The estuary is the portion of a river, towards its mouth, where the influence of the sea or ocean can be felt. Estuaries are more frequent in seas with well-marked tides, whereas deltas are encountered mainly in seas with lower tidal fluctuations, such as the Mediterranean. Among the most important ones are the Nile delta in Egypt, the Gediz, Göksu, Menderes and Cukurova (Seyhan & Ceyhan) in Turkey, the Evros/Meriç in Turkey and Greece, the Axios-Loudias-Aliakmon (Greece), the Guadalquivir and Ebro in Spain, the Rhône in France and the Po delta in Italy.
Deltas and estuaries are complex wetlands, which frequently include mosaics of fresh, brackish and salt marshes, meanders, lagoons, islands, sand-dunes… Under natural conditions, they are (or were) constantly modified through sea currents, floods, sediments brought by the rivers, winds, and now human interventions.
They support numerous food-chains, and are a vital nursery, breeding and feeding habitat for numerous species of fish, shellfish and waterbirds especially.
As the river speed slows down towards their mouth, many pollutants deposit with the sediments in deltas and estuaries and can therefore concentrate in them in the long run. For centuries now, man has sought to control deltas and estuaries by stabilizing their banks and channels. Today, most Mediterranean deltas are enclosed behind dykes, and their courses are no longer subject to their former wide fluctuations.
Furthermore, deltas are today in a phase of high coastal erosion. The main reasons are first a reduction of the erosion within their watershed (abandonment of former agriculture areas in mountains, progression of forest cover…), as well as the retention of sediments behind dams that have been built by the dozens in their watershed. Not being fed any longer by these sediments, the coastline is now under increasing pressure from sea erosion, e;g. in the Nile, Rhone and Gediz deltas.