Further Reading

Hatcher, Robert D. Structural Geology, Principles, Concepts, and Problems. 2nd ed. Englewood Cliffs, N.J.: Prentice Hall, 1995. van der Pluijm, Ben A., and Stephen Marshak. Earth Structure: An Introduction to Structural Geology and Tectonics. Boston: WCB-McGraw Hill, 1997.

deltas Found at the mouths of streams and rivers, deltas are low, flat deposits of alluvium that form broad triangular or irregular-shaped areas that extend into bays, oceans, or lakes. They are typically crossed by many distributaries from the main river and may extend for a considerable distance underwater. Deltas are extremely sensitive coastal environments and are particularly susceptible to the effects of rising sea level and human activities. Since deltas are the sites of rich oil deposits, there is currently a sensitive interplay between meeting the world's energy needs by extracting oil from beneath the fragile delta environment and the environmental concerns about preserving the delta ecosystem.

The velocity of the water and capacity of a river or stream to hold sediment in suspension suddenly drop when it enters the relatively still body of water such as a lake or the ocean. Thus the stream dumps its sediment load here, and the resulting deposit is known as a delta. The term delta was first used for these deposits by Herodotus in the fifth century b.c.e. for the triangular-shaped alluvial deposits at the mouth of the Nile River. The stream first drops the coarsest material, then progressively finer material farther out, forming a distinctive sedimentary deposit. In a study of several small deltas in ancient Lake Bonneville in Utah, Idaho, and Nevada, American geologist Grover Karl Gilbert in 1890 recognized that the deposition of finer-grained material farther away from the shoreline also created a distinctive vertical sequence in delta deposits. The resulting foreset layer is thus graded from coarse nearshore to fine offshore. The bottomset layer consists of the finest material, deposited far out. As this material continues to build outward, the stream must extend its length and forms new deposits, known as topset layers, on top of all this. Topset beds may include a variety of subenvironments, both subaqueous and subaerial, formed as the delta progrades seaward.

Most of the world's large rivers, such as the Mississippi, the Nile, and the Ganges, have built enormous deltas, yet all of these are different in detail. Deltas may have various shapes and sizes or may even be completely removed, depending on the relative amounts of sediment deposited by the stream, the erosive power of waves and tides, the climate, and the tectonic stability of the coastal region. Most deltas are located along passive or trailing continental margins, and few are found along convergent boundaries (exceptions include the Copper River in Alaska and the Fraser River in British Columbia). This is largely because river systems on passive margins tend to be long and to drain huge areas composed of easily eroded soil, carrying large sediment loads. Rivers along active margins tend to be much shorter and cut through bedrock, which is not eroded as easily so yields smaller sediment loads. Additionally, convergent margins do not contain wide continental shelves needed for the delta to be deposited on, but instead are marked by deep-sea trenches where sediments are rapidly deformed and buried.

Most deltas are quite young, having formed since the glaciers melted 18,000-10,000 years ago and sea levels rose onto the continental shelves. During the last glacial maximum when glaciers were abundant for much of the period from 2.5 million years ago until about 18,000 years ago, sea levels were about 395 feet (120 m) lower than at present. During the glacial maximum, most rivers eroded canyons across the continental shelves and carried their sedimentary

Plate 120 Delta

False-color composite of Mississippi River delta from ASTER instrument on NASA's Terra satellite, May 24, 2001 (USGS EROS Data Center Satellite Systems Branch as part of "Earth as Art II" image series, NASA)

Mississippi

Mississippi

Sediment input

Sediment input

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Wave energy flux

Wave energy flux

Säo Francisco

Copper

Tidal energy flux

Tidal energy flux

Säo Francisco

Copper

6 Infobase Publishing

Diagram illustrating different types of deltas formed as a result of different relative influence of sediment supply, tidal energy flux, and wave energy flux. The shapes of deltas characteristic of each are shown on the edges of the diagram, and names of other deltas are plotted in the space inside the diagram, in positions that reflect the relative strength of each component for each delta.

load to the deep oceans. As the glaciers melted, sea level rose onto the broad continental shelves of many continents, where wide and thick delta sediments have space to accumulate. Nearly all of the active parts of deltas are younger than 18,000 years, but many have older, deeper parts that formed during older sea-level high stands (some from interglacial periods) that have subsided deep below sea level. As sea levels were initially rising fast as the glaciers were undergoing rapid melting, the river mouths were moving so rapidly inland that deltas did not have time to form. The rate of sea-level rise slowed significantly around 6,000 years ago, and most of the world's deltas began to grow significantly since that time. This history of sedimentation is reflected in the Mississippi River delta, which has components that are older than several million years, but the active lobes only began forming about 6,000 years ago.

