Drainage Basin by chenmeixiu

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									Drainage Basin


Part II
Geomorphological Process
Contents
   Weathering Subsystem
   Slope Subsystem
   Channel Subsystem
Weathering Subsystem
   What is weathering
   Physical weathering
   Chemical weathering
   Biological / Organic weathering
   Case study – weathering in desert
What is weathering
   Weathering refers to the process of
   disintegration (physical break down) or
   decomposition (chemical decay) of solid
   rocks in situ at or near the earth’s
   surface.
   It is a very slow process.
Main condition affecting the scale and
intensity of weathering
 Characteristics or rocks
   Mineral composition, joints, hardness of the rocks
 Climatic conditions
   Changes of temp. intensity of precipitation,
   frequencies of freezing,….
 Vegetation cover
   Types and density of vegetation cover….
 Topographical conditions
   Gradients, south-facing slope…., windward an
   leeward slopes…..
Physical weathering
  It is the disintegration of solid rocks into smaller
  fragments without involving any change in the
  chemical composition of rocks
  Changes in size and shape
  It encourages chemical weathering by increasing
  rock’s surface area.
  It is the most predominant in areas with great
  diurnal range of temperature. (desert regions and
  high mountains)
Conditions for mechanical weathering
   Alternate heating and cooling.
     Different degree of expansion.
      • Bare rock surface are highly heated by sunshine, the outer
        layer expand , but the inner layer expand little.
      • The different degree of expansion develops a series of joints
        on the exposed surface, rock tends to break into blocks
      • Different minerals have different coefficient of expansion.
      • The alterante heating and cooling produces alternate
        expansion and contraction lead to the cracking and granular
        disintegration.
      • During the night time, cooling by raditation causes water
        freezes in joints, which exert an enormous expansive force.
     Freeze and Thaw action:
      • The volume of water increase 8% when it freezes, and exerts a
        pressure force of about 200kg per m2.
Conditions for mechanical weathering
   Alternate of dry and wet:
     Water absorption causes rocks to swell but when
     they dry out they contract. Alternate wetting and
     drying results alternate expansion and contraction.
   Pressure release / unloading:
     All intrusive rock are formed under great pressure.
     When they exposed to the air through erosion
     process, they will break-up for pressure release.
   Plants and animals:
     They constitutes another important mechanical
     force to disrupt rocks.
Types of Physical weathering
  Exfoliation / Onion Scaling
    Rocks with homogeneous structure.
    Repeated heating and cooling by daily temperature
    changes.
    Repeated expansion and contraction create stress in rock
    and produce radial and concentric cracks.
    The outer layers eventually peel off to form exfoliation.
Types of Physical weathering
  Granular Disintegration
    Rocks composed by rather coarse grains or
    heterogeneous structure.
    Repeated heating and cooling by temperature changes,
    which causes alternate expansion and contraction.
    Minerals disintegrate grain by grain.
Types of Physical weathering
 Block disintegration
   Well-jointed rock (eg. Granite)
   Great diurnal range of temperature with more than 10oC
   between freezing point (0oC)
   Repeated heating and cooling by temperature changes,
   which causes alternate expansion and contraction.
   Cracks and joints are widened or breaks down into small
   blocks or fragments.
   It may be accompanied by frost action and chemical
   weathering.
Block disintegration
Types of Physical weathering
  Frost action / Freeze and Thaw Action
    Diurnal range of temperature fluctuating above 0oC.
    During daytime, water seeps into cracks or joints of rocks.
    Night time, temperature drops to below freezing point,
    water freezes in joints or cracks, which expands and
    widen the cracks.
    Alternate freez and thaw, the rocks break down into
    smaller, angular fragments
Freeze thaw action
Freeze thaw action
Chemical Weathering
   Introduction
   Processes of Chemical Weathering
   Types of Chemical Weathering
Chemical weathering
  Introduction
    Chemical weathering is the decomposition or decay
    of solid rocks as a result of chemical reactions
    between the rock minerals and moisture, rain water,
    sea water and organic acids produced by plants and
    animals,
    Chemical weathering can be accelerated by high
    temperature.
    It also can accelerated by physical weathering which
    breaks rocks up and increase the surface exposed to
    possible chemical weathering.
Processes of Chemical weathering
 Oxidation
   It is the process of the combination of oxygen.
