Islands, Archipelagos, and Delta Formations

An island is land surrounded by water, an archipelago is a group or chain of islands, a delta is a low landform built where a river deposits sediment as it enters a standing body of water, and a floodplain is the flat land beside a river that is flooded when the river overflows. In brief: islands form by volcanic eruption, coral growth, or separation from the mainland when sea level rises. Archipelagos such as Indonesia, the Philippines and Japan include thousands of islands. A delta forms when the river slows and drops its load faster than waves and tides remove it; the shape depends on the balance between river, waves and tides, giving arcuate, bird’s-foot or cuspate deltas. Deltas and floodplains are fertile, densely populated and vulnerable to flooding.

These landforms are among the most productive and the most threatened by sea-level rise. This guide describes how islands, archipelagos, deltas and floodplains form, compares the types in a table, works through calculations of sediment build-up and sea-level change, outlines their history and corrects common mistakes.

How do islands and archipelagos form?

There are three main kinds of islands. Continental islands are parts of a continental shelf that were joined to the mainland and were separated by rising sea level after the last ice age, such as Great Britain, Sri Lanka and Tasmania. Volcanic (oceanic) islands are built up from the sea floor by eruptions, either above a hot spot (the Hawaiian Islands) or along subduction zones as island arcs (Japan, the Aleutians, the Lesser Antilles). Coral islands are built by reef-forming organisms: atolls are rings of coral around a lagoon, usually on a sinking volcano, as Charles Darwin proposed; the Maldives are an example. Other types are barrier islands (long sandbars parallel to a coast) and river islands. An archipelago is a group of islands, often formed by the same process: Indonesia has more than 17,000 islands, the Philippines about 7,600, and the Maldives about 1,200. Islands have unique ecosystems, and small low-lying islands are most at risk from sea-level rise.

How do deltas form?

A river carries sediment (sand, silt and clay). When it enters a lake or sea, its speed falls and it deposits its load, first the coarse sediment and then the fine, which builds a fan-shaped area at the river mouth. The river splits into several channels (distributaries), and fine particles of clay may clump together (flocculation) in salt water and settle quickly. A delta grows when sediment supply is greater than the removal by waves, tides and currents. The shape depends on these forces: arcuate (fan-shaped) deltas, such as the Nile, form where waves reshape the front; bird’s-foot deltas, such as the Mississippi, form where the river dominates and the channels extend far out; cuspate deltas (the Tiber) are shaped by waves from opposite directions; tidal deltas, such as the Ganges–Brahmaputra delta (the largest in the world, at roughly 100,000 km²), are shaped by strong tides. Deltas are very fertile and support rice farming and fisheries, but they are low and sink gradually (subsidence) as sediments compact, especially where dams hold back sediment and groundwater or oil is extracted.

What are floodplains?

A floodplain is the flat area on either side of a river channel that is covered by water when the river floods. It is formed by the deposition of silt during floods and by the sideways movement of meanders, which leaves features such as oxbow lakes and natural levées (raised banks made of coarse sediment deposited closest to the river). The soil is fertile because of the renewed silt, which has made floodplains the sites of agriculture and cities, such as the Nile Valley and the Mississippi floodplain. The risk of flooding is managed by levees, dams, floodways and land-use planning, but they also stop the natural deposition of silt.

Island and delta types compared

Worked example: sediment and sea level

1. Delta growth (hypothetical numbers): a river delivers 400 million tonnes of sediment a year to a delta of 100,000 km². With a bulk density of 1.5 tonnes per m³, the volume is 400,000,000 ÷ 1.5 = 267 million m³ a year.
2. Layer thickness: if spread evenly over the delta (100,000 km² = 10¹¹ m²), the layer adds 267,000,000 ÷ 100,000,000,000 = 0.0027 m, or 2.7 mm a year.
3. Compared with sea-level rise: if the sea level is rising at about 4 mm a year and the delta also sinks by 3 mm a year, the relative rise is 7 mm a year. The deposit of 2.7 mm a year does not keep up, so the delta is drowning slowly, 7 − 2.7 = 4.3 mm a year.
4. Atoll elevation: an island whose highest point is 2.4 m above sea level (the Maldives are the lowest country, with a maximum natural elevation of about 2.4 m) would lose 4 mm × 100 = 0.4 m of relative height in 100 years at a rate of 4 mm a year, leaving 2.0 m. Storm surges and waves cause a risk long before the land is covered.
5. Archipelago count: the Philippines has about 7,600 islands, which is 7,600 ÷ 17,000 = 45 percent of the number in Indonesia.

How have these landforms shaped history?

The Greek historian Herodotus described Egypt as “the gift of the Nile”, noting the importance of the river’s silt. The deltas of the Nile, the Tigris–Euphrates, the Yellow River and the Ganges supported early civilizations. In the Netherlands, much of the land in the Rhine–Meuse delta was reclaimed from the sea using dikes and polders since the Middle Ages; roughly a quarter of the country is below sea level. Island nations such as Britain and Japan developed maritime power, and remote islands in the Pacific were settled by Polynesian navigators. Today, delta megacities such as Dhaka, Shanghai and Bangkok face subsidence and sea-level rise.

Which mistakes are common?

Mistake 1: Thinking that all islands are volcanic or coral

Many large islands, such as Great Britain and Madagascar, are continental fragments.

Mistake 2: Assuming that all deltas are triangle-shaped

The shape depends on the river, waves and tides, and they can be fan-shaped, finger-like or pointed.

Mistake 3: Believing that building levees makes the floodplain safe

Levees reduce the frequency of flooding, but a failure can cause deep floods, and they stop the natural deposit of silt.

Mistake 4: Ignoring subsidence when discussing sea-level rise

The relative sea level on a delta depends on both the rise of the sea and the sinking of the land.