Deltas exhibit a range in conditions and environments from terrestrial and river-dominated at their landward boundaries to marine and wave and tide-dominated at their fronts. The mere presence of a delta along a coast indicates that the amount of sediment input by the river is greater than the amount of sediment that can be removed by the action of waves, tides, currents, wind, and submarine slumping. The distributaries and main channel of the rivers forming deltas typically move to find the shortest route to the sea, and this causes shifting of the active locus of deposition on deltas. Inactive areas, which may form lobes or just parts of the delta, typically subside and are reworked by tidal currents and waves. The processes involved in the growth or seaward progradation of deltas result in the formation of many environments, including those influenced by subaerial, intertidal, and subaqueous processes, and include freshwater, brackish, and saltwater conditions. Most deltas can be divided into three main parts: the landward delta plain, the delta front, and the prodelta in the subtidal to deep continental shelf environment.

The delta plain is really a coastal extension of the river system. It comprises river and overbank sedimentary deposits in a flat, meandering stream-type of setting. These environments are at or near (or in some cases below) sea level, and it is essential that the overbank regions receive repeated deposits of muds and silts during flood stages to build up the land surface continuously as the entire delta subsides below sea level by tectonic processes. Deltas deprived of this annual silt by the construction of levees gradually sink below sea level. If homes were built on delta flood plains without levees, however, they would gradually be buried in mud, as opposed to sinking below sea level behind the false protection of a levee.

False-color image of Lena delta in Russia acquired by Landsat 7's Enhanced Thematic Mapper plus sensor on February 27, 2000 (USGS EROS Data Center Satellite Systems Branch)

The stream channels are bordered by natural levee systems that may rise several feet (1-2 m) above the floodplain; these areas are often the only places above water level during river flood stages. In many outer delta plains the only places above sea level are the natural levees. During floods the levees sometimes break, creating a crevasse splay that allows water and muddy sediment to flow rapidly out of the channel and cover the overbank areas, plus any homes or other human infrastructure built in this sensitive area.

The delta front environment, located on the seaward edge of the delta, is an extremely sensitive environment. It is strongly affected by waves, tides, changing sea level, and changes in the flux or amount of sediment delivered to the delta front. Many delta fronts have an offshore sandbar, called a distributary mouth bar, or barrier island system, parallel to the coast along the delta front. Some deltas, such as the Mississippi, are losing huge areas of delta front to subsidence below sea level, due to combined effects of a decrease in sediment supply to the delta front, tectonic subsidence, sea-level rise, human activities such as oil drilling and building levees, and severe erosion from storms such as Hurricanes Hugo, Katrina, and Ike.

Deltas have been classified various ways over time, including by schemes based on their shapes and on the processes involved in their construction. High-constructive deltas form where the fluvial transport dominates the energy balance on the delta. These deltas dominated by riverine processes are typically elongate, such as the modern delta at the mouth of the Mississippi, which has the shape of a bird's foot, or they may be lobate, such as the older Holocene lobes of the Mississippi that have now largely subsided below sea level.

High-destructive deltas form where the tidal and wave energy is high and much of the fluvial sediment gets reworked before it is finally deposited. In wave-dominated, high-destructive deltas sediment typically accumulates as arcuate barriers near the mouth of the river. Examples of wave-dominated deltas include the Nile and the Rhône. In tide-dominated, high-destructive deltas, tides rework the sediment into linear bars that radiate from the mouth of the river, with sands on the outer part of the delta sheltering a lower-energy area of mud and silt deposition inland from the segmented bars. Examples of tide-dominated deltas include the Ganges, and the Kikari and Fly River deltas in the Gulf of Papua New Guinea. Other rivers drain into the sea in places where the tidal and wave current is so strong that these systems completely overwhelm the fluvial deposition, removing most of the delta. The Orinoco River in South America has had its sediment deposits transported southward along the South American coast, with no real delta formed at the mouth of the river.

Where a coarse sediment load of an alluvial fan dumps its load in a delta, the deposit is known as a fan-delta. Braid-deltas are formed when braided streams meet local base level and deposit their coarsegrained load.

Deltas create unique, diverse environments where fresh and saltwater ecosystems meet, and swamps, beaches, and shallow marine settings are highly varied. They contain some of the most productive ecological areas in the world. Deltas also form some of the world's greatest hydrocarbon fields, however, as the muds and carbonates make good source rocks and the sands make excellent trap rocks. Thus there is a delicate struggle between preserving natural ecosystems and using the planet's resources that must be maintained on the deltas of the world. Resting at sea level, delta environments are also the most susceptible to disaster from hurricanes and coastal storms.

See also basin, sedimentary basin; beaches and shorelines; sedimentary rock, sedimentation; subsidence.

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