 Hydrolysis
   Free hydrogen ions in water enter into the mineral structure and
   create a new compound.
   Eg. Feldspar in Granite to Kaolinite.
 Hydration
   The whole water molecule combines with the mineral.
 Carbonation
   Carbon dioxide (rain water pH=5.7) is capable of reaction with
   certain minerals
   It is particularly effective in limestone with humid climate.
 Solution
   Soluble minerals (rock salt,…)are dissolved directly in water.
Types of Chemical weathering
  Spheroidal weathering
    Chemical reaction is affected by penetrating water.
    A well-jointed rock allows this to go on reading.
Honeycomb weathering
 Rocks with
 heterogeneous structure
 and containing soluble
 minerals.
 By the processes of
 oxidation, hydrolysis,
 hydration, carbonation
 and solution.
 Very common in coastal
 area.
Honeycomb weathering
Features Produced by Chemical
Weathering
   Weathering Profile of Granite
     Ground surface is weathered for a longer period.
     Further inwards, the rock remains more fresh and
     stable. And It is possible to see a graduation of four
     weathered zones.
Weathering Profile of Granite
Weathering Profile of Granite
   Tors
Tors
  Tors may be the result of the surface rotting of
  granite through the action of acidulation
  rainwater penetrating along joints into the body
  of the granitic mass.
  The pattern of tors is controlled by the joints.
  When the overlying weathered materials were
  moved away (eroded) and the corestones
  exposed to the air, tors are formed.
Tors
Biological / Organic weathering
   Biological weathering is the physical
   disintegration or chemical decomposition of
   rocks in situ by organic agents – plants and
   animals.
   It is effective in regions with a continuous
   vegetation cover and burrowing animals.
   In desert regions and polar regions, it is
   insignificant for limited plants and animals.
Biological weathering - plants
Biological weathering - Plants
   The growth of plants roots and their
   penetration into rocks are sufficiently
   effective to widen cracks and joints.
   Rocks may be weathered by orgranic
   acids secreted by roots of plants and
   from decayed plants.
Biological weathering - animals
  Animals
    Burrowing animals may dig or turn up and loosen the
    joints of rocks.
    Earthworms and termites also loosen and expose the
    surface materials for weathering.
    Wastes secreted by animals or derived from dead
    animals (organic acids) help chemical weathering
  Human
    Human activities often cause large scale disintegration
    of rocks (mining, quarrying, excavation for building….)
    Careless removal of vegetation by man exposes large
    surface area to weathering processes. (deforestation
    for farming, for lumbering, for firewood; abandon
    farmland, overgrazing, hill fires,…..)
Case study – weathering in desert

   Traditional concept:
     Little rainfall, strong winds and large daily range of
     temperature.
     Mechanical weathering (block disintegration and
     granular disintegration ) is dominant.
     Exfoliation is very common in deserts.
     Chemical weathering only takes place by the drawing
     of strong solutions to the surface by capillary and
     forms duricrust (a hard compact layer) on the land
     surface.
Case study – weathering in desert
  New founding
    Barton:
     • He found that weathering of the stonework was in general more
       pronounced in the Delta than higher up the Nile Valley where has
       the maximum heating and cooling effects.
    Griggs’s experiment:
     • He gave the granite to 90000 fifteen-minutes cycles of alternate
       heating and cooling over a temperature range of nearly 90oC.
     • He found the rock totally undamaged at the end.
     • When he gave some water in the experiment, the whole block
       very quickly disintegrated.
    Chemical weathering involving water was the real
    destroyer of rocks in the desert regions.
Weathering regions - 1
Weathering regions -2
Slope Sub-system
   Contents
     Slope as a system
     Slope Profiles
     Slope development / Slope evolution
     Geomorphic processes on a slope
Slope as a system
   All landforms are made up of slopes.
   They originate by a combination of
   tectonic (endogenetic) and erosional
   activities (exogenetic).
Slope as a system
   Slope can be divided into 3 types:
     Tectonic slopes:
      • Formed by earth movement: folding and faulting.
     Erosional slopes:
      • Primary slopes:
         – the slope reduced by agents of erosion. Eg. Slip-off slope in
           river, U-shaped valleys in glaciated area, cliffs in coastal region.
      • Secondary slopes:
         – modified by weathering or mass wasting.
