Temps de lecture :4 minutes
Deltas and estuaries constitute the intermediary area between the river and marine environments. A delta is a sedimentary overhang of land on the sea; an estuary is an indentation GLOSSARY at the mouth of a river which allows, to the contrary, the sea to advance into the land. They are generally characterized by variations in salinity –the presence of a freshwater on one side and marine saltwater on the other– as well as by great biomass and production, due mainly to the abundance of nutrients caught up in the water and sediment. This production is estimated at 680g/m3/year, compared to 42 in the oceanic domain and 120 for the continental seas.
Deltas around the world
Approximately forty large deltas exist, the largest being that of the Amazon, covering 467,000 km2, followed by the Gange-Brahmapoutre (Bangladesh), measuring 105,600 km². Among the other large deltas, two are located in China, two in Vietnam (including the Mékong, which supports 18 million inhabitants), six in Russia (e.g. Lena, Volga, Indigirka, Ob), six in North America (e.g. Mississippi, Yukon, Colville), six in Africa (e.g. Niger, Nile, Zambezi, Congo), and four in Europe (Po, Danube, Rhône, Ebre).
Ecosystems of their own…
Deltas and estuaries are characterized by remarkably high numbers of plants and animals. They are favorable to the development of rich milieus such as salt marshes and mangroves forests. Numerous marine species (fish, molluscs, crustaceans) coexist there, from freshwater, brackish and marine origins, as well as mammals (otters, dolphins, manatees) and aquatic reptiles (crocodiles, sea turtles). The young of numerous halieutic GLOSSARY species make prolonged stays in estuaries and this is an obligatory transition habitat for some migratory species, such as salmon, shads and eels. In particular, these regions are important for migrating birds, which make much-needed stops to exploit the abundant food found in estuaries and deltas, joining the large variety of resident birds.
These territories have multiple functions: indispensable coastal nurseries for the replenishment of stores of fish; purification, stocking, transformation and regulation of materials, pollutants and other inputs increasingly being produced; areas of major habitat for intertidal GLOSSARY avifauna and flora linked to wetlands; regulation of precipitation and local humidity by evapotranspiration. These areas are important sources of food for humans but are also frequently used for regulated and unregulated sewage disposal. Aquaculture and fishing are intensely developed in these rich aquatic environments. All kinds of farming is developed around the banks of estuaries and deltas, including rice, nourishing billions of individuals.
… and threatened
Port and continental industries, oil drilling, fishing, aquaculture, tourism… all represent multiple threats to these environments.
– Mining of oil and gas, abundant in the deltas because of their geology, creates mud filled with hydrocarbons.
– The pollution transported by the rivers eventually reaches these systems. The waste from industry, agriculture and cities is concentrated in the sediment, notably heavy metals such as cadmium, lead, zinc and mercury, and also the detergents, pesticides and other biocide products, chlorine, microbial pollutants (pathogenic microorganisms and toxic plankton). The cadmium content in wild oysters in Gironde, while having greatly decreased since the 1980s, remains high: from 10 to 30mg/g of meat (regulatory maximum content is 1 mg/kg).
– The decanting and rerouting of rivers, principally for irrigation and navigation, severely damages the dynamic of the deltas and estuaries, as some rivers no longer flow into the sea. The rerouting of the Colorado, for example, reduced the flow upon its arrival in the Gulf, modifying the landscape and disturbing the ecosystem, resulting in decreased populations of fish, invertebrates and marine mammals.
– The dams, originally created to regulate the flow of water and to serve as a reserve during dry periods, notably for agriculture, can cause delta erosion, water-bourne illnesses (cholera, typhoid, polio, meningitis, hepatitis A and E, diarrhea), disruption of fishing activities, loss of water by evaporation, proliferation of algae, and eutrophization.
– Overfishing harms populations of fish, with certain species being threatened by extinction (e.g. shad, sturgeon).
– Some deltas are threatened with submersion by the rise in sea level due to global warming, or by their recession, a partially natural phenomenon (due to the thickening of sediment accumulated over time), in part aggravated by anthropogenic activity (oil drilling, drilling to the water table).
The Nile Delta, the longest river in the world, runs through some of the most densely populated areas in the planet, with 1000 hab/km2. It has undergone the effects of erosion caused by the Assouan Dam. A few kilometers from the dam, hotels encircling the beach have collapsed, and the lighthouse, built on dry land, now swims under 6 meters of water more than a kilometer from the shore. These disruptions, associated with poor management of water, have resulted in decreased production of irrigated crops.
– Aubry Chantal, Deltas du monde, éditions de la Martinière, Paris, France, 2004.
– S. Fanchette – Hérodote n°121 Menaces sur les deltas – “De l’importance des liens géographie physique/géographie humaine pour comprendre les risques de submersion des deltas surpeuplés”
– Deltas du monde, Dieter Kelletat, Deltaforschung, WB, Darmstadt.