river deltas

7/28/2019 River Deltas

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River Delta


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What is a River Delta?

A delta is a landform that isformed at the mouth of

a river where that river flowsinto an ocean, sea, estuary,

lake, reservoir, flat arid area,or another river.


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How are River Deltas Formed?

Deltas are formed from thedeposition of the sedimentcarried by the river as the flowleaves the mouth of the river.

Over long periods of time, this

deposition builds thecharacteristic geographicpattern of a river delta.


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Anatomy of aRiver Delta

Subaqueous delta -That part of the deltathat is below the low-

tide mark.Seaward, finer andfiner soil particles arefound.The seaward-mostportion has the finestsoils (clays) and is

called the prodelta.


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II. Subaerial delta - Thatpart above the low-tide mark.

A. Lower delta plain -That part governed byriverine/marineinteraction. Extends

landward to the limitof tidal influence. Allsalt and brackish floraand fauna are withinthis area.

B. Upper delta plain -That part governed byriverine depositionalprocesses.


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On the same figure, one can see thefollowing two additional categories:

1. Active delta - Thatpart that is stillactive and has water

channeling throughit carrying sediment.

2. Abandoned (orinactive) delta - That

part that was onceactive, but now hasreduced or no fluvial

activity.


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What are the nutrients in a RiverDelta?

Many nutrients are present in a River Delta,including many sedimentry mineral elements.The main ones are Nitrogen, Phosphorous andSulphur, which are all responsible for plant

growth. Others such as Carbon and hyddrogen are

present in chemical bonds to form

carbohydrates in plants and animals andproteins.

But mainly, geologically, Silicon in the form ofsand and sandstone. Calcium in limestone and

chalk.


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Arcuate delta Has many active, short distributaries

taking sediment to their mouths.

The receiving waters are rather

shallow and have relatively evenwave action arriving perpendicular tothe shore with minimal longshore

current. As the sediment exits the many

distributary mouths, the waves push it

back, so the coastline is rather smooth.


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Bird-foot delta Tend to have one or a very few

major distributaries near theirmouths.

The receiving basin has currents thatcarry the sediment away as it exits

the distributary mouth. There is a broad, shallow shelf that

deepens abruptly, so the trend is to

grow long and thin like a bird's toe.


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Cuspate delta Usually has one distributary

emptying into a flat coastline with

wave action hitting it head-on. This tends to push the sediment

back on both sides of the mouth,

with a "tooth" growing out ontothe shelf.


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Estuarine delta

This type of delta has a river thatempties into a long, narrowestuary that eventually becomesfilled with sediment.


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Ecological Threats to Deltas Human activities, including diversion of

water and the creationof dams for hydroelectric power or tocreate reservoirs can radically alter deltaecosystems.

Dams block sedimentation which can causethe delta to erode away.

The use of water upstream can greatly

increase salinity levels as less fresh waterflows to meet the salty ocean water.

While nearly all deltas have been impacted

to some degree by humans

river deltas

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