     Depositional slopes:
      • It is formed by aggregation and may be either convex or
        concave. Eg. barchan, sand dunes in desert areas
Slope as a system
   Inputs of slope
     Energy:
      • Solar radiation, falling raindrops, winds
     Mass:
      • All forms of water
          – Rainfall, snow-melt, springs and seepages
      • Inorganic minerals from bedrock
      • Organic materials from vegetation and animals
   Outputs of slope
     Energy:
      • Loss of heat
     Mass:
      • Water, weathered debris, solutes and organic waste, which
        leave the system by streams or other transporting media (eg.
        Wind) at the slope base.
Slope as a system
   Slope systems are sustained by inputs
   of energy and mass, which may be
   balanced by outputs, giving a steady
   state or equilibrium condition.
   Slopes are controlled by increased or
   decreased inputs and outputs, so as to
   maintain its equilibrium.
Slope as a system
  Slopes reflect the interaction of 3 factors.
    Earth movement (tectonic activities)
     • Rate of uplift or subsidence
    Rock types
     • Resistance to weathering and erosion
    Weathering and transport processes operating
    on the slope
     • Vegetation cover, animals and human activities
    Effects of agents (erosion & deposition):
     • Running water, glaciers, winds, sea waves….
Slope elements
  Slope profiles may be divided into a
  series of slope units for analysis
Four units model
 A. Wood divided a slope into 4 elements
   Waxing slope:
    • It is the convex curve of the hill crest.
   Free Face / Cliff
    • It is a vertical or very steep rock-face.
   Constant slope:
    • It maintains a constant angle of rest.
    • It is formed by the debris fall from free face and gradually
      accumulates to building up a heap of scree (talus).
   Waning slope:
    • It is below the constant slope, which is formed by fine materials.
    • It is also the washing slope because it is derived from the
      material washed down from constant slope.
    • The low- angle wash slopes will gradually coalesce to form a
      depositional pediment.
Slope units
Talus or Scree
Nine units model
Slope development / evolution
   The forms of slopes develop through time
   and the factors (rock structure, lithology,
   soil, climate, vegetation and human
   activities).
   There are few models of slope
   development or slope evolution.
     Slope Decline
     Slope replacement
     Slope parallel retreat
Slope decline
   American geographer, W.M. Davis (1899)
   From NW Europe and NE USA
   Normal (Humid) climates
   Concept of the “cycle of erosion”.
   Steepest slopes at beginning of process with a
   progressively decreasing angle in time to give a
   convex upper slope and a concave lower slope.
   Slopes will continuous to decline and develop or
   evolve from youthful stage, maturity stage, old
   age stage and finally to become a low relief
   peneplain (almost a plain).
Slope decline
Slope replacement
  By W. Penck (1924)
  Evidence from the Alps and Andes (tectonic
  areas)
  The slope of maximum angle decreases as the
  gentler lower slopes.
  Waxing slope will be replaced by free face
  Free face will be replaced by constant slope
  Constant slope will be replaced by waning slope.
Slope replacement
Slope replacement
Slope Parallel Retreat
 By L.C. King (1948, 1957)
 From South Africa
 Semi-arid regions and sea cliffs with wave-cut
 platforms
 Sedimentary rocks structure
 The slope units retreat by the same amount
 (proportion) so that the whole profile retains but
 leaves an extending concave unit (pediment) at its
 foot.
 This sequence is controlled by the rate of retreat of
 free face which is controlled by geology and
 climate (weathering and transport processes)
Slope Parallel Retreat
Slope Parallel Retreat
Slope Parallel Retreat
Geomorphic Processes on Slope
   Weathering, mass movement and erosion
   are the major processes in shaping slopes.
   On hard rocks which weather very
   slowly – weathering limited slope
   High potential weathering but outputs
   from slope are restricted – transport
   limited.
Slope as a process-response system
Mass movement - landslide
 Mass movement involve the transport of debris
 under the influence of gravity.
 It depends on the instability (stresses and strength)
 of slope.
 Loss strength (frictional resistance and cohesion)
   Heavy rainfall
   Severe weathering
   Oversteepening of hillslope (excavation)
 Increase stress
   Heavy rainfall (increase weight of slope)
   Earthquakes or volcanic activities
   Building on slope increase loading
   Vibration from passing heavy vehicle
Landslides
Landslides
Slope processes(non-cohesive materials)
   Gravels and sands
   Landforms: alluvial fans, screes (talus),
   sand dunes and glacial outwash features.
