deltas and estuaries n.
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Deltas and Estuaries. Deltas. From the Greek letter, based on the shape of the Nile River delta Occur where river supplies a large enough amount of sediment Empties directly to sea (not into estuary; estuary is filled in)

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  • From the Greek letter, based on the shape of the Nile River delta
  • Occur where river supplies a large enough amount of sediment
  • Empties directly to sea (not into estuary; estuary is filled in)
  • Delta forms if sediment supply is greater than erosion by waves and tides
delta physiography
Delta Physiography
  • Delta Plain
    • Flat lowland above sea level, active and abandoned distributary channels (sands)
    • Interdistributary vegetated or flooded (muds)
    • Topset beds
  • Delta Front
    • Shoreline and broad submerged portion of delta
    • Slopes gently seaward
    • Sandy muds
    • Foreset beds
delta physiography1
Delta Physiography
  • Prodelta
    • Deep, most seaward; Muds
    • Bottomset beds, settle from suspension
delta types
Delta Types
  • Function of waves, tides and sediment supply
  • River-dominated delta
    • Waves and tides have little influence
    • Sediments deposited at river mouth
    • Classic example is Mississippi River delta
delta types1
Delta Types
  • Wave-dominated
    • High wave energy
    • Dispersal of sediment away from mouth of river
    • Eg. Rhone River, in the North Sea
delta types2
Delta Types
  • Tide-dominated
    • Tidal currents rework sediments into long linear bars
    • Fan out from river mouth
    • Eg. Ganges-Brahmaputra in Bangladesh
  • Importance
    • Natural navigable harbor
    • Buffer zone between marine and freshwater environments; Important biologically
    • Heavily impacted by humans
  • Definition
    • ‘estuary’ from latin aestuarium which means tidal
    • Semi-enclosed body of water with inflow of both salt and fresh water (contrast with lagoon, which is just salt water)
  • Estuaries can be divided into three parts:
    • Head- where river enters estuary, eg Hillsborough Bay
    • Main estuary
    • Mouth- seaward end of estuary, eg Egmont Key
origin and evolution
Origin and Evolution
  • Young
    • Formed during the last sea level rise
    • Generally short-lived (geologically)
      • The fill in
      • River sediment input
      • Import of sand and mud from offshore
      • Degree of infilling is a function of sea level change and sedimentation rate
origin and evolution1
Origin and Evolution
  • 4 types
    • drowned river valleys (coastal plain estuary)
    • fjord, glacially-carved river valley that is drowned.
      • Often have a glacial moraine at mouth (sill).
      • May create anoxic conditions in deep fjord due to lack of circulation
    • bar-built estuary, created by barrier at estuary mouth
    • tectonic estuary, created due to subsidence along fault,
      • eg Tomales Bay, San Francisco
  • Based on physical oceanographic characteristics (circulation)
    • These controlled by freshwater inputs, tides and winds
  • Salt-wedge estuaries (stratified estuaries)
    • River-dominated, weak marine inflow due to small tide
    • Vertical salinity stratification, salt below, fresh above
  • Fully-mixed estuaries
    • Tide-dominated
    • Well-mixed, vertically homogenous, OR
    • Lateral salinity gradient, incoming saltwater, outgoing fresh
    • Generally wider than 0.5 km
  • Partially-mixed estuary
    • Characteristics of both
    • Vertical salinity gradient, salt-brackish-fresh
estuarine circulation
Estuarine Circulation
  • The Coriolis effect and estuarine circulation. (counterclockwise circulation in the northern hemisphere) explain
  • May effect sedimentation
  • Marine sediments on one side, river sediments on other

San Salvador, Bahamas