A river delta is a dynamic landform created where a river flows into a larger, slower-moving body of water, such as an ocean, sea, or lake. This fan-shaped accumulation of terrestrial material is built from the sediment load transported downstream by the river system. Deltas are constantly evolving environments where the forces of deposition and erosion continually reshape the landscape.
Essential Conditions for Formation
The formation of a river delta requires conditions that permit the sustained accumulation of sediment. The primary condition is an abundant supply of material carried by the river. This sediment, eroded from the upstream drainage basin, must be delivered to the river mouth faster than the receiving basin can remove it.
Another condition relates to the energy level of the receiving basin, such as a sea or lake. For a delta to prograde, or build outward, the water must be relatively calm, characterized by low wave action and minimal tidal range. High-energy environments, featuring strong waves or large tidal fluctuations, tend to disperse the incoming sediment, preventing sustained buildup.
The slope of the continental shelf or lake bottom also influences the process. A wide and shallow shelf provides a suitable area for silt and sand to settle. Furthermore, the river’s flow must slow down sufficiently upon meeting the standing water, allowing the sediment to drop out of suspension. This balance between sediment input and the basin’s ability to redistribute the material determines whether a delta forms or if the river mouth becomes a funnel-shaped estuary.
The Mechanics of Sediment Deposition
The construction of a delta begins with a reduction in the river’s flow velocity as the confined channel water expands into the open water of the basin. This deceleration diminishes the water’s ability to keep transported sediment suspended. As the current slows, the coarsest and heaviest particles, typically sands and gravels, settle out first.
This initial deposition occurs at the river mouth, forming submerged features known as mouth bars. As these bars grow, they obstruct the main channel, forcing the river’s flow to split and seek new paths. This channel bifurcation creates the characteristic network of branching waterways, called distributary channels, across the delta surface.
The repeated clogging and shifting of these channels is known as avulsion. Avulsion allows the delta to spread sediment laterally across a wide area. The main channel follows a path until its gradient becomes too shallow, prompting a sudden switch to a shorter, steeper route. This cyclical process leads to the formation of distinct, overlapping deltaic lobes.
Sedimentary Layers
Geologically, the delta is built up in three distinct sedimentary layers that advance into the receiving basin.
The first layer is the bottomset bed, composed of the finest sediments, like clay and silt, that travel farthest from the river mouth. These particles settle slowly in the deeper, quieter water of the prodelta zone in thin, nearly horizontal layers.
Overlying the bottomset beds are the foreset beds, which represent the main body of the delta front. These layers consist of coarser material, typically sand and mud, deposited on the steeply inclined underwater slope of the advancing delta lobe. The angle of these beds can range from less than one degree in large marine deltas to more than 20 degrees in deltas built by coarse bedload.
Finally, the topset beds form the uppermost, nearly flat surface of the subaerial delta plain, the part visible above the water line. This layer is composed of sediments deposited by shifting distributary channels and flood events. As the delta progrades, the topset beds bury the underlying foreset and bottomset beds, resulting in an outward-building sequence of sedimentary rock layers.
Classifying Delta Shapes
Once sediment is deposited, external forces from the receiving basin dictate the delta’s shape. Scientists classify deltas based on which of the three main forces—river, wave, or tide—exerts the strongest influence on the deposited material.
- River-dominated delta: This type forms in basins with low wave energy and a small tidal range, allowing the river’s depositional processes to dominate. The Mississippi River Delta is a classic example, extending long, finger-like distributary channels far offshore. This characteristic “bird’s foot” shape is maintained by the river building natural levees that confine the flow and push sediment further out.
- Wave-dominated delta: This occurs where wave action is strong enough to rework and redistribute the incoming sediment along the coastline. The Nile River Delta exhibits this pattern, where waves smooth the shoreline into a rounded, arcuate, or cuspate shape. Sediment is pushed back toward the shore and spread laterally, resulting in fewer distributary channels and the formation of beach ridges and barrier islands.
- Tide-dominated delta: This develops in areas subject to a large tidal range and strong tidal currents. These powerful currents push water and sediment both upstream and downstream, significantly modifying channel patterns. This action typically creates numerous wide, funnel-shaped distributary channels separated by long, linear sand bars and islands, leading to a dendritic, or tree-like, drainage pattern, such as the one found in the Fly River Delta.