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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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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
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 Physiography • Prodelta • Deep, most seaward; Muds • Bottomset beds, settle from suspension
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 Types • Wave-dominated • High wave energy • Dispersal of sediment away from mouth of river • Eg. Rhone River, in the North Sea
Delta Types • Tide-dominated • Tidal currents rework sediments into long linear bars • Fan out from river mouth • Eg. Ganges-Brahmaputra in Bangladesh
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)
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
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 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
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
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
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
Mangroves San Salvador, Bahamas
