Delta

Depositional Fluvial Landforms

A delta is a Depositional Fluvial Landform inhumid geomorphic region except for Nile delta ofEgypt which is in arid region.

A delta is a feature of deposition formed when a riverflows into a standing body of water.

Deposition occurs as the velocity drops on entering thestanding body of water, this reducing the capacity andcompetence of the river bed load and suspendedmaterial are ‘dumped’.

Delta forms when velocity drops as the river enters a standing body of water, typically this is at the coast as the river flows into the sea.

An example of a delta formed when a river flows into the sea in south west Greenland

Flocculation occurs as fresh water mixes with saltwater- e.g where a river flows into the sea -(then calledbrackish water), and is the process by which tinyparticles in suspension like clay and silt (whichunder normal fresh water conditions would likelynever fall out of suspension) coagulate with the salt inthe water by chemical reaction and become heavyenough to ‘sink’ and be deposited.

Delta is a site of deposition

Formation of a delta depends on the estuarine dynamics

It also depends on the velocity of river

In long term processes size & shape of a delta depends on rock characteristics, rate of erosion, rate of rainfall in the upper river basin and in the ways river flwos.

Singh, S, 2005

Deltas only form where rate of deposition exceeds rate of sediment removal. These conditions are likely where:

The calibre of the load of the river is very large (therefore requiring a high amount of energy to keep in transport).

The costal area into which the river dumps its load has a small tidal range and weak currents ; meaning there is limited wave action and therefore little subsequent transport of sediment taking place after its initial deposition allowing large deltas to form

Suitable place in the form of shallow sea or lake shores

Long courses of river; means long river system

Relatively calm seaSingh, S, 2005

Accelerated rate of erosion in the catchment area of the river

Large amount of sediment supply

Geologically inactive sea coast; means almost stable condition of sea bottom

Singh, S, 2005

Hyperpycual: The density of river water is greater than the sea water

Homopycual: The density of river water is same as the sea water

Hypopycual: The density of river water is less than the sea water

Delta is a very complex depositional system including three morphological process i.e. fluvial, terrestrial and marine environment.

Delta’s are usually composed of three types of deposit

Topset beds: The larger and heavier particles are the first to be deposited as the river looses energy

Foreset beds: Medium calibre particles travel a little further before they are deposited as more steep angled ‘wedges’ of sediment

Bottomset Beds: The very finest particles travel the furthest before very low velocity/flocculation leads to their deposition

Singh, S, 2005

Classification of delta by dominance Wave dominated delta –i.e. Nile delta, Egypt

Tide dominated delta –i.e. Ganges delta, Bangladesh, India

Estuarine dominated delta –i.e. Mackenzia, Hudson delta

Classification of delta by shape Arcuate –i.e. Ganges delta, Nile delta

Birds Foot –i.e. Mississippi delta

Cuspate –i.e. Ebro delta in Spain

Delta type:Arcuate deltaTide dominated delta

http://eol.jsc.nasa.gov/debrief/STS066/rep2.htm

Ganges Delta, Bangladesh

NASA satellite image in false color

Nile Delta, Egypt

Delta type:Wave dominated delta Arcuate delta

http://earthobservatory.nasa.gov/IOTD/view.php?id=50500

River Deltas, Lake Ayakum, Tibet

Delta type:Arcuate delta

Mississippi Delta

Delta type:Birds Foot

Jesse Allen, University of Maryland’s Global Land Cover Facility, andLANDSAT 1

Okavango Delta, Botswana

Delta type:Arcuate deltaTide dominated delta

http://www.flickr.com/people/8942661@N02

Colorado River Dry Delta

http://gallery.usgs.gov/photos/10_15_2010_rvm8Pdc55J_10_15_2010_0#.Ur0mcvfTnrd

Web: Image [slide 12]: http://eol.jsc.nasa.gov/debrief/STS066/rep2.htm

Image [slide 13] : NASA satellite image in false color

Image [slide 13]: http://earthobservatory.nasa.gov/IOTD/view.php?id=50500

Image [slide 14]: Jesse Allen, University of Maryland’s Global Land Cover Facility,

andLANDSAT 1

Image [slide 15]: http://www.flickr.com/people/8942661@N02

Image [slide 16]: http://gallery.usgs.gov/photos/10_15_2010_rvm8Pdc55J_10_15_2010_0#.Ur0mcvfTnrd

Info: www.wikipedia .com

www.slideshare.com

Books: Singh, S; Geomorphology, Prayag PustakBhawan, 2005

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