Deltas

Deltas are dynamic sedimentary environments that undergo rapid changes over very short periods of time. Found in lakes and shallow ocean waters, they are rich in organic material and provide food and shelter for fish and wildlife.

Principal Terms

crevasse: a break in the bank of a distributary channel causing a partial diversion of flow and sediment into an interdistributary bay

delta: a deposit of sediment, often triangular, formed at a river mouth where the wave action is low and the river’s current slows suddenly

distributary channel: a river that is divided into several smaller channels, thus distributing its flow and sediment load

geoarchaeology: the technique of using ancient human habitation sites to determine the age of landforms and when changes occurred

interdistributary bay: a shallow, triangular bay between two distributary channels; over time, the bay becomes filled with sediment and colonized with marsh plants or trees

natural levee: a low ridge deposited on the flanks of a river during a flood stage

prodelta: a sedimentary layer composed of silt and clay deposited under water; it is the foundation on which a delta is deposited

sediment: fragmented rock material such as gravel, sand, silt, or clay that is deposited by a river to form a delta

wave energy: the capacity of a wave to erode and deposit; as wave energy increases, erosion increases

Formation and Shape

Deltas contain many valuable resources. Government agencies such as the U.S. Fish and Wildlife Service study the surface properties of deltas because of the enormous wetlands and abundant wildlife that occupy these landforms. Geologists study deltas because they are favored places for the accumulation of oil and gas resources. This low topographical feature serves society in many ways, and that is why it has been the object of intense study.

Deltas are deposits of sediments, such as sand or silt, that are carried by rivers and deposited at the shoreline of a lake, estuary, or sea. As the river meets the water body, its velocity is greatly decreased, which reduces its ability to support the sediment load that it has been carrying. As a result, the river sediment becomes deposited. If the accumulated sediment is not removed by waves or currents, a delta will accumulate and continue to extend itself into the lake or ocean. The term “delta” is used to describe this depositional landform because it is often triangular. It is believed that the Greek historian Herodotus coined the term with the shape of the capital Greek letter delta in mind. Herodotus visited Egypt, where the Nile Delta is located, and he correctly defined the shape of that delta. Not all deltas are triangular, however.

The Ganges Delta, the Colorado River Delta, the Mississippi River Delta, and many other deltas have different shapes. The shape postulated by Herodotus is, in fact, somewhat unusual, but it is applicable to the Nile River Delta. The Nile Delta has a smooth but curved shoreline –and so is designated an “arcuate” delta, whereas the Mississippi Delta has been extended into the Gulf of Mexico and has spreading channels resembling the digits of a bird’s foot. Current and wave action at the shoreline can cause sediment to be distributed to the left and right of a river channel, forming smooth beaches on either side. Such a delta acquires a shape more like a cone, the point of which projects toward the sea, and is called a “cuspate” delta. The Tiber River, which empties into the Mediterranean Sea, is a classic example of this type of delta. Rivers such as the Seine, in France, may deposit sediment in elongated estuaries, forming shoals and tidal flats.

Earth scientists have noted that the shapes of deltas are associated with several conditions, such as the character of river flow, the magnitude of wave energy and tides, and the geologic setting. The bird-foot delta of the Mississippi River has extended itself well into the Gulf of Mexico because the river carries and deposits a high volume of sediment on a shallow continental shelf. The wave and tidal forces are low, and the delta deposit is not redistributed along the shoreline or swept away. Conversely, a cuspate delta, such as that of the Tiber, is a product of strong waves moving over a steep continental shelf. Persistent high wave energy redistributes the sediment often, forming beaches and dunes along the delta shoreline. The Nile Delta is an arcuate delta characterized by moderately high wave energy and a modest tide range. Occasional high wave conditions deposit beaches and dunes along the arc-shaped delta front of the Mediterranean Sea. Tides also play a direct role in the creation of deltas. Deltas in estuaries are formed because of high tidal ranges coupled with low wave conditions. The Seine estuary, with its distinctive mud flats exposed at low tide, provides a good example.

Landforms

Although deltas have different shapes that reflect differences in the intensity of river, wave, current, and tidal processes, certain landforms may be identified as characteristic of delta formation. Submarine features are deposited below sea level, and subaerial features form at or just above sea level. As a river empties into the sea, the finest sediments, usually very fine silt or clay, are deposited offshore on the sea floor. This submarine deposit forms the foundation on which the delta sits and is appropriately referred to as a “prodelta deposit.” The deposit can often be detected on navigation maps as a relatively shallow, semicircular deposit under the water.

As deposition continues, the prodelta deposit is covered with the extending subaerial delta, which is composed of coarser sediments. Deltaic extension occurs along the distributary channels. During higher river flow, the distributary channels overflow, depositing natural levees along their sides. The digitate distributary pattern of the Mississippi Delta illustrates this process well. As the distributaries extend to deeper water, the shallow areas between the distributaries are better developed. These areas, known as interdistributary bays, are shallow landforms colonized by aquatic plant life. Over time, deposition occurs in the interdistributary bays through breaches in the natural levees. As the river mouth distributaries enter a flood stage, the lower regions of a natural levee are broken, and fine suspended sediments are introduced into the interdistributary bay area. Such overbank splays, or crevasse splays, are primarily responsible for the infilling of a delta. The crevassing is usually a very rapid but short-lived process, occurring during a high river stage and operating over ten to fifteen years. With the passage of time, the open-water interdistributary bays become silt-filled and colonized. Eventually, however, the marshy bays may subside due to compaction of the sediment, creating water areas once again.

