to be transported in steep mountain valleys
             • river emerges onto lowland
               • sudden decrease in gradient
                 • slows water & it loses its energy
                   • water not trapped by steep sided wadi walls
                     • spreads out over wide area
                       • creating friction & rapid deposition
                         • spreads into numerous distributaries
                           • makes the cone shaped deposition
                             • sediment graded with coarest material
                               • close to mountain
                               • finer silts
                                 • foot of fan

4. Badlands

   • some semi-arid desert margins
   • water/wind erosion has carved out dramatic landscape
     • wadis
     • gullies
     • alluvial fans
     • pipes
       • natural arches
         • formed when pipes flow through soft bedrock
       • formed when water flows into cracks
     • hoodos
       • sedimentary rock relicty hills pillar shaped
         • formed when resistant cap rock has protected softer column of rock below freom being washed away in sheetflow
   • more vegation than arid areas
     • but still sparse
   • E.g. Badlands National Park Dakota
   • Formation
     • precipitation erratic
       • heavy rain sometimes
         • rapid surface run off
           • replenishment of ephemeral streams
             • in wadis
     • vegetation sparse
     • so dry soils and regolith
       • not stabilised by roots
         • exposed to water and wind erosion
     • Combination of these factors
       • high rates of erosion
     • sedimentary bedroc