Deltas by shuifanglj


									Slides for November 3, 2003

                                                          River Delta
All images courtesy of NASA (
                    The Nile Delta

Early geographers, reminded of the Greek letter delta when noting its
            roughly triangular shape, named it accordingly.
• Deltas are transitional environments
  – they are neither fully marine or terrestrial
• Deltas are built by the accumulation of river-
  borne sediment
  – when the accumulation exceeds sea level rise
• Deltas often contain subordinate systems
  – marshes, swamps, and tidal flats
• Deltas are sensitive to sea level change and
  alterations to their basins
  – they are heavily impacted by human activities
            Introduction Continued
• Deltas occur on every continent on trailing edge
  continental margins (but the U.S. Atlantic Coast has
  no real deltas; most East Coast rivers empty into estuaries
  where their sediment is then dispersed)
   – marginal seas and protected coasts are especially
     conducive to delta formation
   – leading edge continental margins are not conducive
      • continental shelves are too narrow and deep to allow
        sediments to accumulate, wave energy is too high and tends
        to sweep away river sediments, and nearby mountains make
        for narrow rivers which carry relatively small amounts of
Fresh river water spreads into salt water
               Delta Age
• Deltas are geologically young features
• Most modern deltas postdate the period of
  high sea level rise about 5,000 years ago
               The Mississippi Delta
Jetties have
been built                                                  Note the
and                                                         large
dredging                                                    amounts
occurs to                                                   of
allow large                                                 sediment
vessels to                                                  that
navigate                                                    surrounds
the delta                                                   the delta

A bird’s foot delta located in Louisiana adjacent to the Gulf of Mexico
                The Atchafalaya Delta
                                        This is one
                                        of the few
This delta is                           where the
considered to                           Louisiana
be the                                  coastline is
Mississippi’s                           growing
largest                                 The
distributary                            Atchafalaya
                                        diverts large
                                        quantities of
                                        water from the
             Delta Environments
• Delta Plain: the land-ward, very flat extension of
  the delta
  – distributary: branching of the main river channel into
    a series of smaller channels
     • these smaller channels distribute water and sediment across
       the delta plain and delta front
  – interdistributary bays and marshes: broad relatively
    flat areas between distributaries with either open
    water (bays) or marsh (herbacious wetland)
 Delta Environments Continued
• Delta plain
  – levees: the built-up areas adjacent to the
     • where sediments accumulate because this area is
       flooded more frequently (sediments are often
       coarser on levees as the coarse material drops out of
       suspension before the finer material (i.e. silt))
  – crevasse: cuts through the levee
  – crevasse splay: cuts through the levee allow
    sediments to spill across the delta plain in a
    broad fan-like shape
      Delta Environments Continued
• Delta front: where the seaward edge of the delta
  plain merges with the sub-tidal portion of the delta
  – sand is deposited first and is therefore closer to the delta
  – distributary mouth bars may form with this sandy
• Prodelta: the seaward extension of the delta, which
  is largely built of fine sediments (silt & clay)
  – waves and in some cases tides rework this material
  – largely below sea-level
Structure of a Delta
Slides for November 5, 2003
                 Delta Formation
• A large river flow is required to deliver the
  sediment for delta development
• Deltas can be river, tide, or wave dominated or
  can be influenced by a mixture of these forces
  – these forces determine the fate of the river-borne
    sediment and the resultant morphology of the delta
  – river-dominated (Mississippi Delta)
     • a well developed delta plain, often with one lobe (the most
       recent one) projecting out beyond the general delta front
  – tide-dominated (Ganges Delta-largest)
     • strong tidal currents mold the delta into elongate forms that
       are parallel to river flow and perpendicular to the coast
     • often resemble estuaries due to the presence of embayed salt
       marshes, swamps, and tidal flats
                    large mangrove forest
                    (largest remaining habitat of
                    the Bengal tiger)

Mississippi Delta      Ganges Delta
River vs. Tide Dominated Deltas
                    large mangrove forest
                    (largest remaining habitat of
                    the Bengal tiger)

