A river delta can form anywhere that a stream flows into open water shallower than the stream. Streams carry sediments and other matter that they deposit when they reach the shallow basin. The delta is an accumulation of these sediments near where the stream meets the open water [source: America's Wetland]. There are four main types of river deltas:
- Arcuate deltas are triangular in shape, like the Nile River's delta. This shape, resembling the Greek alphabet's fourth letter, gave deltas their name. Arcuate deltas usually have smooth coastlines because of the wave action and how they are formed.
- Bird-foot deltas have long finger-like projections that reach out to the sea, like the Mississippi River's delta. These deltas might have broad, shallow shelves. The bird-foot delta is named for its long thin shape, much like a bird's toe.
- Cuspate deltas extend to the sea as a V shape with long curving sides, like the Tiber River's delta. The cuspate delta tends to form when one river or stream empties into a flat coastline where it encounters a direct hit from waves. This action forms its tooth-like shape.
- Estuarine deltas form when a river flows into a large estuary that will fill with sediment, as is the case with France's Seine River.
Deltas may be the dumping-off points for streams, but they aren't just rivers' junkyards. They're important to nearby ecosystems. The sediment that collects in deltas is rich and fertile, often hosting fossil fuels and supporting wildlife [source: Earth Scan Laboratory]. These deltas usually lie below sea level. As sea levels continue to rise, the ocean waters are swallowing up fragile deltas.
This also poses a problem for the people who live in delta cities; they rely on the transportation links that the rivers and seas around deltas offer for their social and economic ways of life [source: RNW]. For example, Alexandria, Egypt, is home to 4 million people and a center of national trade, largely because of its port [source: Zayed]. The Coastal Research Institute says that 63.8 yards (58 meters) of coastline have vanished each year since 1989 in one area of the Nile's arcuate delta. If flooding occurs, it could soak up to 12 percent of Egypt's farmland and displace people from nearby communities [source: Zayed].
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