   The movement occurs largely through the
   sliding or rolling of individual particles.
   It may be triggered by minor events such as
   rainfall or vibration.
Alluvial fan
Slope processes (cohesive materials)
   Soil and clay
   The cohesion is derived from electro-
   chemical bonds between fine particles and
   the surface tension effects of water films
   in the pore spaces.
   Main features
     Rapid movement
      • Slumping and mudflows
     Slow movement
      • Soil creep and solifluction
Slumping / Rotational slips
  It occurs along clearly-defined concave
  (curved) sliding plane.
  Very common in humid climate.
Mudflows
 It may operate on very low angles slope.
 High moisture content of the material reduces the strength
 (frictional resistance and the cohesion almost to zero.)
 Very common in desert after heavy rain and volcanic
 eruption with heavy rainfall.
Soil creep
  It can be found on all slopes
  It was formed by the effects of gravity,
  temperature fluctuations (Freeze-thaw) and
  variations in moisture (wet-dry periods) content
  within the soil may all act to cause displacement
  of particles.
Soil creep
Slope processes in hard-rock slopes
   Rockfalls
   It builds up a constant slope (talus or
   scree)
Soil erosion
  Water plays a significant role to soil erosion
  in humid regions
  Rain splash can redistribution the materials
  without any transport on horizontal surface.
  It can move the materials downslide on a
  slope
  Rain splash     rills    gullies      badland
Rills
Gullies
Badland
Other forms of slide
Channel Sub-system
   Channel as a sub-system
   Stream Velocity
   Channel Processes (Geomorphological
   work of stream)
   Channel Form
   Rivers work in the two landscapes
Channel as a sub-system
  Stream channels are systems
    Inputs:
     • Water
         – Direct precipitation
         – From tributaries
         – From seepage from the river banks
     • Solid materials
         – Debris from stream banks and bed
    Outputs:
     • Water
         – Losses of water throughout the length of the channel by seepage
         – By evaporation
         – To sea
     • Solid materials
         – To sea
     • Channel forms
Channel as a sub-system
Stream Velocity
   It is the most important factor affecting
   the channel / stream processes (Erosion,
   Transportation and Deposition)
   Mean velocity of a river increases
   downstream.
Reasons – Along upper course
   Channel gradient is great but river
   velocity is low
     Low discharge (Q=VA)
     River course is irregular
     Turbulence flow
      • Channel floor is uneven and broken by potholes
      • Much energy is needed to overcome the
        roughness of channel floor.
Reasons – lower course
   Channel gradient is small but river
   velocity is increased
     Rise in river discharge
      • Joining of major tributaries
     Straighter course in spite of the presence
     of marked meanders
     Laminar Flow
      • Reduction in friction along the channel floor for
        the deposition of fine sediments
Stream velocity
Erosion, transportation and deposition
Erosion, transportation and deposition
  Critical tractive force
    Minimum force to entrainment of grains from bedrock on
    the sides or floor of channel.
  Critical (erosion) velocity
    Lowest velocity to obtain the critical tractive force (to move
    grains from channel bed)
  Erosion
    The easiest eroded material is of diameter of about 0.5mm.
    Coarse materials (medium sand to boulders) require greater
    velocity.
    Very fine materials are also difficult to erode for strong
    binding by chemical bonds.
    Smooth channel bed formed by fine materials is more
    resistant to erosion.
Erosion, transportation and deposition
    Transportation velocity
      Between the critical erosion velocity and the curve
      of deposition of particles
    Fall /Settling velocity
      Velocity at which materials in transport are
      dropped and deposited on the channel bed.
      It is high for the larger and heavier particles but
      extremely low for clay (transported in suspension)
Channel Processes
(Geomorphological work of river)
   The morphology of natural river channel
   is determined by the interaction of
   flowing water and solid materials.
   Channel Processes
     Erosion
     Transportation
     Deposition
Erosion – along the river courses
   Stream erosion is the progressive removal of
   mineral material from the floor and sides of the
   channel, whether bedrock or regolith.
   Upper Course
     Erosion is the dominant process for the steep
     channel gradient.
   Middle Course
     Erosion is a bit reduced as some depositions occur
     where channel beds are flat.
   Lower Course
     Erosion becomes far less important than deposition.
