A river delta is a landform created when a river enters a larger, relatively still body of water, such as an ocean, sea, or lake. The land is built up from the huge volume of sand, silt, and clay that the river has carried downstream over millennia. Delta formation is a continuous battle between the river’s power to deposit sediment and the receiving body’s energy to redistribute it.
Essential Conditions for Delta Formation
The river must carry a significant amount of material, referred to as a high sediment load, which is the foundational ingredient for the new land. Rivers with long courses and extensive tributary systems, like the Mississippi or the Nile, are effective at gathering and transporting this massive volume of sediment.
The receiving body of water must be relatively shallow and sheltered to allow the sediment to settle and accumulate. If the continental shelf drops off steeply into very deep water, the material will slide away, preventing the buildup of a stable delta plain. A shallow, gently sloping offshore area is necessary for the underwater platform, known as the subaqueous delta, to form.
The environment requires a low-energy setting where the sediment is not immediately swept away. High wave action, strong ocean currents, or large tidal ranges will quickly erode and disperse the material before it can consolidate into a delta. The river’s ability to deposit material must consistently exceed the rate at which marine forces can remove it for the landform to survive and grow.
The Process of Sediment Deposition
When the river water carrying the sediment reaches the larger body of water, its velocity rapidly decreases. This sudden loss of speed causes the river to lose its ability to keep the heaviest particles suspended. The coarser, heavier sediment, like sand and gravel, drops out almost immediately at the river mouth, forming submerged mounds called mouth bars.
The finer materials, primarily silt and clay, are carried further out into the basin before they settle. In marine deltas, the saltwater environment plays a unique role in speeding up the deposition of these fine particles through a process called flocculation. Clay particles normally repel each other in freshwater, but the dissolved salts in the ocean cause them to clump together into larger, heavier aggregates called flocs.
These flocs sink much faster than individual clay particles, leading to extensive deposition of mud on the seabed beyond the river mouth. This forms an underwater slope, known as the delta front, which gradually builds outward into the receiving basin. This forward growth, or progradation, extends the river’s channel and eventually raises the submerged land above the water level, forming the recognizable delta plain.
Types of Delta Shapes
The final shape of a delta is determined by the balance of power between the river’s sediment input and the external forces of waves and tides in the receiving basin. Deltas are broadly classified based on which of these three forces is dominant. The river-dominated delta forms where the river’s discharge and sediment load greatly overpower the marine forces.
River-dominated deltas result in a “bird’s foot” or lobate shape, with long, projecting channels called distributaries that extend far into the water body. The Mississippi River Delta is a classic example of this form, where the river’s momentum dictates the shape, creating finger-like extensions of land.
A wave-dominated delta forms when wave action is strong enough to rework and smooth the deposited sediment along the coastline. This creates a curved or arcuate shape with a symmetrical, convex shoreline. The Nile River Delta exhibits this fan-like pattern, where strong Mediterranean Sea waves redistribute the river’s sediment to create a broad, uniform arc.
Tide-dominated deltas are shaped by high tidal ranges and strong, reversing tidal currents. These currents erode and redistribute the sediment, often creating long, parallel sand ridges and a funnel-shaped mouth. The Ganges-Brahmaputra Delta, the largest in the world, is an example of this type, where the powerful Bay of Bengal tides create a complex network of channels and tidal flats.