Mississippi Delta      Ganges Delta
        Delta Formation Continued
• wave-dominated
  – smooth shorelines with relatively few distributaries
  – tend to be smaller than other types of deltas as wave
    action limits growth
  – limited marsh or swamp areas
• mixed or intermediate
  – deltas where one force (wave, river, or tidal) does not
  – the Nile River Delta shows characteristics of river and
    wave domination as it has both well developed
    distributaries and a smooth delta front
Seasonal variation: Mississippi, ex.
• Peak discharge > 3x base flow of
  Mississippi at the river’s mouth
• During low flows, salt wedge moves
  upstream; sedimentation patterns low in
  delta resemble those of a salt-wedge
  estuary, with sedimentation occurring
  higher in delta
• During high flows, salt wedge is pushed
  out, turbulent mixing occurs, sedimentation
  on delta bar crest
Rhone River Delta
Wave Dominated Delta

    Rhone River Delta
                             Nile Delta
developed                                                            Smooth
distributaries                                                       delta front

        A mixed or intermediate delta influenced by both river and wave forces
   Delta Progradation/Retrogradation
• Delta growth depends on drainage basin conditions
  – the most important factors for the vitality of the delta are
    soil type and rainfall
     • large amounts of rainfall and loose soils increase delta growth
  – changes in climate, topography, and sediment supply
    may alter the growth of the delta
  – humans can alter all of these important factors
 Delta Progradation/Retrogradation
• Progradation: the growth of a delta seaward
  – this occurs by the accumulation of sediments in
    zones that shift seaward over time
     • sandy materials landward, then silt, and then clay
  – if you bored through a delta you would find clay
    materials toward the bottom (essentially the prodelta
    that was over-ridden) and coarser materials toward
    the surface
  – deltas will continue to prograde as long as the
    sediment supply exceeds the rate of relative sea level
          More Delta

• Retrogradation
  – occurs when the delta begins to disappear
    because the supply of sediment is not enough to
    offset sea level rise
     • this can happen for a variety of reasons
        – sea level rises too fast
        – drought in the river basin decreases river flow
        – sediment supply declines due to climate change or human
    Evolution of a
Mississippi Delta Splay
Atchafalaya Delta
Delta plain   Delta Cycle
               Delta front

             Human Impact

• People can affect Deltas in many ways
• For example, people often create dams
  which block sediment from reaching the
  – Both the Nile and the Mississippi Deltas are
    examples of this phenomenon
    The Aswan Dam and the Nile Delta
The dam                                                                  Dam
provides                                                                 (1971)
flood control,
irrigable land                                                      Lake Nasser
area, and
much needed
power (about
40 percent of
the country’s

 The Aswan Dam has disrupted the Nile Delta’s normal development. The dam traps
  sediments formerly used to build the delta and agricultural area, allows saltwater
      intrusion, and reduces the amount of nutrients present in the Nile’s water.
The Aswan Dam and the Nile Delta
Human Impact, cont’d: Mississippi
• Construction of levees on the river forces
  sediment downstream that would normally
  be deposited in the flood plain
• Construction of levees on the delta starves
  surrounding marsh area of sediment
• Marsh starved for sediment is prone to loss,
  e.g. retrogradation
• Oil & Gas extraction is causing subsidence,
  exacerbates marsh loss
• Excess sedimentation in distributaries leads
  to need for continuous dredging
              People and the Mississippi
                                                                        deposited by
  The channel                                                           distributaries
  frequent                                                               Note the
  dredging so                                                            large
  that large                                                             sediment
  vessels can                                                            plumes
  navigate the

The Mississippi River carries 200 million tons of sediment annually to the Gulf of
Mexico. Much more sediment would be carried if not for the dams and levees
constructed on the Mississippi for flood control, navigation, & other purposes.
                The Danube Delta

This delta
attracts many
                                                      The Danube
tourists who
                                                      is the second
                                                      longest river
watching the
                                                      in Europe
delta’s birds
and other

  The Danube Delta is the second largest in Europe (4300 square
   kilometers) and is home to one of the biggest reed beds in the
  world, vast stretches of other wetland types, and much wildlife.
           The Irrawaddy River Delta
Sediment                           mangrove
plumes                             forest

                                  mud and
This river                        sand have
delta in                          been
Burma is                          deposited
one of the                        here
world’s                           during the
great rice                        last 2
producing                         million
regions                           years
              Sacred River Delta
                                                 The delta
                                                 is a site of
                                                 some of
                                                 the earliest
                                                 in India
Both Rivers
are sacred
to the
              Godavari and Krishna River Delta

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