A delta is a low-lying landform at the mouth of a river, often known for fertile soil and abundant vegetation. Use of the term "delta" began with the Greek historian Herodotus, who made the observation that the triangular shape of the Nile River's delta was somewhat similar to the shape of the Greek alphabet's fourth letter -- the delta glyph.
River deltas form slowly, over long periods of time, when waterborne particles of mud, silt, sand and gravel settle to the bottom of a river at its mouth. These materials are often known as sediment or alluvium. They usually accumulate wherever a river's rate of flow begins to slow down -- often, this happens when the river enters a large and relatively calm body of water, such as a gulf. As the movement of the water becomes less urgent, particles floating in the water settle to the bottom and eventually build up into a raised mass that extends downstream. When this happens at the mouth of a river, the accumulation of sediment extends seaward. This pile of sediment grows over many years, until the sediments eventually accrete into a landform that rises above the surface of the water -- though most deltas remain elevated only barely above sea level. As the tiny particles that form a river delta collect, they tend to arrange themselves into a three-tiered landmass. The farthest inland is the upper delta plain, which has the highest elevation, remains above the reach of high tide and maintains a freshwater environment that is able to support diverse ecosystems, such as coastal plains and wetlands. Beyond the upper delta plain is the lower delta plain, which is affected by the incursion of tides and saltwater. Past the lower delta plain, the delta continues as a subaqueous land mass that tapers off into the ocean.
Most rivers cut across their deltas as several branches, called distributaries, though some rivers flow through their deltas as one channel. Other rivers don't form deltas at all, often because of strong coastal currents and tides, the land's submergence or tremendously deep waters offshore where the river empties.
When a delta does form, it tends to be very hospitable to vegetation, including food and other commercial crops, for several reasons. For one, the river that builds the delta helps provide adequate fresh water to the surrounding soil. Also, the soil of the delta itself is usually rich in nutrients, since the sediment from which it is made probably arrived replete with organic matter and useful minerals from upstream. The resulting fertility has made deltas like that of the Nile and the Ganges very important to the development of agricultural civilizations throughout history.
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