Deltas tidal waves

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					                 Deltas




                                                          Mangoky
                                                          River Delta
All images courtesy of NASA (http://earth.jsc.nasa.gov)
        The Nile Delta: a Prototype




Early geographers, reminded of the Greek letter delta when noting its
            roughly triangular shape, named it accordingly.
                  Introduction
• 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
• 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
        sediment
                 Delta Age
• Deltas are geologically young features
• Most deltas postdate the period of high sea
  level rise about 5,000 years ago
• However, this is not always the case.
  – For example, the Mississippi River Delta is
    built upon much older deltas that are several
    million years old (although its newest lobe is
    only about 600 years old)
               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
channel




A bird’s foot delta located in Louisiana adjacent to the Gulf of Mexico
                The Atchafalaya Delta
                                        This is one
                                        of the few
                                        areas
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
                                        Mississippi
             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
    channel
     • 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
    front
  – distributary mouth bars may form with this sandy
    material
• 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
                 Delta Formation
• 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)
     • freshwater is over-powered by tidal currents
     • 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
River vs. Tide Dominated Deltas
                                   large mangrove forest
                                   (largest remaining habitat of
                                   the Bengal tiger)




Mississippi Delta   Prone to devastating Ganges Delta
                    floods
        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
    dominate
  – the Nile River Delta shows characteristics of river and
    wave domination as it has both well developed
    distributaries and a smooth delta front
Wave Dominated Delta
                        Wave
                        action
                        has
                        smoothed
                        the
                        coastline




    Rhone River Delta
                             Nile Delta
Well
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 first, 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
    rise
          More Delta
  Progradation/Retrogradation!

• 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
          influence
Delta plain   Delta Cycle
               Delta front




Pro-delta
             Human-Impact

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


 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
Effects of Sea Level Rise




Sediment once transported by the Nile
River to the coast, average of 100x10 6 t
yr-1, is now almost cut off.
              People and the Mississippi
                                                                         Sediment
                                                                         deposited by
  The channel                                                            distributaries
  requires
  frequent                                                                Note the
  dredging so                                                             large
  that large                                                              sediment
  vessels can                                                             plumes
  navigate the
  channel



Although the Mississippi River still carries 200 million tons of sediment into the Gulf
of Mexico every year, much more sediment would be carried if not for the dams and
levees constructed by people in its drainage basin for flood control & other purposes.
                                   Causes
•   The Mississippi River water level has fluctuated and fluctuates a great deal due to
    weather phenomena, glacial melt, and human use.
    The river valley has been struck by devastating floods many times. Since the early
    1800s, Americans have attempted to control the level of the river using different
    engineering techniques.
•   The most effective means of controlling the river has been damming, especially far
    upstream from the delta. However, these flood control
    measures have led to the worst coastal erosion and wetland loss in the United
    States.
•   Since the turn of the 20th century, the rate of coastal land loss has accelerated
    from 5 mi2/yr in the 1930s to 40 mi2/yr in the 1970s.
•   The current rate of loss is approximately 25 mi2/yr (Penland, 1995). Flood control
    has cut off the sediment supply to the Mississippi Delta, which formerly
    counteracted the subsidence.

•   Diversion control has stopped the delta from delta switching, the process by which
    the Mississippi River has built and maintained its delta plain, causing a net loss of
    land (Penland, 1995). The average amount of shoreline erosion is 10m/yr.
              Other Factors
• Canals: cause of bank erosion and incursion
  of saltwater into low salinity and brackish
  marshes
• Oil and Gas: fluid withdrawal and land
  collapse and subsidence
• Old pipelines: constructed since the 1930s
  – Not really monitored until 1960s.
Land Loss in the Mississippi
           Delta
                The Danube Delta
This delta
attracts many
                                                      The Danube
tourists who
                                                      is the second
enjoy
                                                      longest river
watching the
                                                      in Europe
delta’s birds
and other
inhabitants



  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.
Mekong River
Basin Maps
Tonle Sap
     • Annual flooding from the
       Mekong swells the lake from
       2500 km2 to over 13,000 km2
     • Fish spawning and migration
       cycle with the floods
     • 40% of Mekong fish catch is in
       Cambodia (10% in the lake
       alone)
     • 80% of animal protein
       consumed in Cambodia is from
       freshwater fish
           The Irrawaddy River Delta
Sediment                           mangrove
plumes                             forest


                                  Fertile
                                  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
                                                 European
                                                 settlements
                                                 in India
Both Rivers
are sacred
to the
Hindu
Religion
              Godavari and Krishna River Delta

				
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posted:3/17/2011
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