Deltas are features found where rivers that contain much sediment flow into the sea; velocity is reduced and deposition of material occurs. They are 3-dimensional features with most deposition occurring below the water surface.
Estuaries are where fresh water (rivers) and salt water (sea) interact. They attract industry and settlement due to their provision of:
- Flat land.
- Water supply.
- Deep water.
For example, Avonmouth on the Severn estuary.
Not all rivers have deltas at their mouths and they vary greatly on size and shape. According to geomorphologists, there are 3 main estuary shapes:
- Accurate: Rounded, convex edges. For example, the Nile.
- Cuspate: Material evenly spread from the river.
- Birds foot: Spread over a wide area with many ‘distributaries’ extending out to sea. For example, Mississippi.
Deltas build up over time and on the diagram, 3 beds are visible:
- Bottom set: Have finest material, carried furthest in suspension and sinks to sea bed.
- Foreset: More readily deposited, coarser material. Builds out to sea.
- Topset: Deposits from river with the most coarse material.
Factors influencing formation of deltas:
- Amount and type of sediment available.
- Variations in volume of water discharged from river.
- Aspect and geometry of coast.
- Coastal processes in operation, for example, wave action.
- Changes in coast level.
- Impacts of climate on growth of vegetation and marine organisms.
Deltas are especially favourable for agriculture, due to the deposition of fine sediment, but at the same time are places of high flood risk as shown by the Ganges Delta.
Essentially where rivers and coasts meet. They are often important wildlife habitats, but equally attract human settlement: in the UK, approximately 20 million people live near to estuaries, due to their flat building land. They exist where either a coastline has been raised – meaning that the lower area of the river is drowned or the coastline has subsided. The fact that salt marshes can develop distinguishes then from Rias.