Deltas are wetlands that form as rivers empty their water and sediment into another body of water. The Nile delta, created as it empties into the Mediterranean Sea, has a classic delta formation. The upper delta, influenced by the Nile’s flow, is the most inland portion of the landform. The wide, low-lying lower delta is more influenced by the waves and tides of the Mediterranean.
A delta is sometimes divided into two parts: subaqueous and subaerial. The subaqueous part of a delta is underwater. This is the most steeply sloping part of the delta, and contains the finest silt. The newest part of the subaqueous delta, furthest from the mouth of the river, is called the prodelta.
The subaerial part of a delta is above water. The subaerial region most influenced by waves and tides is called the lower delta. The region most influenced by the river’s flow is called the upper delta.
Like most wetlands, deltas are incredibly diverse and ecologically important ecosystems. Deltas absorb runoff from both floods (from rivers) and storms (from lakes or the ocean). Deltas also filter water as it slowly makes its way through the delta’s distributary network. This can reduce the impact of pollution flowing from upstream.
Deltas are also important wetland habitats. Plants such as lilies and hibiscus grow in deltas, as well as herbs such as wort, which are used in traditional medicines.
Many, many animals are indigenous to the shallow, shifting waters of a delta. Fish, crustaceans such as oysters, birds, insects, and even apex predators such as tigers and bears can be part of a delta’s ecosystem.
Not all rivers form deltas. For a delta to form, the flow of a river must be slow and steady enough for silt to be deposited and build up. The Ok Tedi, in Papua New Guinea is one of the fastest-flowing rivers in the world. This river becomes a tributary of the Fly River. (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.
Types of Deltas
There are two major ways of classifying deltas. One considers the influences that create the landform, while the other considers its shape.
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. (Utah’s Great Salt Lake is the only remnant of 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.
The term delta comes from the upper-case Greek letter delta (Δ), which is shaped like a triangle. 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. The Tiber River forms a cuspate delta as it empties into the Tyrrhenian Sea near Rome, Italy.
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. The delta of the Sacramento-San Joaquin River in northern California is an inverted delta. The rivers and creeks of the Sacramento and San Joaquin distributary networks meet in Suisun Bay, before flowing to the Pacific Ocean through a single gap in the Coast Range, the Carquinez Strait.
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.” Water from the Okavango River never reaches another body of water. 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 5,000 years.