River deltas

Rivers carry sand and mud as they flow.

Near the sea, the river slows down. The sand and mud sink and pile up. Slowly, they build new land. This land is called a delta.

Rivers are always carrying things: tiny bits of rock, sand and mud called sediment. When a river reaches the sea, a lake or another river, it slows down. Slow water cannot carry as much, so the sediment sinks and piles up. Over time it builds new, low land at the river's mouth. This is a delta.

Deltas get their name from the Greek letter delta, which is shaped like a triangle: Δ. Many deltas are fan- or triangle-shaped, wide at the sea and narrow where the river comes in. Deltas are wetlands, full of water channels, marshes and islands, and they are some of the richest places on Earth for wildlife and farming.

National Geographic defines deltas as wetlands that form as rivers empty their water and sediment into another body of water, such as an ocean, a lake or another river; very rarely, they empty onto land. A delta extends the river's mouth out into the water body it is filling.

The name comes from the upper-case Greek letter delta (Δ), and classic triangle- or fan-shaped deltas are called arcuate, meaning arc-like. Not every river has one. A delta needs a river that is slow and steady enough for silt to settle and build up, and a coast where waves, currents and tides do not sweep that silt away faster than it arrives. Where these conditions hold, deltas become some of the most productive landscapes on Earth: diverse wetland ecosystems, fertile farmland and busy centers of trade.

The river slows down near its end.

Big, heavy bits of sand drop first. Tiny bits of mud float farther out. The river splits into many small streams. Plants grow on the new mud and hold it in place.

As a river nears its mouth, it moves more slowly, and sediment falls to the bottom. The heavier, coarser bits, like sand, settle first. Finer bits are carried farther, and the finest mud, called silt, is dropped beyond the mouth. Silt is full of nutrients that help plants grow.

As sediment builds up, the river cannot stay in one channel. It breaks into smaller, shallower channels called distributaries, which spread out like the fingers of a hand. Part of a delta is underwater and part is above it. The part above water near the river is the upper delta; the part nearer the sea, shaped most by waves and tides, is the lower delta.

Slowing velocity near the mouth makes sediment drop out of suspension, and the build-up lets the river break from its single channel. Under the right conditions it forms a deltaic lobe with a mature distributary network: smaller, shallower channels branching off the mainstream. Sorting follows energy: coarse material settles first, finer sediment travels farther, and the finest, alluvium or silt, is laid down beyond the mouth. That nutrient-rich silt feeds microbes and plants, the producers of the delta's food web.

National Geographic divides a delta into two parts. The subaqueous delta lies underwater; it is the most steeply sloping part and holds the finest silt, and its newest, outermost part is the prodelta. The subaerial delta lies above water: the upper delta is shaped mostly by the river's flow, the lower delta mostly by waves and tides. NASA notes that on new Louisiana deltas, reeds and willows colonize the higher sandbars and their roots help hold the sediment in place.

Deltas come in different shapes.

The Nile's delta is shaped like a fan. The Mississippi's delta looks like a bird's foot! Waves and tides help shape each one.

Deltas are not all the same shape. The Nile in Egypt makes a classic fan, or arcuate, delta where it meets the Mediterranean Sea. Strong waves can shape a pointed, tooth-like cuspate delta, like the Tiber River's near Rome. The Mississippi has a bird-foot delta: a few long channels stretch far out into the sea, like a bird's toes.

The forces at the coast matter too. Waves from the Mediterranean shape the Nile delta. The giant Ganges-Brahmaputra delta in India and Bangladesh is shaped by the rise and fall of tides in the Bay of Bengal. And the Okavango Delta in Botswana is very unusual: it spreads out over a flat plain in the Kalahari Desert, and its water dries up there instead of reaching a sea.

Geographers classify deltas by shape and by the process that controls them. By shape: arcuate, fan-shaped like the Nile's; cuspate, pointed by strong waves, like the Tiber's on the Tyrrhenian Sea; bird-foot, with a few widely spaced distributaries, like the Mississippi's; and inverted, where the distributary network lies inland and a single channel reaches the sea, as in the Sacramento-San Joaquin delta, which drains to the Pacific through the Carquinez Strait.

By process: wave-dominated deltas (the Nile, the Senegal), tide-dominated deltas in places with a large tidal range (the Ganges-Brahmaputra, shaped by Bay of Bengal tides), Gilbert deltas of coarse sediment, usually where rivers enter freshwater lakes (named after geologist Grove Karl Gilbert, who described streams feeding ancient Lake Bonneville, whose remnant is Utah's Great Salt Lake), and estuarine deltas, such as the Yellow River's at the Bohai Sea. Inland deltas, which empty onto a plain, are extremely rare; the Okavango is the best-known, its water spreading across the Kalahari and evaporating.

Not every river makes a delta.

