The Delta. Known as the fourth letter of the Greek alphabet, in the form of a Δ. In another, more notorious perspective, a Delta is a sub aqueous accumulation of riverine sediments at the mouth of a river. The receiving basin may be an ocean, sea or lake. Once this basin is reached, deposition results from the loss of momentum and the ability of the river to carry sediment.

The term “delta” was first applied (circa 450 BC) by the Greek historian Herodotus to the triangular alluvial deposits at the mouth of the Nile River. Deltas form wherever sediment at a river mouth is being supplied faster than it can be removed by marine processes.

Most deltas do not exhibit the classic deltoid form but have a wide range features that reflect the environment of the receiving basin. Numerous variables influence the delta formation. As a result, every delta has a unique form and content of species.

The large variety of water types, substrate and dynamic processes make deltas rich in natural biotopes and nutrients, making deltas among the most diverse natural habitats for flora and fauna. And for people. Historically, deltas have been important for civilisation as regions for agriculture, habitation, navigation, and fishing. More recently, deltas have been exploited for oil, gas, and fresh water.

Today, over half the world’s population lives within 200 km from the coastline. The biggest ports of the world are located in deltas. Eight of the top ten largest cities are located by the coast. Production, income, infrastructure, housing and properties… they have all grown fastest in the delta.

At the same time, these deltas are also very vulnerable to natural disasters such as hurricanes and tsunamis, or a surplus of river discharge, causing floods and damages. And lives! Moreover, the deltas may be especially threatened by the impacts of climate change.