Lower Course Landforms: Floodplains, Levees, and Deltas

In the lower course of a river, the gentle gradient and fine sediment produce landforms dominated by deposition: floodplains (wide, flat valley floors), levees (raised banks along the channel), deltas (sediment fans at the river mouth) and estuaries (tidal river mouths where fresh and salt water mix). In brief: when a river floods, it spreads out, slows down and deposits silt, building the floodplain; the coarsest material falls beside the channel and builds natural levees. Where the river enters a sea or lake and the sediment supply exceeds the removal by waves and tides, a delta forms; where tides are strong and the sediment is carried away, an estuary forms instead.

The lower course is where most people live along rivers, because of the fertile land and flat terrain, which also makes it prone to flooding. This guide describes each landform and how it forms, compares them in a table, works through flood probability and sediment build-up, outlines the history of their study and corrects common mistakes.

How do floodplains form?

A floodplain is the flat area of land next to a river that is covered by water when the river overflows its banks. It is formed in two ways: by lateral erosion and migration of meanders, which widens the valley floor and spreads a thin layer of sediment over it, and by deposition of silt (alluvium) during floods. When the river is over the banks, the flow spreads out over a wide area, the depth and velocity drop sharply, and the fine sediment settles, adding a thin layer each time. Over thousands of years, this builds a flat, fertile surface that may be many meters thick. The floodplain often contains features such as oxbow lakes, meander scars and backswamps (wet areas behind the levees). Floodplains are the most important agricultural land in many countries: the Nile, the Ganges, the Mississippi and the Yangtze all support dense populations.

How do levees form?

When a river overflows, the water leaving the channel immediately loses speed, and the coarsest part of its load (sand and silt) is dropped right next to the channel, while the finer material is carried farther out. After many floods, this builds raised ridges along the banks called natural levees, typically a few meters high, and their slopes dip gently away from the river. Levees can raise the river channel slightly above the floodplain, so that the tributaries may run parallel to the main river (a yazoo stream) before they find a place to join. People build artificial levees (embankments) to raise the banks and protect land; they reduce the risk of small floods, but they confine the river, speed up the flow and can fail catastrophically in extreme floods, as happened in New Orleans in 2005 and along the Mississippi in 1927.

How do deltas and estuaries form?

A delta forms where a river carrying a lot of sediment enters a calm body of water, such as a lake or a sea with weak tides and waves. The flow slows, the sediment is deposited, the channel splits into distributaries, and the land builds seaward. The sediment deposits are layered: coarse bottomset, foreset and topset beds. Examples are the Nile, the Mississippi, the Ganges–Brahmaputra and the Mekong deltas; their shapes are arcuate, bird’s-foot or cuspate. An estuary forms where the river meets the sea in a funnel-shaped, tidal mouth, usually in a drowned river valley, and where the tides are strong enough to carry the sediment away rather than allowing it to accumulate. In an estuary, fresh and salt water mix, and the salinity changes with the tides. Estuaries have mudflats and salt marshes, which are rich in wildlife and food for birds, and they make good natural harbors (the Thames, the Hudson, the Gironde). Some estuaries have a very large tidal range, such as the Severn in the UK, and the Bay of Fundy in Canada, where tides can be over 15 m.

Landforms compared

Worked example: floods and sediment

1. Return period: a “100-year flood” has a 1 in 100 chance (1 percent) of occurring in any year. It does not occur once every 100 years on schedule.
2. Probability over 30 years: the chance that no such flood happens in a year is 0.99. Over 30 years, the chance of none is 0.99³⁰ = 0.74, so the chance of at least one is 1 − 0.74 = 26 percent, over a period about as long as a typical mortgage.
3. Over 100 years: 1 − 0.99¹⁰⁰ = 1 − 0.366 = 63 percent, which is why a 100-year flood is not a rare event on this time scale.
4. Levee growth (hypothetical numbers): if each flood adds 2 cm of sediment on the crest of a natural levee, then 100 floods add 2 cm × 100 = 200 cm = 2 m.
5. Floodplain build-up: if a floodplain accumulates 1 mm of silt a year, it takes 1 mm × 5,000 = 5,000 mm = 5 m of sediment in 5,000 years.
6. Flood storage: a flood that covers 10 km² of floodplain with 1.5 m of water stores 10 × 10⁶ m² × 1.5 m = 15 × 10⁶ m³ = 15 million m³, which is water that does not flow downstream at the same time, so building on a floodplain reduces its storage and raises the flood risk downstream.

How have people used lower course landforms?

Floodplains and deltas were the cradles of the first agricultural civilizations (the Nile, the Tigris and Euphrates, the Indus and the Yellow River), where the annual floods renewed the soil. Levees were built from ancient times; the Chinese built dikes along the Yellow River over 2,000 years ago, and that river is still prone to disastrous floods. The Great Mississippi Flood of 1927 led to the Flood Control Act of 1928 and a large program of levees and floodways. Estuaries have served as sites for major ports, such as London, New York and Rotterdam, and today the protection of salt marshes and the restoration of floodplains are used as natural flood defenses.

Which mistakes are common?

Mistake 1: Thinking a “100-year flood” occurs only once every century

It means a 1 percent chance in any year. Two such floods could occur in consecutive years.

Mistake 2: Assuming that levees remove the flood risk

They reduce the frequency of flooding but increase the damage when they fail, and they can raise flood levels elsewhere.

Mistake 3: Confusing deltas and estuaries

Deltas are built up by deposition of sediment. Estuaries are tidal river mouths where the sediment is carried away.

Mistake 4: Treating floods only as disasters

Natural floods renew the floodplain with fertile silt and water. The damage is mainly where people have built on floodplains.