Some rivers are too fast. At some coasts, big waves wash the mud away. Strong tides can carry it away too.

Some rivers never build a delta. A delta needs a river that flows slowly and steadily enough for mud to settle. The Ok Tedi in Papua New Guinea is one of the fastest-flowing rivers in the world, and it does not make one.

Waves and tides can also stop a delta from growing. The Columbia River carries huge amounts of sediment into the Pacific Ocean, but strong waves and currents sweep it away as soon as it lands. National Geographic says the Amazon, the largest river in the world, has no delta because the Atlantic Ocean's tides are too strong for silt to pile up at its mouth.

Delta building is a contest between supply and removal. National Geographic gives three ways it can fail. The river may be too fast: the Ok Tedi in Papua New Guinea, one of the world's fastest-flowing rivers, forms none, although the Fly River it feeds builds a rich delta in the Gulf of Papua. Waves may win: the Columbia River delivers enormous amounts of sediment to the Pacific, but strong waves and currents carry it off as soon as it is deposited. Or tides may win: National Geographic describes the Amazon, the largest river in the world, as having no delta because Atlantic tides are too strong for silt to build one.

The same balance of supply and removal decides whether an existing delta grows or shrinks, as the story of the Mississippi delta shows.

Lots of animals live in deltas. Fish, crabs, oysters, birds and insects live there. Even tigers and bears can live in deltas!

Delta soil is good for farms. Many people grow rice and catch fish in deltas.

Deltas are wetlands, and like most wetlands they are full of life. Fish, oysters, birds and insects live in their shallow, shifting waters, and even big predators such as tigers and bears can be part of a delta's wildlife. Deltas soak up floodwater from rivers and storms, and they filter water as it trickles slowly through all the little channels.

Because their soil is rich, deltas are good for farming. The biggest delta in the world, the Ganges-Brahmaputra delta, empties into the Bay of Bengal, and almost all of Bangladesh sits on it. Fish, rice and tea are important products there. Deltas are also busy places for trade: Vancouver in Canada sits on the delta of the Fraser River.

As wetlands, deltas are diverse and ecologically important. National Geographic notes that they absorb runoff from river floods and coastal storms, and filter water as it moves slowly through the distributary network, which can reduce the impact of pollution from upstream. Their animals range from fish, oysters and insects to apex predators such as tigers and bears.

The same silt makes deltas fertile farmland. The world's largest, the Ganges-Brahmaputra delta in India and Bangladesh, carries almost the whole of Bangladesh, with fish and other seafood, rice and tea among its leading products. Deltas are also hubs of trade: Vancouver is built on the Fraser delta, and the Pearl River Delta in southern China, which includes Hong Kong and Macau, is another heavily urbanized delta. USGS figures show what is at stake in coastal Louisiana alone: its wetlands support more than 30 percent of the United States' commercial fishing catch and protect five important shipping ports.

Deltas can grow, and they can shrink.

Some deltas are getting smaller. Dams stop mud from reaching them. But in one bay in Louisiana, new land is growing! People are working to help deltas too.

Deltas change all the time. Over at least the past 5,000 years, the Mississippi has switched channels again and again, building seven or eight different delta lobes.

Today some big deltas are shrinking. Dams and walls along rivers, called levees, stop mud from reaching them, while the land slowly sinks and the sea rises. Coastal Louisiana lost about 5,200 square kilometers of land between 1932 and 2016, the U.S. Geological Survey reports, though the loss has slowed since the 1970s. The Nile delta is shrinking too, partly because the Aswan Dam holds back silt. But in Atchafalaya Bay, Louisiana, two young deltas are growing, adding new land that first rose above the water in 1973.

Deltas shift by avulsion: when a channel's slope decreases and sediment builds up, the river breaks out and finds a steeper route to the sea, abandoning the old lobe. National Geographic says delta lobe switching has built seven or eight distinct Mississippi lobes over at least the past 5,000 years.

Human changes now dominate. The USGS estimates that Louisiana's coastal parishes lost 5,197 km² (2,006 square miles, ± 443 km²) of land from 1932 to 2016. Loss rates peaked in the 1970s at 83.5 km² a year and fell to about 28 km² a year, an American football field every 100 minutes; the USGS names sea-level rise, subsidence and storm-driven erosion as threats, and says dams, levees and flood-control structures have interrupted the Mississippi sediment that once renewed the wetlands. National Geographic notes that the Nile delta is shrinking as the Aswan Dam traps silt and Mediterranean waves erode it. The exception is Atchafalaya Bay: fed by the Atchafalaya, a Mississippi distributary, and the Wax Lake Outlet dug in 1941, new land appeared in 1973, and Louisiana State University scientists estimate the two deltas grow by about 2.8 km² a year. LSU geologist Harry Roberts called them models for building new land, although geologists quoted by NASA say these gains will do little to offset losses elsewhere.