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Deltas and Estuaries

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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Deltas and Estuaries

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  1. Deltas and Estuaries

  2. 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) • Delta forms if sediment supply is greater than erosion by waves and tides

  3. 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

  4. Delta Physiography • Prodelta • Deep, most seaward; Muds • Bottomset beds, settle from suspension

  5. 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

  6. Mississippi River Delta

  7. Delta Types • Wave-dominated • High wave energy • Dispersal of sediment away from mouth of river • Eg. Rhone River, in the North Sea

  8. Wave-dominated Delta

  9. Delta Types • Tide-dominated • Tidal currents rework sediments into long linear bars • Fan out from river mouth • Eg. Ganges-Brahmaputra in Bangladesh

  10. Estuaries

  11. Estuaries • 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)

  12. Estuaries • 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

  13. 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

  14. 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

  15. Classification • 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

  16. Classification • 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

  17. Classification

  18. 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

  19. Mangroves San Salvador, Bahamas

  20. Man O’ War Key, Roatan, Honduras

  21. Salt Marsh, Big Bend Area, Florida

  22. Salt Marsh, Big Bend Area, Florida

  23. Salt Marsh, Big Bend Area, Florida

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