Deltas

1
Deltas
Mangoky River Delta
All images courtesy of NASA (http//earth.jsc.nasa
.gov)
2
The Nile Delta a Prototype

• Early geographers, reminded of the Greek letter
  delta when noting its roughly triangular shape,
  named it accordingly.

3
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

4
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

5
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)

6
The Mississippi Delta
Jetties have been built and dredging occurs to
allow large vessels to navigate the delta channel
Note the large amounts of sediment that surrounds
the delta
A birds foot delta located in Louisiana adjacent
to the Gulf of Mexico
7
The Atchafalaya Delta
This is one of the few areas where the Louisiana
coastline is growing
This delta is considered to be the Mississippis
largest distributary
The Atchafalaya diverts large quantities of water
from the Mississippi
8
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)

9
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

10
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

11
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

12
River vs. Tide Dominated Deltas
large mangrove forest (largest remaining habitat
of the Bengal tiger)
Mississippi Delta
Ganges Delta
Prone to devastating floods
13
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

14
Wave Dominated Delta
Wave action has smoothed the coastline
Rhone River Delta
15
Nile Delta
Well developed distributaries
Smooth delta front
A mixed or intermediate delta influenced by both
river and wave forces
16
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

17
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

18
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

19
Delta Cycle
Delta plain
Delta front
Pro-delta
20
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

21
The Aswan Dam and the Nile Delta
Aswan Dam (1971)
The dam provides flood control, increased
irrigable land area, and much needed
hydroelectric power (about 40 percent of the
countrys electricity).
Lake Nasser
The Aswan Dam has disrupted the Nile Deltas
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 Niles water.
22
The Aswan Dam and the Nile Delta
23
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.
24
People and the Mississippi
Sediment deposited by distributaries
The channel requires frequent dredging so that
large vessels can navigate the channel
Note the large sediment plumes
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.
25
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.

26
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.

27
Land Loss in the Mississippi Delta
28
(No Transcript)
29
The Danube Delta
This delta attracts many tourists who enjoy
watching the deltas birds and other inhabitants
The Danube is the second longest river in Europe
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.
30
Mekong River
31
Basin Maps
32
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

33
The Irrawaddy River Delta
mangrove forest
Sediment plumes
Fertile mud and sand have been deposited here
during the last 2 million years
This river delta in Burma is one of the worlds
great rice producing regions
34
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