Depositional Fluvial Landforms

Delta’s

A delta is a feature of deposition formed when a river flows into a standing body of water

Deposition occurs as the velocity drops on entering the standing body of water, this reducing the capacity and competence of the river bedload and suspended material are ‘dumped’

Delta’s

Delta’s form 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

Delta’s

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

Flocculation occurs as fresh water mixes with salt water - e.g where a river flows into the sea -(then called brackish water), and is the process by which tiny particles in suspension like clay and silt (which under normal fresh water conditions would likely never fall out of suspension) coagulate with the salt in the water by chemical reaction and become heavy enough to ‘sink’ and be deposited

Flocculation

Deltas only form where rate of deposition exceeds rate of sediment removal – i.e. Where the critical threshold is no longer met and the system has fallen out of dynamic equilibrium. 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

Typical conditions for a Delta

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

Composition of Delta’s

Composition of Delta’s

Delta’s can be categorised into three main shapes

Arcuate

Birds Foot

Cuspate

Types of Delta

•The most common

shape of delta

•Characteristics:

curving shoreline

(smoothed by long

shore drift), distinct

pattern of drainage

(branching of

distributaries), and

typically more

‘gravely’ deposits .

•Example: the Nile

Delta

Arcuate Delta’s

•‘Fingers’ of

deposition build

out into the sea

along the

distributaries

channels giving

an appearance

like a birds claw.

Typically with a

finer sediment

•Example: the

Mississippi delta

Birds Foot Delta

•A cuspate delta

is shaped like a

tooth by gentle

but regular

opposing

currents in the

water body the

river flows into,

or longshore

drift

•Example: The

river Ebro delta

in Spain

Cuspate Delta

Web:Image [slide 3]: http://

upload.wikimedia.org/wikipedia/commons/thumb/e/e7/NarsarsuaqGreenland-ArcuateDelta.jpg/380px-NarsarsuaqGreenland-ArcuateDelta.jpg

Image [slide 10] : http://strata.geol.sc.edu/MARINESEDIMENTS/delta/Nile.jpg

Image [slide 11]: http://www.es.ucsc.edu/~es10/classnotes/images/MississippiDelta.drawn.jpg

Image [slide 12]: http://www.iberianature.com/material/photos/google_earth_spain_2/images/delta_de_ebro.jpg

Info: http://wiki.answers.com/Q/Examples_of_cuspate_deltas

Books: AQA text book, Oxford dictionary of Geography

Bibliography