Deltaic Coasts

What is a delta? How does the morphology of a delta reflect the processes acting to shape it? Why are deltas at risk?

Geologic and archeological evidence clearly indicate that a significant part of the rise of modern societies and culture can be attributed to the development of the modern world's deltas, which started forming during slowed post ice age sea level rise. Numerous lines of evidence indicate that well developed societies were occupying deltas in the time frame of 4,000 to 7,000 years before present because of the many natural resources that deltaic environments generally contain. Resources such as shellfish, fish, furs, and plants were necessarily taken from the wild by our ancestors, and so many ancient societies began proximal to deltaic environments.

The term delta comes from Herodotus, a Greek historian and philosopher who recognized the similarity of the upside down Greek letter delta (Δ) and the shape of the Nile river delta when viewed from the south toward the north (Figure 3.27).

Figure 3.27: Oblique view from space of the Nile River, Nile River Delta, and Sinai Peninsula. The lower body of water is the Red Sea: an open sea to the south (left side of image) that is developing upon actively rifting continental crust at this location. Coasts on either side of the Red Sea would be considered neo-trailing coasts because of the active rifting down the center of the Red Sea rift depression. At the top of the images is the Nile River delta at the mouth of the Nile river, denoted clearly by the meandering line of vegetation (green colored land) through the otherwise barren adjacent lands (brown colored land).
Credit: NASA

What is a delta?

A delta is a subaerial and subaqueous volume of sediment that has accumulated at the mouth of a river as it enters into an open body of water. The largest deltas are the product of very large river systems that are transporting large quantities of sediment. The Ganges-Brahmaputra delta is one of the largest deltaic environments on the planet and carries vast amounts of sediment to the ocean where it is reworked by strong tidal currents of the region to form coast perpendicular but inlets and sediment ridges (Figure 3.28).

Figure 3.28: Satellite image of the South-Asia Ganges-Brahmaputra delta consisting of Bangladesh and the state of West Bengal, India. It is one of the world's largest deltas and covers the coastal region shown in the image. Note the brown water where rivers drain out of the tectonically uplifted Himalayan mountains of this collisional zone. The lack of distinct barrier island systems, as well as the coast perpendicular alignment of channels and islands surrounded by channels, suggests that tidal energy is a dominant process driving the evolution of this system.
Credit: NASA

• Delta Morphologies and Driving Processes
• Deltas in Crisis
• Learning Check Point