Erosion – erosion processes
   Hydraulic action:
     Removal of loose materials by direct force of impact
     of running / flowing water.
   Abrasion (Corrasion):
     Mechanical wearing and tearing of rock particles at
     or being dragged along the channel bed.
   Corrosion (Solution):
     Rocks minerals are dissolved by water.
   Attrition:
     Reduction in size of loads in transport as they strike
     at each other or the channel bed.
Erosion – erosion direction 1
   Headward erosion:
Erosion – erosion direction 1
Erosion – erosion direction 2
  Lateral erosion
    Erosion of the sides of a river channel
    More lateral erosion is found along the
    concave banks than along the convex slip-
    off banks.
    More active along lower course for the
    gentle slope.
    The kinetic energy transfers to lateral
    erosion.
Erosion – erosion direction 2
   Lateral erosion
Erosion – erosion direction -3
 Vertical erosion /
 downcutting
   It is the erosion and
   subsequent deepening
   of the floor.
   It is more active along
   upper course for steep
   slope.
   A narrow V-shape
   valley is develop.
Erosion – erosion direction 3
Erosion – incised meanders
Erosion – incised meanders
Transportation
   Weathering on channel side slopes and plains
   produces loosened masses of materials that
   can be washed into the channel.
   Such moving materials are called loads
   There are four types of load
     Dissolved load / Soluble load
     Suspended load
     Saltation load
     Traction load / Bed load
Transportation
  Dissolved load:
    They are enters the water current by corrosion, and is
    transported in solution by the river water.
    Chemical composition of river water depends on
     • Topography
        – Steep bank-side slopes is likely to be richer in dissolved minerals.
     • Climate
        – High temperature can increase the rate of chemical reaction.
     • Geology
        – Rocks minerals are dissolvable or not.
     • Vegetation
        – Supplier of organic matter.
Transportation
  Suspended load
    They are carried downstream by the irregular
    turbulent in suspension.
  Saltation load
    They are moved forward by the water current in a
    series of leaps and bounds.
  Traction load
    The larger fragment are moved by the water current
    in rolling and sliding on the river bed.
Transportation
Stream Competence
  The largest transported particles that a stream
  is able to move in traction as bed load.
  Normally, stream channel is less competent in
  removeing coarse bedload materials because
  coarser materials are heavier and more irregular
  in shape such that much energy is needed to
  overcome their friction.
  Stream channels are more competent in
  removing bedload at high water flow level for
  increasing in river discharge.
Stream Capacity
   It is maximum amount of load materials
   that a stream can transport.
   It varies according to velocity which
   depends on channel gradient, stream
   discharge and weight of load.
Loads
  Suspended sediment:
    It tends to increase with discharge levels at any
    point on a stream.
  Solute:
    It is the greatest concentrated at low flows, or
    channel water is derived entirely from
    groundwater seepage.
    During periods of higher discharge, solute will
    be diluted by throughflow and overland flow.
  Bedload:
    It increases at higher levels of discharge.
Loads
  Three types of load varies according to
  the nature of load available, the
  discharge and courses of the river.
    Solute load being more important at low
    flows.
    Suspended sediment is transported in
    greater at flood time.
    Bedload moves only once a threshold level
    of discharge has been attained.
Deposition
   The deposition of sediments occurs
   when a stream is no longer to carry
   loads, loss of competence or
   transporting ability.
   It may be the result of
     Decrease in channel gradient
     Decrease in discharge volume
     Increase loads supply in the channel
Deposition
   Decrease in channel gradient
     Stream water enter to its lower course at flat plain
     Stream water enter into a pre-existing depression
     (lake, lagoon, or any type of still water…)
   Decrease in discharge volume
     Discharge will decrease in dry season
     It will decrease by river capture.
     It will decrease by seepage which is very common
     along exotic rivers (River Nile), along a permeable
     channel floor (desert) or flowing into limestone areas.
   Increase loads supply in the channel
     Excessive increase in the supply of load materials
     (serious mass movement)
Channel Form
  The channel form can be regarded as
  the response by the channel inputs.
  It can be considered in terms of
    Cross profile
    Long profile
    Plan
Plan – Channel patterns
   There are three major patterns:
     Straight channel pattern
     Meandering channel pattern
     Braided channel pattern
Straight channel pattern
 It refers to the channel characterized with straight
 banks.
 In fact, it is impossible of natural rivers with straight
 banks for irregularities of river channels.