Geologic History

Although the geologic histories of large deltas, such as the Mississippi, are complex, the succession and behavior of shifting deltas have been determined in some detail. Over the past twenty thousand years, the large continental glaciers that occupied much of the upper midwestern United States began to melt. As Earth’s climate continued to warm, the meltwater was returned to the oceans, and the sea level rose some 100 meters (328 feet), inundating valleys that had previously been cut by streams. The oceans reached their present approximate level about five thousand years ago. The Mississippi and similar valleys were flooded and became elongated bays. Over time, the shallow water bays were choked with sediments that formed broad floodplains extending down the valleys. Once a depositing river extended beyond the confines of its valley, a delta was deposited in deeper water. Because the river was no longer confined, it was free to shift over greater distances. The Mississippi River Delta is actually composed of seven distinct delta lobes extending over an approximate distance of 315 kilometers (196 miles). The oldest delta, Salé Cypremort, was deposited some 4,600 years ago; the most modern delta was deposited within the past 550 years. Older Mississippi Delta lobes, such as the Teche Delta, have subsided since they were deposited, giving an opportunity for a more recent delta (in this case, the Lafourche) to be deposited on top and more seaward of the older feature. The different delta lobes making up the enormous deltaic plain have resulted from a shifting of the Mississippi River well upstream in its valley. This process may be visualized as a hand movement occurring because of a shoulder movement.

Because of significant changes in the shoreline environments, many deltas in marine coastal zones are eroding. The Mississippi River is at the edge of the continental shelf and cannot build out into deep water. Also, subsidence and rising sea levels are causing the delta to erode. The changes in the Mississippi delta have been so severe that the shape of coastlines, like those in Louisiana, are changing. The Nile Delta is eroding as well. With the construction of the High Aswan Dam upstream, there has been a significant decrease in the sediment supplied to the Nile Delta. This lack of sediment, along with rising sea levels, has led to erosion. Some earth scientists have suggested that the wave action in coastal Egypt is cutting back the Nile Delta at a rate of 15 to 30 meters (49 to 98 feet) per year in some areas.

Study of Deltas

Deltas are difficult to study because many of the features are very flat, marshy, or under water. Because deltas change rapidly over time, however, maps and aerial photographs are important tools with which to determine changes. Navigation maps and maps that illustrate the topography of coastal areas around the world have been made for generations. By comparing the size and location of a delta on old maps and new maps, changes can be analyzed. Also, aerial photographs and pictures taken from satellites aid in identifying the erosion and deposition of delta landforms.

Often, older delta lobes were settled by ancient peoples. Through the science of geoarchaeology, it can be determined when changes occurred. As deltas, such as the Mississippi, shift from side to side over time, the human population follows the deltas from place to place. By examining the location of archaeological sites over the past fifteen hundred years, scientists have determined the minimum age of the several deltas forming the Mississippi deltaic plain. The Indian pottery found there reveals that the delta framework was deposited very recently. Cultural remains indicate other environmental changes—in salinity, subsidence, and delta deterioration.

By boring holes into the soft sediments of a delta, geologists can decipher its subsurface aspects. Because oil and natural gas are often associated with deltas, oil companies have bored holes in many delta landforms. Information derived from this method of study reveals the composition of the thick delta sediments and the rate of delta accumulation. In fact, boreholes in some deltas have indicated that older deltas once existed and are now buried beneath younger deltas.

Finally, deltas can be created in the laboratory. In nature, deltas are very large and complex. To make the study of deltas less difficult, scientists use tanks filled with water and sediments. Experiments can be performed that, for example, control the amount of sediment used to build deltas. Relationships between sediments and current velocities may be studied to gather information on such properties as the rate of delta growth. By controlling the phenomena that cause deltas to form, geologists can gain an overview of the behavior and processes of delta development.

Significance

Deltas, with their marshes, bogs, wetlands, and swamps, are not generally aesthetically pleasing; however, depositional landforms have been useful to prehistoric and historic populations in many ways. Deltas, along with estuaries, are among the most biologically productive areas on Earth. Most deltas are colonized with wetland swamps or marshes, which are breeding areas for wildlife. In marine deltas where there is tidal influence, freshwater and saltwater mix. The river brings oxygen and nutritive substances into the delta, and the result is an enormous production of sea life. High biological productivity attracted humans to this land feature. Deltas have often been centers of civilization; the deltas of the Nile, in Egypt, and the Tigris-Euphrates, at the head of the Persian Gulf, have supported important societies. Soils in delta regions are nourished through seasonal flooding, and water tables are high, guaranteeing adequate water with which to irrigate crops, even in the dry season. Food and crop production from tropical deltas is significant because most tropical soils are not very productive. Deltas such as those of the Mekong and the Ganges are outstanding examples of how important these areas can be for communities.

Deltas are transition zones between the land and the sea and between river and marine processes. Their rivers are also links between ocean and continent. Cities such as New Orleans, Venice, Amsterdam, and Rotterdam owe their prosperity to their delta geography. Such cities, known as entrepôts, thrive on marine traffic entering a country or on overland traffic exiting the country. However, because of climate change, these areas are especially suseptable to rising sea levels and must be monitored accordingly. As of 2021, 500 milion people worldwide live in delta regions, a number that is expected to rise. Because of this, climate and environmental organizations are working to meet the changing landscapes in these areas to keep people safe and the land prosperous.

Because deltas are areas of vast accumulations of sediments, they generate building material for future mountains. The young mountains of the world, such as the Alps and the Himalayas, parallel coastal areas and are composed of sedimentary rocks. Marine fossils frequently found in such rocks reveal that they not only are composed of sediment but also were once deposited under water, later to be thrust upward to great heights.

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