The mouth of a river is where it meets an ocean, a lake or another river. If a river carries a great deal of silt, gravel, clay and sediment as it travels, and this settles out at its mouth, that area of land is called a delta. The word “delta” comes from the Greek letter, which looks like a triangle. As such, a river delta typically makes up a triangle-shaped piece of land.

Deltas are located at river mouths. They usually exist at the mouth of a river entering an ocean. However, deltas can also be found where rivers meet a lake. While less common, sometimes deltas occur inland. Deltas can be found all over the world. However, not every river forms a delta. Fast-moving rivers do not tend to form deltas.

Deltas can be found at the mouths of rivers that carry large amounts of sediment. This sediment is called alluvium. Typically the river will slow down at this point. The delta takes the shape of a triangle or fan where the alluvium enters the ocean or lake. This fan is also called a deltaic lobe. The part of the delta that is underwater is called subaqueous, whereas the part above water is called subaerial.

River deltas can come in different types, depending on the amount of sediment they carries with it and the type of water they empty into. Shape also determines delta type.

One type of river delta is the wave-dominated delta, which is affected by strong waves. An example of this is the Nile River, which is strongly influenced by the Mediterranean Sea. Waves from the Mediterranean are stronger than the river’s force, which pushes sediment in such a way that the coast is smooth. The Nile River delta can also be classified as an arcuate delta, which is a triangular-shaped delta.

If a river delta is greatly affected by tides, that is called a tide-dominated delta. River deltas found in freshwater lakes are called Gilbert deltas.

When rivers do not fully empty into the ocean and form estuaries instead, those wetland environments are considered estuarine deltas.

A cuspate delta is a tooth-shaped delta.

Bird-foot deltas are deltas that have smaller portions of a river broken apart, called distributaries. They are so named because they resemble a bird foot. Bird-foot deltas get their shape from the force of the river being greater than that of ocean waves, so that alluvium deposits more quickly.

Inverted deltas occur when only one river reaches a sea or bay, but upstream several distributary rivers exist.

Inland deltas are far less common, and occur when a river empties onto dry land.

The largest river delta in the world is at the mouth of the Ganges River, straddling the boundaries of India and Bangladesh at the Bay of Bengal. The Ganges-Brahmaputra-Meghna Delta is a tide-dominated delta that spans an enrmous 220 miles, and also includes several other rivers.

The Mississippi River Delta is probably the most famous river delta in the United States. It is a bird-foot delta. The Mississippi River Delta hosts millions of people while also supporting a unique ecosystem. This delta is comprised of wetlands, barrier islands and estuaries that provide homes for many kinds of plants and animals.

Also in the United States, the Sacramento-San Joaquin River is an example of an inverted delta.

The Fraser River in Vancouver, Canada, the Rhine-Meuse-Scheldt Delta in the Netherlands and Pearl River Delta in China all coincide with large populations of people. The Yellow River in northern China is an example of an estuarine delta, and the Tiber River is an example of a cuspate delta located in Italy.

Far northern Lena Delta in Russia and the Yukon and Mackenzie Deltas in Canada are especially susceptible to increasing temperatures in the Arctic.

The Amazon and Columbia Rivers do not form true deltas because they meet strong waves at their mouths, preventing the settlement of alluvium needed.

In Botswana, the Okavango Delta represents an inland delta that flows into the Kalahari Desert.

River deltas do not cover much actual land area. However, they tend to host many resources that both people and animals rely on; some river delta areas host millions of people. River deltas provide food, ports and transportation for many countries.

River deltas are some of the most biologically productive places in the world. The soil tends to be rich, and plants flourish there. River deltas support fish nurseries, fisheries, crustaceans, forests and crops such as tea and rice. Ecosystems like wetlands and mangrove forests depend on a stable river delta. Animals like birds, insects and sometimes even large predators rely on the ecosystem of a delta.

River deltas are one of nature’s cleanup crews. The deltas work to absorb storm and flood runoff as well as to clean water traveling through them.

Changes in the environment and climate can alter river deltas. When channels are dug into rivers, or wetlands are developed, greater amounts of erosion occur. This means more sediment will wash out to sea much faster, rather than slowly filling in the delta land. Increased sea level and stronger hurricanes also threaten the sensitive delta areas. More water can inundate further inland when deltas begin to sink.

Some river deltas experience overdevelopment by people or restriction by water management such as dams. This causes ecosystems to undergo drastic change in relatively short periods of time. For example, the Colorado River was dammed in the 20th century, preventing it from flowing into the Sea of Cortez in Mexico. Many species that once lived in the Colorado River Delta suffered from complete disappearance of their original habitat. In the case of the Nile River Delta, overmanagement combined with Mediterranean waves have led to rapid erosion of the delta that cannot be quickly restored.

The Mississippi River alone experiences an alarming amount of erosion today. The land loss of the Mississippi is simply occurring at a much faster rate than it did before the industrial age, and any rising seas will yield a profound effect on the Mississippi River Delta. The degradation of the Mississippi River Delta results from river dredging for canals, less sediment being deposited, inundation by saltwater and erosion by waves from the Gulf of Mexico. The delta has also been altered by the construction of dams and levees along its course to prevent flooding of communities. Engineers have proposed creating new delta land over the next several decades by engineering river diversions.

Restoring deltas takes considerable amounts of time and money. Even with coastal restoration attempts, deltas around the world may not be able to withstand such rapid change and population growth. Nevertheless, scientists are working hard to find ways to protect river deltas while keeping their surrounding populations sustainable. New methods are being used to predict how river deltas change shape by measuring ratios of river flow rate, sediment fluctuation and ocean wave influence. When engineers know these kinds of data, they can understand the effects of manmade structures such as dams and levees on river deltas. The future of river delta regions depends on more research and funding to keep these important areas healthy and productive for the future.