Where in the World? What geographical feature is the green triangle?
Deltas are river sediments deposited when a river enters a standing body of water such as a lake, a lagoon, a sea or an ocean. They are fundamentally features of river deposition, not marine deposition.
When a river enters a lake it is called a lacustrine delta. Deposition is enhanced if the water is saline: since salty water causes small clay particles to flocculate (the meeting of fresh and salt water produces an electric charge which causes clay particles to coagulate and to settle on the seabed).
Conditions favouring deltaic accumulation are:
- A river with a high sediment load, such as the Nile or the Mississippi.
- Usually a large river. This condition is necessary for marine deltas otherwise the action of the sea might disperse the sediment.
- Reasonably shallow water offshore. Very deep water inhibits delta building. For example, the Congo, which virtually debouches into a submarine canyon, has no delta.
- A coast on which the wave energy is low, such as the Mediterranean or the Gulf of Mexico, though here again how low will depend on other factors such as sediment supply and tidal range.
There are many delta types, but the three ‘classics’ are:
Fan-shaped, having a rounded, convex outer margin; these are found in the areas where regular longshore drift of other currents keep the seaward edge of the delta trimmed and relatively smooth in shape.
Pointed like a tooth or cusp shaped by regular but opposing, gentle water movement.
Where the river brings down enormous amounts of fine silt, deposition can occur in a still sea area, along the edges of the distributaries for a vast distance offshore.
- What is a delta?
- Why is deposition enhanced in salt water?
- List three conditions that favour the formation of deltas
- Use the Earth for web project linked to above to find each of the following deltas and discover which body of water they flow into and what delta type they are.
|River Delta||Body of water into which the river flows||Cuspate, Bird’s Foot or Arcuate|
- Read the news article ‘Global Warming Causing Mediterranean Sea to Rise, Threatening Egypt's Lush Nile Delta’ article linked to above and answer the following questions in detail.
- How much of Egypt’s land area is the Nile Delta?
- How many people live on the Nile Delta?
- What affect did the building of the Aswan Dam have on the Nile Delta?
- List four consequences that sea level rise would have upon the Nile Delta and Egypt.
- What is the Egyptian government doing about the developing situation?