 The stream line of flow is usually in a winding path
 or sinuous pattern.
Sinuosity
  Sinuosity = actual channel length / straight line distance.
Sinuosity
Meandering channel pattern
  If forms when pronounced bends or loops
  develop in the course of river.
  It may be the result of
    Helical flow
     • When stream water moves in a winding pattern, it produces a
       strong centrifugal force which causes a helical flow in concave
       outside bank.
     • It results in effective erosion in concave bank.
    Slope gradient is reduced in lower course.
    Large proportion of loads is carried in suspension form
    There is significant local bank erosion and deposition.
Braided channel pattern
 River channel is subdivided into two or more
 bifurcated channel which are separated by bars of
 alluvial materials (usually visible in dry seasons).
 It occurs when
   Too many bedloads in the river
   The bedloads are too coarse to move.
   The discharge is variable
Long Profile
   The graded river which is capable of
   existing in a state of balance, or
   dynamic equilibrium, with the rate of
   erosion being equal to the rate of
   deposition.
Long Profile
Cross Profile
   Discharge increases in a downstream direction.
   The channel adjusts to the change by deepening and
   widening (cross-section form)
   The width / depth ratio also changes (increases)
   downstream as width increases more rapidly than depth.
Work done by rivers in the two
landscape
   Tropical Rainforest
   Tropical Desert
Tropical Rainforest
   Intense chemical weathering under the hot humid
   condition.
   Erosion:
     Small streams of steep gradient have bouldery
     channels but the boulders experience little downstream
     movement.
     Lateral erosion is very serious in middle and lower
     course for gentle slope and large discharge.
   Loads and transportation
     Greater portion is in the form of dissolved solids.
     Little coarse sediments move as bed load, because the
     cover of vegetation holds back all
     Finest soil particles from transport by overland flow.
Tropical Rainforest
   Deposition:
     Deposition is very common in the lower
     course, meandering channels, flood plains
     and deltas can be found in lower lower
     course.
Tropical Desert

   Introduction
   Streamflow characteristics in the arid
   lands
   Streamflood
Introduction
   River work is very effectiveness because of
   the meagreness of vegetation in dry desert.
   Without a thick vegetation , large quantities
   of coarse rock debris are swept into streams
   to transform a dry channel into a flood
   stream.
   Sediment concentration can be extremely
   high up to 50%.
   Stream flow in deserts usually accounts for
   most of the water from precipitation.
Introduction
   Much of the precipitation is held as surface
   runoff rather than infiltrating deeply into the
   ground:
   Reasons
     Lack of vegetation
     Lack of water-absorbent organic layer in desert soil
     Presence of hardpans (salt cover) in the topsoil.
     Clayey surface of some deserts.
     Rainfall is too intense for a large amount of water to
     percolate.
Streamflow characteristics
   Precipitation tends to run off into wadis which
   are normally dry out occasionally subjected to
   large flows of water and sediment.
   Rock-cut gorges may developed on
   pediments and fans at the foot of mountains.
   Floods are rather occasional and even fewer
   floods in the almost totally arid parts.
Streamflood
   Rainfall in deserts in usually of high intensity,
   most of the rainfall is available as runoff and
   enters the dry stream courses.
   Where streams flow across plains of gravel and
   sand, water is lost from the channels by
   seepages to water table as underground water.
   The role of water in erosion, transportation,
   deposition should not be ignored or made in
   secondary to the action of wind, once though to
   be the most important geomorphological agent
   in desert .
Geomorphological work of
stream-floods
    Large amounts of debris due to weathering, slow
    mass movements and wind action.
    Debris of all sizes from clay particles to boulders
    are moved until eventually they reach the upland
    edges.
    Stream floods can also cause erosion of stream
    channels themselves.
    Streamfloods in deserts are more important than
    the floods of humid regions in erosion. It is
    because weathered materials are not cohesive in
    dry environment and few plants to hold the soil
    together.
Geomorphological work of
stream-floods
    Lateral erosion of wadis channels is also due to
    streamfloods.
    Drainage density is very high (eg. 350 km/km2 in
    parts of arid North America), but in sandy deserts
    with high infiltration rate and hence little runoff,
    drainage intensity will be low.
    Deposition occurs and braided channels are
    conspicuous because of the heavy load of sediments
    carried by the streams.

								
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