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Delta Characteristics
While differences abound, deltas typically all share three primary characteristics. First, they are fed by rivers with large watersheds, or drainage basins. For example, the Mississippi River, which feeds the Mississippi Delta, has a watershed that covers 1.85 million square miles, or roughly 40 percent of the US. Second, they are fed by rivers that carry large amounts of clastic sediment, ranging in size from fine particles to larger rock fragments. Third, due to their slow development, deltas occur on shorelines that are geologically stable, meaning no tectonic activity.
Delta Anatomy
Deltas consist of three primary parts. The upper delta plain represents the first zone, found closest to the river. This zone is dominated by the riverine depositional process. The lower delta plain represents the second, or middle, zone. The lower delta plain resides within the tidal range and is influenced by both riverine deposition and the tidal forces. Both the upper and lower plains make up what is known as the subaerial delta, meaning they are located above the low-tide limit and are exposed to the atmosphere. The subaqueous delta is the outermost zone of the delta, lying underwater at all times. It is comprised of the finest sediments and is primarily shaped by oceanic agents.
Types of Deltas
Deltas come in four basic types. The arcuate delta features a broad, shallow subaqueous zone that is symmetric, giving it a fan-like appearance. The Nile River Delta is an example of an arcuate delta. The bird's foot delta develops around one, or very few, distributive canals that run along ridges. This results in long, thin sedimentary buildups that give these deltas the appearance of bird's toes or claws. The Mississippi River Delta is an example of a bird's foot delta. Cuspate deltas form from one distributary along a flat coastline. Wave action pushes most of the sediment back along the coastline, except for a portion right around the distributary. This buildup slowly extends, protruding like a tooth. The Tiber River Delta is an example of a cuspate delta. Estuarine deltas are formed by riverine deposits within long narrow estuaries, creating a delta inside the coastline. The Seine River Delta is an example of an estuarine delta.
Forces Shaping Deltas
Deltas can be shaped by various combinations of forces. The first developmental force is the river itself. River-dominated deltas are typically wide and triangular. The second developmental force is supplied by tidal actions. Tidal-dominated deltas are typically bell shaped, with tidal bars that run perpendicular to the shoreline. The third developmental force is provided by wave action. Wave-dominated deltas are primarily fan shaped and are impacted very little by tides or waves.