Sign In or Create an Account. User Tools. Sign In. Advanced Search. Skip Nav Destination Article Navigation. Close mobile search navigation Article navigation. Volume 49, Number Previous Article Next Article. Article Navigation. Research Article October 01, Tanner William F. Florida State University, Tallahassee, Florida. Vincent Island, about kilometers southwest of Tallahassee, Florida. Image of the Day Land Water. The Ebro River Delta, located along the eastern coast of Spain, is one of the largest wetland areas in the western Mediterranean region.
EO Explorer. Click to view larger and see the legend. Delta sediments are shown as bluish fingers on the geological map of Indonesia. Can you find the delta at the eastern edge of the map? Hint: The delta looks like lots of short fat fingers poking out into the ocean.
Static map. Deltas have different shapes depending on how much sediment is deposited by the river compared to how much sediment is eroded and redeposited by waves and tides. A river dominated delta, such as the map below of the Mississippi delta, Louisiana, USA, is sometimes called a bird's foot delta because of its shape. Long, narrow mounds of sediment called levees build up along the sides of the narrow distributary channels.
The Fly, on the other hand, does form a rich delta as it empties into the Gulf of Papua, part of the Pacific Ocean. A river will also not form a delta if exposed to powerful waves. The Columbia River in Canada and the United States, for instance, deposits enormous amounts of sediment into the Pacific Ocean, but strong waves and currents sweep the material away as soon as it is deposited.
Tides also limit where deltas can form. The Amazon, the largest river in the world, is without a delta. The tides of the Atlantic Ocean are too strong to allow silt to create a delta on the Amazon. There are two major ways of classifying deltas.
One considers the influences that create the landform , while the other considers its shape. There are four main types of deltas classified by the processes that control the build-up of silt: wave -dominated, tide -dominated, Gilbert deltas, and estuarine deltas.
In a wave-dominated delta, the movement of waves controls a delta's size and shape. The Nile delta shaped by waves from the Mediterranean Sea and Senegal delta shaped by waves from the Atlantic Ocean are both wave-dominated deltas. Tide-dominated deltas usually form in areas with a large tidal range , or area between high tide and low tide. The massive Ganges-Brahmaputra delta, in India and Bangladesh, is a tide-dominated delta, shaped by the rise and fall of tides in the Bay of Bengal.
Gilbert deltas are formed as rivers deposit large, coarse sediments. Gilbert deltas are usually confined to rivers emptying into freshwater lakes. They are usually steeper than the normal flat plain of a wave-dominated or tide-dominated delta. This type of delta was first identified by the geologist Grove Karl Gilbert, who described mountain streams feeding ancient Lake Bonneville.
Estuarine deltas form as a river does not empty directly into the ocean, but instead forms an estuary. An estuary is a partly enclosed wetland that features a brackish water part-saltwater, part-freshwater habitat. The Yellow River forms an estuary, for instance, as it reaches the Bohai Sea off the coast of northern China. Deltas with this triangular or fan shape are called arcuate arc-like deltas.
The Nile River forms an arcuate delta as it empties into the Mediterranean Sea. Stronger waves form a cuspate delta, which is more pointed than the arcuate delta, and is tooth-shaped. Not all deltas are triangle-shaped. A bird-foot delta has few, widely spaced distributaries, making it look like a bird's foot.
The Mississippi River forms a bird-foot delta as it empties into the Gulf of Mexico. Another untraditional looking delta is the inverted delta. The distributary network of an inverted delta is inland, while a single stream reaches the ocean or other body of water. Inland deltas, which empty into a plain, are extremely rare. The Okavango delta in Botswana is probably the most well-known—and so unusual it is recognized as one of the "Seven Natural Wonders of Africa.
The delta spreads water and silt across a flat plain in the Kalahari Desert before being evaporated. An abandoned delta forms as a river develops a new channel, leaving the other to dry up or stagnate.
This process is called avulsion. Avulsion occurs when the slope of a channel decreases and the sediment build-up increases. These forces allow the channel to overflow its banks or levees and find a steeper, more direct route to the ocean or other body of water. The process of avulsion in deltaic lobes is called delta lobe switching. Over time, delta switching can create entirely new deltaic lobes.
Delta switching has resulted in seven or eight distinct deltaic lobes of the Mississippi River over, at least, the past 5, years.
Deltas are incredibly important to the human geography of a region. They are important places for trade and commerce , for instance. The Fraser delta helps make Vancouver one of the busiest, most cosmopolitan ports in the world, where goods from the interior of Canada are exported, and goods from around the world are imported.
The Pearl River Delta, sometimes called the Delta of Guangdong, is another heavily urbanized river delta. The Pearl River delta is one of the fastest-growing centers of China's economy.
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